Acids, Bases and Salts – Notes

Introduction

There are about 115 known chemical elements.
They combine to form thousands of compounds.
Based on chemical properties, compounds are mainly classified into:
1. Acids
2. Bases
3. Salts

To identify acids and bases, we use indicators.

Indicators

An indicator is a dye that changes colour in acidic or basic solutions.

1. Litmus (Natural Indicator)

Extracted from lichens.
Neutral colour = Purple.
Available as red litmus and blue litmus.

Reactions:

Acid → Turns blue litmus red.
Base → Turns red litmus blue.

Examples:

Orange juice turns blue litmus → red → Acidic.
NaOH solution turns red litmus → blue → Basic.

👉 Note: Water-soluble bases are called alkalis.

2. Methyl Orange (Synthetic Indicator)

Neutral colour = Orange.
Acid → Red
Base → Yellow

3. Phenolphthalein (Synthetic Indicator)

Neutral colour = Colourless.
Acid → Colourless
Base → Pink

4. Universal Indicator

A mixture of indicators.
Shows a range of colours depending on pH value of the solution.

Natural Indicators

Turmeric (Haldi):
- Acid → Yellow
- Base → Red/Brown
- Example: Curry stains turn reddish-brown when scrubbed with soap (basic).
Red Cabbage Extract:
- Acid → Red
- Base → Green
Flower Petals (Hydrangea, Petunia, Geranium):
- Change colour depending on acidic or basic medium.
- Example: Hydrangea → Blue (acidic), Pink (basic).

Olfactory Indicators (Work with smell)

Their odour changes in acidic or basic solutions.

Examples:

Onion:
- Base (NaOH) → Onion smell disappears.
- Acid → Smell remains.
Vanilla Extract:
- Base (NaOH) → Vanilla smell disappears.
- Acid → Smell remains.

Sample Problem (NCERT)

Q: You are given 3 test tubes containing:

Distilled water
Acidic solution
Basic solution
You have only red litmus paper. How will you identify them?

Solution:

Put red litmus in all test tubes:
- The solution that turns red → blue = Base.
Use this blue litmus formed to test the other two:
- The solution that turns blue → red = Acid.
The solution that does not affect any litmus = Neutral (Distilled water).

Quick Recap Table

Indicator	In Acid	In Base
Litmus	Blue → Red	Red → Blue
Methyl Orange	Red	Yellow
Phenolphthalein	Colourless	Pink
Turmeric	Yellow	Red/Brown
Red Cabbage	Red	Green
Onion Smell	Present	Disappears
Vanilla Smell	Present	Disappears

Acids – Notes

1. What are Acids?

Substances with sour taste (e.g., lemon, tamarind, orange).
Turn blue litmus → red.
Found in fruits, vegetables, and also prepared in laboratories.

Example:

Lemon 🍋 → Contains citric acid → sour taste.
Orange 🍊 → Contains citric acid + ascorbic acid (Vitamin C).

2. Types of Acids

(A) Organic Acids (Natural Acids)

Found in plants and animals.
Weak acids (safe to eat/drink in natural form).

Examples:

Acetic Acid – in vinegar (sirka).
Citric Acid – in citrus fruits (lemon, orange).
Lactic Acid – in curd, sour milk.
Tartaric Acid – in tamarind, unripe grapes.
Oxalic Acid – in tomatoes.
Formic Acid – in ant sting, nettle sting.

👉 Used in food ingredients (pickles, ketchup, baking powder, fizzy drinks).

(B) Mineral Acids (Inorganic/Man-made Acids)

Made from earth’s minerals.
Mostly strong acids and very dangerous.

Common Examples:

Hydrochloric Acid (HCl)
Sulphuric Acid (H₂SO₄)
Nitric Acid (HNO₃)
Carbonic Acid (H₂CO₃) → Weak mineral acid, used in soda water.

⚠️ Concentrated mineral acids can cause severe burns.

3. Strong Acids vs Weak Acids

Strong Acids (Highly corrosive)	Weak Acids (Mild, safe in small amounts)
Hydrochloric acid (HCl)	Acetic acid (vinegar)
Sulphuric acid (H₂SO₄)	Citric acid (lemon/orange)
Nitric acid (HNO₃)	Tartaric acid (tamarind, grapes)
	Formic acid (ant sting)
	Carbonic acid (soda water)

Strong acids: Dangerous even in dilute form.
Weak acids: Used in food and beverages.

4. Concentrated and Dilute Acids

Concentrated acid → Contains very little water.
Dilute acid → Prepared by mixing acid with more water.

⚠️ Important Safety Rule:

Always add acid to water slowly with stirring.
Never add water to acid → Heat produced suddenly → may cause explosion / acid burns.
Dilution is an exothermic process (releases heat).

Activity Example:

Add drops of concentrated H₂SO₄ to water → Beaker becomes hot → Heat is released.

5. Everyday Uses of Acids

Acetic acid (vinegar): Pickles, ketchup.
Tartaric acid: Baking powder.
Carbonic acid: Soft drinks, soda water.
Citric acid: Preservative in juices, jams.

Quick Recap

Acids taste sour and turn blue litmus red.
Organic acids → weak, natural (safe in food).
Mineral acids → mostly strong, man-made, very corrosive.
Strong acids: HCl, H₂SO₄, HNO₃.
Weak acids: Citric, acetic, tartaric, formic, carbonic.
Always add acid to water, never water to acid (dilution safety rule).

Properties of Acids – Notes

Acids show certain characteristic properties that help us identify and study them.

1. Taste

Acids have a sour taste (e.g., lemon, tamarind, curd, vinegar).
⚠️ Never taste laboratory acids – they are corrosive and dangerous.

2. Effect on Litmus

Blue litmus → Red
(No effect on red litmus).

3. Acids Conduct Electricity

Acid solutions in water conduct electricity → they are electrolytes.
Reason: In water, acids produce ions (H⁺ or H₃O⁺), which carry electric current.

4. Reaction with Metals

Acid + Metal → Salt + Hydrogen gas

Example:


Zn (s) + H_2SO_4 (aq) → ZnSO_4 (aq) + H_2 (g)

Test for H₂ gas → Burns with a ‘pop’ sound.
Reason: Metal displaces hydrogen from acid.

⚠️ Sour food (curd, vinegar, lemon juice) should not be kept in metal vessels → Acids react with metal to form poisonous salts → food poisoning risk.

5. Reaction with Carbonates and Hydrogencarbonates

Acid + Metal carbonate → Salt + CO₂ + H₂O
Acid + Metal hydrogencarbonate → Salt + CO₂ + H₂O

👉 CO₂ test: Pass through lime water → turns milky due to formation.

Excess CO₂ makes solution clear again → forms (soluble).

📌 Applications:

Baking soda (NaHCO₃) neutralises excess HCl in stomach (antacid).
Marble, chalk, limestone, and egg-shells (all ) react with acids to release CO₂.

6. Reaction with Bases (Neutralisation Reaction)

Acid + Base → Salt + Water

Example:


NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)

Both acid and base neutralise each other.
Tested using phenolphthalein:
- Pink in base → turns colourless when neutralised by acid.

📌 This principle is used in antacids (like milk of magnesia) to neutralise stomach acid.

7. Reaction with Metal Oxides

Metal oxide + Acid → Salt + Water
Shows that metal oxides are basic in nature.

Example:


CuO (s) + 2HCl (aq) → CuCl₂ (aq) + H₂O (l)

8. Corrosive Nature

Strong mineral acids (HCl, H₂SO₄, HNO₃) are highly corrosive.
Cause burns on skin, holes in clothes, black spots on wood.
Acids are stored in glass or ceramic containers (not metals).

⚠️ Hazard Warning Sign (on acid containers): Indicates danger of burns to skin, wood, clothes.

Quick Recap Table

Property	Observation / Reaction	Example / Note
Taste	Sour	Lemon, vinegar
Litmus	Blue → Red	Acid test
Conductivity	Conduct electricity	Due to ions in water
With Metals	Salt + H₂ gas	Zn + H₂SO₄ → ZnSO₄ + H₂
With Carbonates	Salt + CO₂ + H₂O	Na₂CO₃ + HCl → NaCl + CO₂ + H₂O
With Bases	Salt + Water (Neutralisation)	HCl + NaOH → NaCl + H₂O
With Metal Oxides	Salt + Water	CuO + HCl → CuCl₂ + H₂O
Nature	Corrosive	Burn skin, holes in clothes

What Do All Acids Have in Common? – Notes

1. Common Feature of Acids

All acids contain hydrogen.
When dissolved in water, acids dissociate (ionise) to produce hydrogen ions:


  H+ (aq)

👉 Definition:
An acid is a substance which dissociates in water to produce hydrogen ions .

2. Hydrogen Ions and Hydronium Ions

Free H⁺ ions cannot exist alone in water.
They combine with water molecules to form hydronium ions (H₃O⁺):


  H^+ + H2O \rightarrow H_3O^+

For convenience, acids are written as producing H⁺(aq).

3. Examples of Acids Producing H⁺ Ions

Hydrochloric Acid (HCl):


   HCl (aq) \rightarrow H^+ (aq) + Cl^- (aq)

Sulphuric Acid (H₂SO₄):


   H_2SO_4 (aq) \rightarrow 2H^+ (aq) + SO_4^{2-} (aq)

Nitric Acid (HNO₃):


   HNO_3 (aq) \rightarrow H^+ (aq) + NO_3^- (aq)

Acetic Acid (CH₃COOH):


   CH_3COOH (aq) \rightarrow CH_3COO^- (aq) + H^+ (aq)

👉 In all cases, hydrogen ions are released in water, giving acidic behaviour.

4. Compounds That Contain Hydrogen But Are Not Acids

Not all hydrogen-containing compounds are acids.
Example:
- Glucose (C₆H₁₂O₆)
- Alcohol (C₂H₅OH)
They contain hydrogen but do not ionise in water to produce H⁺ ions.
Hence, they do not show acidic properties.

5. Key Conclusion

All acids contain hydrogen.
Acidity is due to the release of H⁺(aq) ions in water.
Not all hydrogen compounds are acids (only those that release H⁺ in solution).

Quick Recap Table

Substance	Dissociation in Water	Acidic?
HCl	H⁺(aq) + Cl⁻(aq)	Yes
H₂SO₄	2H⁺(aq) + SO₄²⁻(aq)	Yes
HNO₃	H⁺(aq) + NO₃⁻(aq)	Yes
CH₃COOH	H⁺(aq) + CH₃COO⁻(aq)	Yes
Glucose (C₆H₁₂O₆)	No ionisation → no H⁺	No
Alcohol (C₂H₅OH)	No ionisation → no H⁺	No

To Show That All the Compounds Containing Hydrogen are Not Acids

Aim:

To prove that not all hydrogen-containing compounds behave as acids.

Materials Required

Beaker
Rubber cork with two iron nails fixed in it
Battery (6 volt), switch, and bulb
Solutions of:
1. Dilute Hydrochloric acid (HCl)
2. Dilute Sulphuric acid (H₂SO₄)
3. Glucose (C₆H₁₂O₆)
4. Alcohol (C₂H₅OH)

Procedure

Arrange the circuit as shown:
- Two iron nails are connected to a battery, bulb, and switch.
- The nails are dipped into the solution kept in the beaker.
Pour dilute hydrochloric acid solution in the beaker and switch on the current.
Repeat the experiment using:
- Dilute sulphuric acid solution
- Glucose solution
- Alcohol solution

Observations

With HCl solution, the bulb glows → HCl solution conducts electricity.
With H₂SO₄ solution, the bulb glows → H₂SO₄ solution conducts electricity.
With Glucose solution, the bulb does not glow → glucose solution does not conduct electricity.
With Alcohol solution, the bulb does not glow → alcohol solution does not conduct electricity.

Explanation

Acid solutions conduct electricity because they produce ions in water.
Example:


  HCl (aq) \rightarrow H^+ (aq) + Cl^- (aq)

Glucose and alcohol contain hydrogen but do not ionise in water → no ions → no conduction → bulb does not glow.
Hence, not all hydrogen compounds are acids.

Additional Note

Distilled water does not conduct electricity because it has no ions.
Rain water conducts electricity because it dissolves CO₂ from air, forming carbonic acid (H₂CO₃), which produces ions:


  H_2CO_3 \rightarrow 2H^+ (aq) + CO_3^{2-} (aq)

Conclusion

All acids contain hydrogen, but only those compounds which produce H⁺ ions in water behave as acids.
Compounds like glucose and alcohol contain hydrogen but are not acids because they do not ionise in water to produce H⁺ ions.

✅ These notes clearly cover experiment setup → results → explanation → conclusion, making them perfect for school exams

Acids Do Not Show Acidic Behaviour in the Absence of Water

Key Idea

Acids show acidic behaviour due to the presence of hydrogen ions, H⁺(aq).
Acids produce H⁺ ions only in the presence of water.
In the absence of water, acids do not release hydrogen ions and hence do not show acidic properties.

Experiment: Using Hydrogen Chloride Gas (HCl)

Apparatus / Materials

Boiling tube
Sodium chloride (NaCl)
Concentrated sulphuric acid (H₂SO₄)
Rubber cork with delivery tube
Dry blue litmus paper
Moist (wet) blue litmus paper

Procedure

Take about 1 g sodium chloride (NaCl) in a dry boiling tube.
Add concentrated sulphuric acid carefully → this produces hydrogen chloride gas (HCl gas).




   NaCl + H_2SO_4 \; \rightarrow \; NaHSO_4 + HCl (g)

Hold a dry blue litmus paper at the mouth of the tube.
Hold a moist blue litmus paper at the mouth of the tube.

Observations

Dry blue litmus paper → No colour change → HCl gas does not show acidic behaviour without water.
Moist blue litmus paper → Turns red → HCl gas shows acidic behaviour in the presence of water.

Explanation

Dry HCl gas contains no hydrogen ions → no acidic behaviour.
When HCl dissolves in water, it ionises:


  HCl (g) + H_2O (l) \; \rightarrow \; H^+ (aq) + Cl^- (aq)


  HCl + H_2O \; \rightarrow \; H_3O^+ + Cl^-

Conclusion

Acids do not show acidic behaviour in the absence of water.
Water is essential for the ionisation of acids into hydrogen ions (H⁺/H₃O⁺), which are responsible for their acidic properties.

✅ These notes cover definition → experiment → observation → explanation → conclusion, making them easy to learn and perfect for exams.

Strong and Weak Acids

1. Strong Acids

Definition

An acid which completely ionises in water to produce a large amount of hydrogen ions (H⁺/H₃O⁺) is called a strong acid.

Examples

Hydrochloric acid (HCl):


HCl (aq) \;\;\rightarrow\;\; H^+ (aq) + Cl^- (aq)


H_2SO_4 (aq) \;\;\rightarrow\;\; 2H^+ (aq) + SO_4^{2-} (aq)


HNO_3 (aq) \;\;\rightarrow\;\; H^+ (aq) + NO_3^- (aq)

Key Points

Single arrow (→) shows complete ionisation.
“Strong” refers to degree of ionisation, not concentration.
Properties of strong acids:
- Produce many H⁺ ions.
- React rapidly with metals, carbonates, etc.
- Show high electrical conductivity (bulb glows brightly).
- Strong electrolytes.

2. Weak Acids

Definition

An acid which partially ionises in water to produce only a small amount of hydrogen ions (H⁺/H₃O⁺) is called a weak acid.

Examples

Acetic acid (CH₃COOH):


CH_3COOH (aq) \;\;\rightleftharpoons\;\; CH_3COO^- (aq) + H^+ (aq)

Sulphurous acid (H₂SO₃)

Key Points

Double arrow (⇌) shows partial ionisation.
Properties of weak acids:
- Produce fewer H⁺ ions.
- React slowly with metals, carbonates, etc.
- Show low electrical conductivity (bulb glows dimly).
- Weak electrolytes.

3. Effect of Dilution

When concentrated acid is diluted with water, the concentration of H⁺ (aq)/H₃O⁺ ions per unit volume decreases.
This reduces the strength of the acid solution (but does not change whether it is strong or weak by nature).

4. Uses of Mineral Acids

(a) Sulphuric Acid (H₂SO₄)

Manufacture of fertilisers (e.g., ammonium sulphate).
Used in paints, dyes, detergents, plastics, synthetic fibres, explosives.
Used in car batteries.

(b) Nitric Acid (HNO₃)

Manufacture of fertilisers (e.g., ammonium nitrate).
Explosives (e.g., TNT – trinitrotoluene).
Dyes and plastics.

(c) Hydrochloric Acid (HCl)

Removing oxide film from steel (before galvanising).
Removing scale deposits from boilers.
Used in textile, dye, food, and leather industries.

Comparison Table: Strong vs Weak Acids

Feature	Strong Acids (HCl, H₂SO₄, HNO₃)	Weak Acids (CH₃COOH, H₂CO₃, H₂SO₃)
Ionisation in water	Complete (→)	Partial (⇌)
Amount of H⁺ ions	Large	Small
Reactivity	Very fast	Slow
Conductivity	High (bulb glows brightly)	Low (bulb glows dimly)
Nature as electrolytes	Strong electrolytes	Weak electrolytes

✅ With this format, students can understand, memorise, and revise quickly.

Bases – Notes for Students

1. What are Bases?

Substances like caustic soda (NaOH), lime (CaO/choona), and washing soda (Na₂CO₃) are bitter in taste and soapy/slippery to touch.
Such substances are called bases.

👉 Definition:
A base is a chemical substance that:

Has a bitter taste.
Turns red litmus paper blue.
Neutralises acids (cancels their effect).

2. Examples of Bases

Metal oxides: Sodium oxide (Na₂O), Calcium oxide (CaO).
Metal hydroxides: Sodium hydroxide (NaOH), Calcium hydroxide [Ca(OH)₂].
Others: Ammonium hydroxide (NH₄OH), Metal carbonates & hydrogencarbonates (Na₂CO₃, CaCO₃, NaHCO₃).

3. Alkalis (Water-Soluble Bases)

Not all bases dissolve in water.
The bases that do dissolve in water are called alkalis.

👉 Definition:
A base soluble in water is called an alkali.

Common Alkalis:

Sodium hydroxide (NaOH)
Potassium hydroxide (KOH)
Calcium hydroxide [Ca(OH)₂]
Ammonium hydroxide (NH₄OH)
Magnesium hydroxide [Mg(OH)₂]

💡 Alkalis are more useful because most chemical reactions happen in aqueous solutions.

4. Common Property of All Bases

When a base dissolves in water → it produces hydroxide ions (OH⁻ ions).

Examples:

NaOH → Na⁺ + OH⁻
KOH → K⁺ + OH⁻
Mg(OH)₂ → Mg²⁺ + 2OH⁻

👉 It is the OH⁻ ions that give bases their basic (alkaline) properties.

5. Concentration of Bases

Adding more water to a base solution dilutes it (decreases OH⁻ concentration per unit volume).

6. Types of Bases

(a) Strong Bases

Completely ionise in water → produce a large amount of OH⁻ ions.
Examples: NaOH, KOH.

(b) Weak Bases

Partially ionise in water → produce a small amount of OH⁻ ions.
Examples: NH₄OH, Ca(OH)₂, Mg(OH)₂.

Quick Recap / Key Points

✅ Bases → Bitter, soapy, turn red litmus blue, neutralise acids.
✅ Alkalis → Water-soluble bases (NaOH, KOH, NH₄OH, etc.).
✅ All bases produce OH⁻ ions in water.
✅ Strong bases (NaOH, KOH) = completely ionised.
✅ Weak bases (NH₄OH, Ca(OH)₂, Mg(OH)₂) = partially ionised.

Properties of Bases (Alkalis)

1. Bases have a bitter taste

All bases taste bitter (e.g., NaOH solution).
⚠️ Note: Never taste chemicals in labs—this is only for theory.

2. Bases feel soapy to touch

They are slippery or soapy when rubbed between fingers.
Example: Sodium hydroxide (NaOH) solution feels slippery like soap.

3. Bases turn red litmus blue

This is a test property of bases.

4. Bases conduct electricity in solution

Bases ionise in water → produce ions (charged particles).
These ions help the solution conduct electricity.
Hence, bases are electrolytes.

5. Bases react with some metals to form Hydrogen gas

Not all metals react, but some like zinc (Zn) do.
Example:


  2NaOH (aq) + Zn (s) \xrightarrow{\text{Heat}} Na_2ZnO_2 (aq) + H_2 (g)

The salt formed is sodium zincate (Na₂ZnO₂).

6. Bases react with acids to form salt and water (Neutralisation)

Example:


  2NaOH (aq) + H_2SO_4 (aq) → Na_2SO_4 (aq) + 2H_2O (l)


  H^+ (aq) + OH^- (aq) → H_2O (l)

7. Bases react with non-metal oxides to form salt and water

Non-metal oxides are acidic in nature.
Example:


  Ca(OH)_2 (aq) + CO_2 (g) → CaCO_3 (s) + H_2O (l)

Uses of Bases

Sodium hydroxide (NaOH) – Used in making soap, paper, rayon (synthetic fibre).
Calcium hydroxide [Ca(OH)₂] (Slaked lime) – Used in making bleaching powder.
Magnesium hydroxide [Mg(OH)₂] – Used as an antacid to cure indigestion.
Sodium carbonate (Na₂CO₃) – Used as washing soda, also for softening hard water.
Sodium hydrogencarbonate (NaHCO₃) – Used as baking soda in cooking, in baking powder, as an antacid, and in soda-acid fire extinguishers.

Quick Recap (Exam Points) ✅

Bases → Bitter, Soapy, Turn red litmus blue, Conduct electricity.
Reactions:
- With metals → Salt + H₂ gas.
- With acids → Salt + Water (Neutralisation).
- With non-metal oxides → Salt + Water.
Uses: NaOH → soap & rayon; Ca(OH)₂ → bleaching powder; Mg(OH)₂ → antacid; Na₂CO₃ → washing soda; NaHCO₃ → baking soda.

🌟 Strength of Acid and Base Solutions – The pH Scale

🔹 Ionisation of Water

Water (H₂O) is slightly ionised into:
In pure water:
- [H⁺] = [OH⁻]
- Therefore, water is neutral (neither acidic nor basic).

🔹 Acids in Water

Acids produce H⁺ ions in water.
More H⁺ ions → acidic solution.
Even acids have some OH⁻ ions (from water), but [H⁺] > [OH⁻].

🔹 Bases in Water

Bases produce OH⁻ ions in water.
More OH⁻ ions → basic solution.
Even bases have some H⁺ ions (from water), but [OH⁻] > [H⁺].

🔹 pH Scale (Introduced by Sorenson, 1909)

pH = “power of hydrogen” → measure of H⁺ concentration.
Range: 0 to 14
pH ∝ 1/[H⁺]
- High [H⁺] → Low pH → Strong acid
- Low [H⁺] → High pH → Strong base

⚡ pH is a number only (no units).

📊 Rules of the pH Scale

Neutral Substances (pH = 7)
- Examples: Pure water, NaCl solution, sugar solution.
- No colour change with litmus, methyl orange, or phenolphthalein.
Acids (pH < 7)
- More acidic = Lower pH.
- Strong acids: pH = 0, 1, 2, 3
- Weak acids: pH = 4, 5, 6
- Indicators:
  - Blue litmus → Red
  - Methyl orange → Red
Bases (pH > 7)
- More basic = Higher pH.
- Weak bases: pH = 8, 9, 10
- Strong bases: pH = 11, 12, 13, 14
- Indicators:
  - Red litmus → Blue
  - Phenolphthalein → Pink

🔹 Relation of pH to Strength

From pH 7 → 0: [H⁺] increases → Acid strength ↑
From pH 7 → 14: [OH⁻] increases → Base strength ↑

📌 pH Values of Some Common Substances

Substance	pH	Nature
Conc. HCl	0	Strong acid
Dil. HCl	1	Strong acid
Gastric juice	1.4	Strong acid
Lemon juice	2.5	Acidic
Vinegar	4.0	Weak acid
Tomato juice	4.1	Weak acid
Coffee	5.0	Weak acid
Soft drinks	6.0	Weak acid
Milk	6.5	Slightly acidic
Pure water	7.0	Neutral
Saliva (before meals)	7.4	Slightly basic
Blood	7.4	Slightly basic
Eggs	7.8	Slightly basic
Toothpaste	8.0	Weak base
Baking soda solution	8.5	Weak base
Washing soda solution	9.0	Weak base
Milk of magnesia	10.5	Base
Household ammonia	11.6	Strong base
Dil. NaOH	13.0	Strong base
Conc. NaOH	14	Very strong base

🌈 Summary (For Quick Revision)

pH < 7 → Acidic (more acidic as pH ↓)
pH = 7 → Neutral
pH > 7 → Basic (more basic as pH ↑)
Strong acids: 0–3
Weak acids: 4–6
Weak bases: 8–10
Strong bases: 11–14

👉🌟 Universal Indicator

🔹 Why We Need Universal Indicator

Ordinary indicators (like litmus, methyl orange, phenolphthalein) only show:
- Whether a substance is acidic or basic.
- They cannot tell how strong or weak the acid/base is.
Example:
- Litmus shows both vinegar and sulphuric acid as acids.
- But it cannot tell that sulphuric acid is much stronger than vinegar.

✅ To measure the strength of acids and bases, we use the Universal Indicator.

🔹 What is a Universal Indicator?

Universal indicator = A mixture of many indicators (dyes).
It gives different colours at different pH values (0–14).
Works by showing the concentration of H⁺ ions in a solution.
Can be used as:
- Solution (drops added to liquid)
- Paper strips (dipped into solution)

🔹 How It Works

Add the solution (acid/base) to universal indicator paper/solution.
It produces a colour change.
Compare the colour with the pH colour chart provided.
The pH value tells us whether it is:
- Strong acid
- Weak acid
- Neutral
- Weak base
- Strong base

📊 Colours of Universal Indicator at Different pH Values

pH	Colour	Type of solution
0	Dark red	Strong acid
1–2	Red	Strong acid
3	Orange-red	Acid
4	Orange	Weak acid
5	Orange-yellow	Weak acid
6	Greenish-yellow	Weak acid (near neutral)
7	Green	Neutral
8	Greenish-blue	Weak base
9	Blue	Weak base
10	Navy blue	Base
11	Purple	Strong base
12–14	Dark purple/Violet	Strong base

🔹 Examples of Use

If universal indicator turns dark red → pH ≈ 0 → Strong acid (e.g., conc. HCl).
If colour is orange → pH ≈ 4 → Weak acid (e.g., vinegar).
If colour is green → pH = 7 → Neutral (e.g., pure water).
If colour is blue → pH ≈ 9 → Weak base (e.g., baking soda solution).
If colour is violet → pH ≈ 14 → Strong base (e.g., NaOH).

🔹 Advantages of Universal Indicator

✅ Shows not just acidic/basic nature but also strength of acids and bases.
✅ Covers the entire pH scale (0–14).
✅ Simple to use in labs (both as paper and solution).

🌈 Quick Recap (For Exams)

Litmus: only acid or base (no strength).
Universal Indicator: shows pH value & strength of acids/bases.
pH Colour Range:
- 0–3 → Strong acids (red)
- 4–6 → Weak acids (orange/yellow)
- 7 → Neutral (green)
- 8–10 → Weak bases (blue)
- 11–14 → Strong bases (purple/violet)

Importance of pH in Everyday Life

👉 The pH scale measures how acidic or basic a solution is (0–14).

pH < 7 → Acidic
pH = 7 → Neutral
pH > 7 → Basic

The pH plays a vital role in many activities of our daily life such as digestion, tooth care, agriculture, environment, and even self-defence in plants and animals.

1. pH in Our Digestive System

Our stomach produces HCl (pH ≈ 1.4) which helps in digesting food.
Sometimes, excess acid causes indigestion, pain, and irritation.
To cure this, we take antacids (mild bases) such as:
- Magnesium hydroxide (Milk of Magnesia)
- Sodium hydrogencarbonate (Baking soda)
These neutralise the extra acid and give relief.

2. pH Change and Tooth Decay

Bacteria in the mouth break down sugars → produce lactic acid.
If pH falls below 5.5, tooth enamel (calcium phosphate) starts corroding → tooth decay.
Prevention:
- Rinse mouth after eating.
- Use toothpaste (pH ≈ 8) → neutralises excess acid.
- Reduce sugary food intake.

3. pH and Plants & Animals

(i) Soil pH and Plant Growth

Most plants grow best when soil pH ≈ 7.
Too acidic soil → add lime (CaO), slaked lime (Ca(OH)₂), or chalk (CaCO₃).
Too basic soil → add decaying organic matter (manure/compost).
Farmers test soil pH and adjust it for better crop yield.

(ii) pH and Survival of Animals

Human body works best in pH range 7.0 – 7.8.
Aquatic animals (fish, etc.) survive in water within a narrow pH range.
Acid rain (pH < 5.6) lowers water pH → aquatic life threatened.
To protect, CaCO₃ is added to lakes → neutralises acid.

4. Self-Defence by Animals and Plants

Bee sting → injects acidic liquid.
- Relief: apply mild base (baking soda).
Wasp sting → injects alkaline liquid.
- Relief: apply mild acid (vinegar).
Ant sting → injects methanoic acid.
- Relief: apply baking soda.
Nettle plant leaves → inject methanoic acid through stinging hairs.
- Relief: apply baking soda or dock plant leaf (contains base).

Quick Revision Points (Exam Tips)

Stomach acid pH ≈ 1.4 → digestion.
Tooth decay starts below pH 5.5.
Plants grow best at soil pH ≈ 7.
Human blood pH = 7.35 – 7.45 (slight variation → illness).
Acid rain harms aquatic life (neutralised with CaCO₃).
Bee sting = acidic, Wasp sting = basic.

📘 Question Bank on BASES – Class 10 Science

A. Multiple Choice Questions (MCQs)

Q1. Which of the following is a base?
a) HCl
b) NaOH
c) CO₂
d) H₂SO₄
✅ Answer: b) NaOH

Q2. Bases produce which ions in aqueous solution?
a) H⁺
b) OH⁻
c) Na⁺
d) Cl⁻
✅ Answer: b) OH⁻

Q3. Which of the following turns red litmus to blue?
a) Acids
b) Bases
c) Salts
d) Water
✅ Answer: b) Bases

Q4. Milk of magnesia is used as an antacid because:
a) It is acidic
b) It is neutral
c) It is a weak base
d) It is a salt
✅ Answer: c) It is a weak base

Q5. Which of the following is not an alkali?
a) KOH
b) NaOH
c) Ca(OH)₂
d) Cu(OH)₂
✅ Answer: d) Cu(OH)₂ (not soluble in water, so not an alkali)

B. Very Short Answer Questions (1 mark)

Q1. Define a base.
✅ Answer: A base is a substance which produces hydroxide ions (OH⁻) when dissolved in water.

Q2. Give one example each of a strong base and a weak base.
✅ Answer: Strong base – NaOH; Weak base – NH₄OH.

Q3. What is an alkali?
✅ Answer: A base which is soluble in water is called an alkali.

Q4. Name one base used in soap making.
✅ Answer: Sodium hydroxide (NaOH).

Q5. Write the formula of slaked lime.
✅ Answer: Ca(OH)₂

C. Short Answer Questions – Type I (2 marks)

Q1. Differentiate between a base and an alkali.
✅ Answer:

Base: A substance that produces OH⁻ ions in water (may or may not be soluble). Example: Cu(OH)₂.
Alkali: A water-soluble base. Example: NaOH, KOH.

Q2. What happens when a base reacts with an acid? Write a general equation.
✅ Answer: They neutralize each other to form salt and water.
Equation: Acid + Base → Salt + Water

Q3. Why is sodium hydroxide called a strong base?
✅ Answer: Because it ionises completely in water to give a high concentration of OH⁻ ions.

D. Short Answer Questions – Type II (3 marks)

Q1. Write three uses of bases in daily life.
✅ Answer:

NaOH is used in making soap.
Ca(OH)₂ is used in whitewashing walls.
Mg(OH)₂ (milk of magnesia) is used as an antacid.

Q2. Explain the reaction of bases with:
(a) Metals
(b) Non-metal oxides
(c) Ammonium salts
✅ Answer:
(a) Base + Metal → Salt + H₂ gas
Example: 2NaOH + Zn → Na₂ZnO₂ + H₂↑
(b) Base + Non-metal oxide → Salt + Water
Example: 2NaOH + CO₂ → Na₂CO₃ + H₂O
(c) Base + Ammonium salt → Ammonia + Water + Salt
Example: NaOH + NH₄Cl → NaCl + H₂O + NH₃↑

Q3. Give one test to identify bases in the laboratory.
✅ Answer: Bases turn red litmus blue or turn phenolphthalein pink.

E. Long Answer Questions (5 marks)

Q1. Explain the chemical properties of bases with suitable examples.
✅ Answer:

Reaction with acids (Neutralisation):
Base + Acid → Salt + Water
Example: NaOH + HCl → NaCl + H₂O
Reaction with metals:
Base + Metal → Salt + H₂
Example: 2NaOH + Zn → Na₂ZnO₂ + H₂↑
Reaction with non-metal oxides:
Base + Non-metal oxide → Salt + Water
Example: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Reaction with ammonium salts:
Base + Ammonium salt → Salt + Water + Ammonia
Example: NaOH + NH₄Cl → NaCl + H₂O + NH₃
Reaction with indicators:
Bases turn red litmus → blue, phenolphthalein → pink, methyl orange → yellow.

Q2. What are bases and alkalis? Explain with examples. Also, list the differences between strong and weak bases.
✅ Answer:

Bases: Substances that release OH⁻ ions in water. Example: NaOH, Ca(OH)₂.
Alkalis: Water-soluble bases. Example: NaOH, KOH.

Strong bases: Completely ionise in water (e.g., NaOH, KOH).
Weak bases: Partially ionise in water (e.g., NH₄OH, Mg(OH)₂).

Differences:

Strong Base	Weak Base
Completely ionised	Partially ionised
High OH⁻ concentration	Low OH⁻ concentration
Strongly corrosive	Less corrosive

Question Bank on Acids (Class 10 Science – CBSE)

A. Multiple Choice Questions (MCQs)

Which of the following will turn blue litmus red?
a) NaCl solution
b) Glucose solution
c) HCl solution
d) Alcohol solution
Answer: (c) HCl solution
Which of the following is a weak acid?
a) HCl
b) H₂SO₄
c) CH₃COOH
d) HNO₃
Answer: (c) CH₃COOH
The acidic behaviour of acids is due to:
a) Oxygen ions
b) Hydrogen ions (H⁺/H₃O⁺)
c) Hydroxide ions (OH⁻)
d) Chloride ions
Answer: (b) Hydrogen ions (H⁺/H₃O⁺)
Which of the following will conduct electricity?
a) Dilute HCl solution
b) Dilute H₂SO₄ solution
c) Dilute HNO₃ solution
d) All of these
Answer: (d) All of these
Dry HCl gas does not change the colour of dry litmus paper because:
a) It has no Cl⁻ ions
b) It has no H⁺ ions in absence of water
c) It has no acidic nature
d) It is neutral
Answer: (b) It has no H⁺ ions in absence of water

B. Very Short Answer Questions (1 mark each)

Define acids as per Arrhenius theory.
Answer: Acids are substances which produce hydrogen ions (H⁺/H₃O⁺) in aqueous solution.
Give one example of a strong acid and one weak acid.
Answer: Strong acid – HCl, Weak acid – CH₃COOH.
Why does rainwater conduct electricity but distilled water does not?
Answer: Rainwater dissolves CO₂ and other acidic gases forming ions, while distilled water has no ions.
Which ion is responsible for acidic nature of substances?
Answer: H⁺ ion (or H₃O⁺ ion).
Why does HCl show acidic behaviour only in presence of water?
Answer: Because HCl dissociates into H⁺ and Cl⁻ ions only in aqueous solution, not in dry state.

C. Short Answer Questions – Type I (2 marks each)

Why is HCl considered a strong acid while CH₃COOH is a weak acid?
Answer: HCl ionises completely in water to produce large amount of H⁺ ions (strong acid), while CH₃COOH ionises partially giving fewer H⁺ ions (weak acid).
Write the chemical reaction to show the ionisation of HCl in water.
Answer:




   HCl (aq) \rightarrow H^+ (aq) + Cl^- (aq)


   HCl + H_2O \rightarrow H_3O^+ + Cl^-

Why does a solution of alcohol and glucose not conduct electricity though they contain hydrogen?
Answer: Because they do not produce ions (H⁺ or other ions) in water, hence no conduction.

D. Short Answer Questions – Type II (3 marks each)

Explain with an activity that all compounds containing hydrogen are not acids.
Answer:
- Set up an electric circuit with bulb, battery, switch, and beaker.
- Pour HCl solution → bulb glows (conducts electricity).
- Pour H₂SO₄ solution → bulb glows.
- Pour glucose or alcohol solution → bulb does not glow.
  Conclusion: All H-containing compounds are not acids; only those producing H⁺ ions in water are acids.
Differentiate between strong acids and weak acids with examples.
Answer:

Strong Acids (e.g., HCl, H₂SO₄)	Weak Acids (e.g., CH₃COOH, H₂CO₃)
Completely ionise in water.	Partially ionise in water.
Produce large H⁺ ions.	Produce few H⁺ ions.
High conductivity, fast reactions.	Low conductivity, slow reactions.

E. Long Answer Questions (5 marks each)

With the help of an experiment, show that acids do not show acidic behaviour in the absence of water.
Answer:
- Take NaCl + conc. H₂SO₄ → HCl gas forms.
- Pass dry HCl gas over dry blue litmus → no change.
- Pass dry HCl gas over moist blue litmus → turns red.
  Explanation: HCl gas does not produce H⁺ ions without water. In presence of water, it forms H₃O⁺ ions → shows acidic behaviour.
Explain the uses of mineral acids in industries.
Answer:
- Sulphuric acid (H₂SO₄): used in fertilisers, dyes, detergents, car batteries, plastics.
- Nitric acid (HNO₃): used in fertilisers, explosives (TNT), dyes, plastics.
- Hydrochloric acid (HCl): used to clean steel, remove boiler scale, textile and leather industries.
Explain why strong acids are good conductors of electricity while weak acids are poor conductors. Support your answer with chemical equations.
Answer:
- Strong acids (e.g., HCl): complete ionisation → many H⁺ ions → high conductivity.


     HCl \rightarrow H^+ + Cl^-


     CH_3COOH \;\;\rightleftharpoons\;\; CH_3COO^- + H^+

SALTS

👉 Definition:
A salt is a compound formed when the hydrogen (H) of an acid is replaced by a metal or ammonium ion (NH₄⁺).

Example:

HCl (Hydrochloric acid) → Replace H with Na → NaCl (Sodium chloride)
HCl + NaOH → NaCl + H₂O

Thus, salts are formed by the reaction of an acid with a base (neutralization reaction).

General Characteristics of Salts

Mostly solid, crystalline compounds.
Have high melting and boiling points.
Usually soluble in water (some are sparingly soluble, e.g., CaCO₃).
Aqueous solutions of salts conduct electricity (electrolytes).
Every salt contains:
- Cation (positive ion) → from base
- Anion (negative ion) → from acid

Example: NaCl → Na⁺ (cation) + Cl⁻ (anion)

Naming of Salts

First part of name → from base.
Second part of name → from acid.

Examples:

Sodium chloride → NaOH (base) + HCl (acid)
Copper sulphate → Cu(OH)₂ (base) + H₂SO₄ (acid)

Types of salts based on acids:

Hydrochloric acid → Chlorides
Sulphuric acid → Sulphates
Nitric acid → Nitrates
Carbonic acid → Carbonates
Acetic acid → Acetates

Important Salts and Their Formulae

Salt	Formula
Sodium chloride	NaCl
Calcium chloride	CaCl₂
Magnesium chloride	MgCl₂
Zinc chloride	ZnCl₂
Sodium sulphate	Na₂SO₄
Potassium sulphate	K₂SO₄
Calcium sulphate	CaSO₄
Magnesium sulphate	MgSO₄
Zinc sulphate	ZnSO₄
Copper sulphate	CuSO₄
Ammonium sulphate	(NH₄)₂SO₄
Sodium nitrate	NaNO₃
Potassium nitrate	KNO₃
Calcium carbonate	CaCO₃
Sodium carbonate	Na₂CO₃
Zinc carbonate	ZnCO₃
Aluminium sulphate	Al₂(SO₄)₃
Sodium acetate	CH₃COONa

Key Points to Remember (Exam Tips)

Common salt = NaCl (used in food, industry).
Salts are formed by neutralization: Acid + Base → Salt + Water.
Salts are ionic compounds (made of cations & anions).
Most salts are soluble and good conductors in solution.
Different acids give different families of salts (chlorides, sulphates, nitrates, carbonates, acetates).

✨ These notes cover all NCERT concepts in a short, crisp, and clear format for exams.

Family of Salts

👉 Definition:
Salts having the same positive ion (cation) or the same negative ion (anion) belong to the same family of salts.

Examples

Same cation (positive ion):

NaCl (Sodium chloride) and Na₂SO₄ (Sodium sulphate) → both belong to the Sodium salt family (Na⁺ ions).

Same anion (negative ion):

NaCl (Sodium chloride) and KCl (Potassium chloride) → both belong to the Chloride salt family (Cl⁻ ions).

Important Families of Salts

Cation-based families: Sodium salts, Potassium salts, Calcium salts, Magnesium salts, Zinc salts, Copper salts, Aluminium salts, Ammonium salts.
Anion-based families: Chloride salts, Sulphate salts, Nitrate salts, Carbonate salts, Acetate salts.

Sample Problem (NCERT Book Question)

Q. Write the formulae of the salts given below and identify the acids and bases from which these salts may be obtained:
Potassium sulphate, Sodium sulphate, Calcium sulphate, Magnesium sulphate, Copper sulphate, Sodium chloride, Sodium nitrate, Sodium carbonate, Ammonium chloride. How many families can you identify among these salts?

Solution

Name of Salt	Formula	Obtained from (Base + Acid)
Potassium sulphate	K₂SO₄	KOH + H₂SO₄
Sodium sulphate	Na₂SO₄	NaOH + H₂SO₄
Calcium sulphate	CaSO₄	Ca(OH)₂ + H₂SO₄
Magnesium sulphate	MgSO₄	Mg(OH)₂ + H₂SO₄
Copper sulphate	CuSO₄	Cu(OH)₂ + H₂SO₄
Sodium chloride	NaCl	NaOH + HCl
Sodium nitrate	NaNO₃	NaOH + HNO₃
Sodium carbonate	Na₂CO₃	NaOH + H₂CO₃
Ammonium chloride	NH₄Cl	NH₄OH + HCl

Families Identified

From the above salts, 10 families can be identified:

Potassium salts
Sodium salts
Calcium salts
Magnesium salts
Copper salts
Ammonium salts
Sulphate salts
Chloride salts
Nitrate salts
Carbonate salts

Key Points (Exam Tips)

Salts with the same cation → same family.
Salts with the same anion → same family.
Example: NaCl, Na₂SO₄, NaNO₃ → All are sodium salts.
Families of salts are useful to classify compounds systematically.

✅ These notes cover all NCERT concepts + solved problem in a short and crisp format.

The pH of Salt Solutions

👉 Salt Formation:
A salt is formed when an acid reacts with a base (neutralisation reaction).

Expected: Salt solution should be neutral (pH = 7).
Reality: Some salt solutions are acidic (pH < 7) or basic (pH > 7) because of hydrolysis.

📌 Hydrolysis = Splitting of a salt by water to give acidic or basic nature.

Types of Salt Solutions

(i) Salts of Strong Acid + Strong Base → Neutral Solution

Example: NaCl (from HCl + NaOH)
Reaction: NaCl (s) + H₂O → No hydrolysis
pH = 7 → Neutral
Does not affect litmus.
Other Example: K₂SO₄ (KOH + H₂SO₄)

(ii) Salts of Strong Acid + Weak Base → Acidic Solution

Example: NH₄Cl (from HCl + NH₄OH)
Reaction:
NH₄Cl + H₂O → NH₄OH (weak base) + HCl (strong acid)
HCl ionises completely → Excess H⁺ ions → Acidic
pH < 7 → Turns blue litmus red
Other Example: (NH₄)₂SO₄ (H₂SO₄ + NH₄OH)

(iii) Salts of Weak Acid + Strong Base → Basic Solution

Example: Na₂CO₃ (from H₂CO₃ + NaOH)
Reaction:
Na₂CO₃ + 2H₂O → 2NaOH (strong base) + H₂CO₃ (weak acid)
NaOH ionises completely → Excess OH⁻ ions → Basic
pH > 7 → Turns red litmus blue
Other Example: CH₃COONa (CH₃COOH + NaOH)

Summary Table: pH of Salt Solutions

Salt	Acid + Base	Nature	pH
Sodium chloride (NaCl)	Strong acid (HCl) + Strong base (NaOH)	Neutral	7
Ammonium chloride (NH₄Cl)	Strong acid (HCl) + Weak base (NH₄OH)	Acidic	< 7
Sodium carbonate (Na₂CO₃)	Weak acid (H₂CO₃) + Strong base (NaOH)	Basic	> 7
Sodium acetate (CH₃COONa)	Weak acid (CH₃COOH) + Strong base (NaOH)	Basic	> 7

Key Exam Points

Strong acid + strong base → Neutral
Strong acid + weak base → Acidic
Weak acid + strong base → Basic
Litmus Test:
- Acidic salt → Turns blue → red
- Basic salt → Turns red → blue

✅ These notes are student-friendly + NCERT-based with definitions, reactions, examples, and a summary table.

Great! Let’s prepare perfect exam notes on Common Salt (Sodium Chloride) that are simple, clear, and NCERT-based 👇

Common Salt (Sodium Chloride)

Introduction

Common salt is a white crystalline substance used in food.
Also known as salt.
Chemical name: Sodium chloride (NaCl)
Nature: Neutral salt.

Laboratory Preparation

Reaction:

NaOH (aq) + HCl (aq) \rightarrow NaCl (aq) + H₂O (l)

❌ Not used on large scale (because NaCl is abundantly available in nature).

Sources of Common Salt

(i) From Sea Water

Sea water contains many dissolved salts.
Major salt = NaCl.
Obtained by evaporation of sea water in shallow pools.
Salt obtained → impure (contains other salts).
Purified before use.
🌍 Main industrial source of salt.

(ii) From Underground Deposits (Rock Salt)

Large underground crystals of NaCl are called rock salt.
Usually brown (due to impurities).
Obtained by mining (like coal).
Rock salt formed when ancient seas dried up thousands of years ago.

Uses of Common Salt (NaCl)

Raw material in industry
- Used for making many important chemicals:
  - Sodium hydroxide (caustic soda)
  - Sodium carbonate (washing soda)
  - Sodium hydrogencarbonate (baking soda)
  - Hydrochloric acid (HCl)
  - Hydrogen, Chlorine, Sodium metal
Cooking food
- Improves flavour.
- Essential for:
  - Proper working of nervous system
  - Muscle movement
  - Production of HCl in stomach (digestion).
Preservative
- Used in pickles.
- Used for curing meat and fish (prevents spoilage).
Soap Industry
- Used in the manufacture of soap.
Melting ice
- Sprinkled on icy roads in cold countries to melt ice.

Key Points to Remember

NaCl is a neutral salt.
Obtained from sea water (evaporation) and rock salt deposits (mining).
Known as raw material for chemical industry.
Has domestic + industrial + biological importance.

✅ These notes are short, crisp, and exam-ready (with reactions, sources, uses, and key points).

Perfect! Let’s make clear, exam-focused notes on Chemicals from Common Salt. I’ll break it into headings, reactions, diagrams (conceptual), and bullet points so students can learn fast and revise quickly.

Chemicals from Common Salt (NaCl)

👉 Common salt is an important raw material for many chemicals.
The three major chemicals obtained from it are:

Sodium hydroxide (NaOH / Caustic soda)
Sodium carbonate (Na₂CO₃ / Washing soda)
Sodium hydrogencarbonate (NaHCO₃ / Baking soda)

We begin with sodium hydroxide.

1. Sodium Hydroxide (NaOH / Caustic Soda)

Preparation (Chlor–Alkali Process)

Raw material: Brine (concentrated NaCl solution).
Process: Electrolysis of brine.
Reaction:


2NaCl (aq) + 2H₂O (l) \xrightarrow{\text{Electricity}} 2NaOH (aq) + Cl₂ (g) + H₂ (g)

Products formed:
- At Anode ( + ) → Chlorine (Cl₂)
- At Cathode ( – ) → Hydrogen (H₂)
- In solution → Sodium hydroxide (NaOH)

👉 This process is called the Chlor–Alkali Process:

"Chlor" → Chlorine
"Alkali" → NaOH

Uses of Sodium Hydroxide

Making soaps and detergents
Manufacture of artificial fibres (rayon)
Paper industry
Purification of bauxite ore (to obtain aluminium)
Degreasing metals, oil refining, dyes, bleaches

Uses of Chlorine (by-product)

Sterilising drinking water & swimming pools
Making bleaching powder
Making hydrochloric acid (HCl)
Making PVC (plastics), pesticides, CFCs, chloroform, dyes, paints
Making dry-cleaning solvents (e.g., trichloroethane)

Uses of Hydrogen (by-product)

Hydrogenation of oils → vegetable ghee (margarine)
Manufacture of HCl
Making ammonia (NH₃) → fertilisers
Making methanol (CH₃OH)
Rocket fuel (liquid hydrogen)

Uses of Hydrochloric Acid (HCl)

(formed by combination of H₂ + Cl₂)

Cleaning iron sheets before tin-plating / galvanisation
Preparing chlorides (e.g., ammonium chloride → dry cells)
Used in medicines and cosmetics
Used in textile, dyeing, tanning industries
Used in making PVC (plastic)

Another Important Compound: Sodium Hypochlorite (NaClO)

Formed when NaOH reacts with Cl₂.
Used as:
- Bleaching agent (household bleach)
- Bleaching fabrics

Quick Concept Map

Common Salt (NaCl) → Brine → Electrolysis →

NaOH (caustic soda) → soaps, rayon, paper
Cl₂ (chlorine) → bleaching powder, PVC, disinfectants
H₂ (hydrogen) → ammonia, methanol, ghee
HCl (from Cl₂ + H₂) → cleaning, medicines, plastics
NaClO → bleach

✅ These notes give preparation, reaction, products, and all uses in one place – perfect for CBSE Class 10 exams.

Great! Let’s turn this into concise, student-friendly notes on Washing Soda (Na₂CO₃·10H₂O).

Washing Soda (Sodium Carbonate Decahydrate)

👉 Washing soda is a hydrated salt of sodium carbonate.

Formula: Na₂CO₃·10H₂O
Called Sodium Carbonate Decahydrate
Anhydrous sodium carbonate (without water) is called Soda Ash (Na₂CO₃).
It is an important chemical obtained from common salt (NaCl).

Production of Washing Soda

Washing soda is produced from sodium chloride in 3 steps (Solvay Process):

(i) Preparation of Sodium Hydrogencarbonate (NaHCO₃)


NaCl + NH₃ + H₂O + CO₂ \; \longrightarrow \; NaHCO₃ \downarrow + NH₄Cl

(ii) Conversion to Sodium Carbonate (Na₂CO₃)


2NaHCO₃ \xrightarrow{\text{Heat}} Na₂CO₃ + CO₂ + H₂O

(iii) Crystallisation into Washing Soda


Na₂CO₃ + 10H₂O \;\; \longrightarrow \;\; Na₂CO₃·10H₂O

Properties of Washing Soda

Transparent crystalline solid.
One of the few metal carbonates soluble in water.
Aqueous solution is alkaline (turns red litmus → blue).
Has detergent (cleansing) properties: removes dirt/grease by converting them into soluble products.

Uses of Washing Soda (Na₂CO₃·10H₂O)

Cleansing agent – used in washing clothes, dry soap powders.
Used to remove permanent hardness of water.
In the manufacture of glass, soap and paper.
Used to make sodium compounds like borax.

Quick Recap

Washing soda = Na₂CO₃·10H₂O (hydrated)
Soda ash = Na₂CO₃ (anhydrous)
Made from common salt via Solvay Process
Important in cleaning, industries, water treatment

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Perfect — let’s turn this into student-friendly, exam-ready notes on Baking Soda (NaHCO₃).

Baking Soda (Sodium Hydrogencarbonate)

👉 Chemical name: Sodium Hydrogencarbonate
👉 Formula: NaHCO₃
👉 Also called Sodium Bicarbonate
👉 Commonly known as Baking Soda

Production of Baking Soda

Baking soda is made from common salt (NaCl) by the Solvay Process:


NaCl + NH₃ + H₂O + CO₂ \;\; \longrightarrow \;\; NaHCO₃ \downarrow + NH₄Cl

Sodium hydrogencarbonate (NaHCO₃) precipitates as a solid because it is only slightly soluble in water.

Properties of Baking Soda

White crystalline solid, sparingly soluble in water.
Mild, non-corrosive base → aqueous solution is weakly alkaline.
Action of heat:


   2NaHCO₃ \xrightarrow{\text{Heat}} Na₂CO₃ + CO₂ + H₂O

Uses of Baking Soda

1. As an Antacid (Medicine)

Neutralises excess stomach acid → relieves acidity & indigestion.

2. In Baking (Baking Powder)

Baking powder = Baking soda (NaHCO₃) + a mild edible acid (e.g., tartaric acid).
Reaction with acid releases CO₂ gas, which:
- gets trapped in dough,
- makes cakes/bread rise and become soft & spongy.
Note: Using only baking soda can leave behind Na₂CO₃, giving a bitter taste.
Acid in baking powder neutralises Na₂CO₃ → prevents bitterness.

3. In Fire Extinguishers

Soda–acid fire extinguisher uses the reaction:


  NaHCO₃ + H₂SO₄ \;\; \longrightarrow \;\; Na₂SO₄ + CO₂ + H₂O

forms a blanket over fire → cuts oxygen supply,
cools the burning substance → extinguishes fire.

Important Notes

Baking soda ≠ Baking powder
- Baking soda → single compound (NaHCO₃).
- Baking powder → mixture of NaHCO₃ + edible acid (e.g., tartaric/citric acid).
Bleaching agent = removes colour.
Disinfectant = kills germs/bacteria.

Quick Recap

Baking soda = NaHCO₃
Made from common salt
On heating → Na₂CO₃ + CO₂ + H₂O
Uses → antacid, baking, fire extinguishers

✅ These notes cover definitions, equations, properties, and uses in a crisp format for CBSE Class 10 exams.

Here are the best exam-ready notes for Bleaching Powder (CaOCl₂) in a crisp and clear format for CBSE Class 10 students:

Bleaching Powder (Calcium Oxychloride)

👉 Chemical formula: CaOCl₂
👉 Also called: Chloride of Lime

Preparation

Bleaching powder is made by passing chlorine gas over dry slaked lime (Ca(OH)₂):


Ca(OH)₂ + Cl₂ \;\; \longrightarrow \;\; CaOCl₂ + H₂O

Properties of Bleaching Powder

White powder with a strong smell of chlorine.
Soluble in cold water, but some insoluble lime remains.
Reacts with dilute acids to liberate chlorine gas:


CaOCl₂ + H₂SO₄ \;\; \longrightarrow \;\; CaSO₄ + Cl₂ + H₂O

The chlorine gas released is the actual bleaching agent.
Bleaching action is due to the oxidising property of chlorine.
Acts as a storage and source of chlorine, since chlorine gas is hard to store directly.

Uses of Bleaching Powder

Bleaching Agent
- Used in textile industry to bleach cotton & linen.
- Used in paper industry to bleach wood pulp.
- Used in laundries for bleaching washed clothes.
Disinfectant
- Used to disinfect drinking water → kills germs & bacteria.
Manufacture of Chemicals
- Used to prepare chloroform (CHCl₃).
Treatment of Wool
- Used to make wool unshrinkable.
Oxidising Agent
- Widely used in chemical industries as a strong oxidiser.

Important Notes

Bleaching action of CaOCl₂ = due to chlorine released.
Chlorine oxidises coloured substances → makes them colourless.
Bleaching powder = indirect way to store chlorine safely.

Quick Recap

Formula: CaOCl₂ (Calcium oxychloride)
Prepared from: Chlorine + Slaked lime
Properties: White powder, smells of chlorine, liberates Cl₂ with dilute acids
Uses: Bleaching, disinfecting water, chemical manufacture, oxidiser

✅ These notes are short, crisp, and cover preparation, properties, and uses — exactly what examiners look for.

Here are perfect exam notes for Plaster of Paris (POP), written in a crisp and clear student-friendly format:

Plaster of Paris (POP)

👉 Chemical name: Calcium sulphate hemihydrate
👉 Formula: CaSO₄ · ½H₂O (also written as 2CaSO₄ · H₂O)
👉 Common name: POP
👉 Name origin: First made by heating gypsum found near Paris.

Preparation

Plaster of Paris is made by heating gypsum (calcium sulphate dihydrate, CaSO₄·2H₂O) to 100°C (373 K):


CaSO₄·2H₂O \; \xrightarrow{100°C} \; CaSO₄·½H₂O + 1½H₂O

Controlled heating is important.
If gypsum is heated above 100°C, it loses all water → forms anhydrous CaSO₄ (dead burnt plaster), which does not set with water.

Properties of POP

White powder.
Sets into a hard mass when mixed with water (about half an hour).




   CaSO₄·½H₂O + 1½H₂O \;\; \longrightarrow \;\; CaSO₄·2H₂O

(POP hydrates back to gypsum → forms a hard solid).

Slight expansion in volume on setting → useful for making moulds/casts.
Must be stored in moisture-proof containers → otherwise it slowly sets by absorbing moisture and becomes useless.

Uses of POP

Medical use – For making plaster casts to keep fractured bones in position; also in dentistry for impressions.
Toys & Decoration – Used for making toys, statues, cosmetics, decorative materials, and blackboard chalks.
Fire-proofing – Used as a fire-resistant material.
Laboratory use – For sealing air-gaps in apparatus (air-tight arrangements).
Construction – Used for smooth finishing of walls before painting and for ornamental designs on walls & ceilings.

Quick Recap

Formula: CaSO₄·½H₂O
Made from: Heating gypsum at 100°C
Key property: Sets into a hard mass with water (forms gypsum)
Uses: Bone setting, toys, fire-proofing, lab sealing, wall finishing

✅ These notes are exam-ready: short, simple, with reactions, properties, and uses — perfect for quick revision before CBSE exams.

Here are exam-ready notes on Water of Crystallisation & Hydrated Salts in a clear, student-friendly format 👇

Water of Crystallisation : Hydrated Salts

Definition

Some salts contain a fixed number of water molecules as an essential part of their crystal structure.
These molecules are called water of crystallisation.
The salts which contain water of crystallisation are called hydrated salts.

👉 Water of crystallisation is not free water – it does not wet the salt but is built into its crystal lattice.

Examples of Hydrated Salts

Copper sulphate (Blue vitriol)
- Formula: CuSO₄·5H₂O
- Name: Copper sulphate pentahydrate
- Colour: Blue
Sodium carbonate (Washing soda)
- Formula: Na₂CO₃·10H₂O
- Name: Sodium carbonate decahydrate
- Colour: White
Calcium sulphate (Gypsum)
- Formula: CaSO₄·2H₂O
- Name: Calcium sulphate dihydrate
- Colour: White
Iron(II) sulphate (Green vitriol)
- Formula: FeSO₄·7H₂O
- Name: Iron sulphate heptahydrate
- Colour: Green

Importance of Water of Crystallisation

Gives shape to crystals.
Imparts colour to some salts:
- CuSO₄·5H₂O → Blue
- FeSO₄·7H₂O → Green
Without water of crystallisation, salts lose colour and become white powders.
- Example: Heating CuSO₄·5H₂O → gives white CuSO₄ (anhydrous).

Key Points

Pentahydrate = 5 water molecules
Decahydrate = 10 water molecules
Dihydrate = 2 water molecules
Heptahydrate = 7 water molecules

Quick Recap (Exam Tips)

Water of crystallisation = essential water in crystal lattice.
Salts look dry (water is bound, not free).
Colours of hydrated salts:
- CuSO₄·5H₂O → Blue
- FeSO₄·7H₂O → Green
Heating removes water → salt turns white and anhydrous.

✅ These notes are short, crisp, and easy to revise for CBSE Class 10 exams.

Here are perfect exam-ready notes on the Action of Heat on Hydrated Salts in a clear, simple, and student-friendly format 👇

Action of Heat on Hydrated Salts

Key Concept

When hydrated salts are heated strongly, they lose their water of crystallisation.
As a result, they:
1. Lose their regular crystal shape
2. Lose their colour
3. Become white, powdery anhydrous salts

👉 The salts without water of crystallisation are called anhydrous salts.

Example: Copper Sulphate (CuSO₄·5H₂O)

On heating:
- Blue hydrated copper sulphate crystals lose water → form white anhydrous copper sulphate.


   CuSO₄·5H₂O \xrightarrow{Heat} CuSO₄ + 5H₂O

On adding water:
- White anhydrous copper sulphate becomes blue again.


   CuSO₄ + 5H₂O \rightarrow CuSO₄·5H₂O

👉 This is a reversible process.

Experiment: Heating Copper Sulphate Crystals

Take blue copper sulphate crystals in a dry boiling tube.
Heat strongly over a burner flame.
The blue colour disappears, and white powder (anhydrous CuSO₄) is formed.
Small droplets of water are seen on the tube wall → due to water of crystallisation coming out.
Cool the tube, then add a few drops of water.
The white powder turns blue again → hydrated copper sulphate is restored.

Application

Anhydrous copper sulphate is used as a test for water/moisture.
If a liquid turns white anhydrous CuSO₄ → blue, it confirms the presence of water.

Summary

Heating hydrated salts removes water of crystallisation → white anhydrous salts form.
Adding water restores hydrated form and colour.
This principle is used to detect water in liquids.

Action of Heat on Hydrated Salts – Quick Comparison Table

Hydrated Salt	Formula	Colour (Before Heating)	Effect of Heating	Anhydrous Salt	Colour (After Heating)
Copper sulphate	CuSO₄·5H₂O	Blue	Loses 5H₂O	CuSO₄	White
Iron(II) sulphate	FeSO₄·7H₂O	Green	Loses 7H₂O + decomposes → Fe₂O₃ + SO₂ + SO₃	Ferric oxide + gases	Brown (solid)
Sodium carbonate	Na₂CO₃·10H₂O	White crystals	Loses 10H₂O	Na₂CO₃	White (powder)
Calcium sulphate	CaSO₄·2H₂O (Gypsum)	White	Loses 1.5 H₂O	CaSO₄·½H₂O (Plaster of Paris)	White
Magnesium sulphate	MgSO₄·7H₂O	Colourless crystals	Loses 7H₂O	MgSO₄	White powder

✅ Special Note for Exams:

Copper sulphate (Blue → White → Blue) is the most common example.
Iron(II) sulphate not only loses water but also decomposes, releasing SO₂ and SO₃ gases (smell of burning sulphur).
Gypsum → Plaster of Paris on heating at 100°C.

Got it 👍 You want a complete set of CBSE Class 10 Science Exam Questions & Answers (MCQs, Very Short, Short, and Long Answer) covering ALL topics of Salts (like common salts, uses, preparation, properties, plaster of Paris, bleaching powder, water of crystallisation, action of heat, etc.).

Here’s a well-organised exam practice sheet 👇

Salts – CBSE Class 10 Science Exam Practice Questions

A. Multiple-Choice Questions (MCQs)

The chemical formula of washing soda is:
a) Na₂CO₃·10H₂O
b) NaHCO₃
c) NaCl
d) CaCO₃
Answer: a) Na₂CO₃·10H₂O
Plaster of Paris is chemically:
a) CaSO₄·2H₂O
b) CaSO₄·½H₂O
c) CaSO₄
d) CaCO₃
Answer: b) CaSO₄·½H₂O
The compound used for disinfecting drinking water is:
a) Baking soda
b) Bleaching powder
c) Washing soda
d) Common salt
Answer: b) Bleaching powder
The blue colour of copper sulphate crystals is due to:
a) Cu²⁺ ions
b) Water of crystallisation
c) Sulphate ions
d) None of these
Answer: b) Water of crystallisation
Which gas is released when sodium bicarbonate is heated?
a) Oxygen
b) Carbon dioxide
c) Hydrogen
d) Nitrogen
Answer: b) Carbon dioxide

B. Very Short Answer Questions (1 mark each)

Write the chemical name of NaHCO₃.
Answer: Sodium hydrogen carbonate (Baking soda).
Which salt is used for making the surface of walls smooth before painting?
Answer: Plaster of Paris (CaSO₄·½H₂O).
What is the formula of gypsum?
Answer: CaSO₄·2H₂O.
Name one salt which is used as an antacid.
Answer: Sodium hydrogen carbonate (NaHCO₃).
Why is common salt sometimes called rock salt?
Answer: Because it is obtained by mining from underground deposits formed by evaporation of ancient seas.

C. Short Answer Questions – Type I (2 marks each)

What is meant by water of crystallisation? Give one example.
Answer: Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt, essential for its crystal structure. Example: CuSO₄·5H₂O (copper sulphate pentahydrate).
Why is baking powder preferred in cooking instead of baking soda?
Answer: Baking soda (NaHCO₃) produces bitter taste if used alone. Baking powder contains NaHCO₃ + weak acid (like tartaric acid), which neutralises the bitter taste and releases CO₂ for fluffiness in cakes.

D. Short Answer Questions – Type II (3 marks each)

Write the chemical equations for the preparation of:
(i) Washing soda from sodium carbonate
(ii) Plaster of Paris from gypsum.

Answer:
(i) Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
(ii) CaSO₄·2H₂O →(heat at 100°C)→ CaSO₄·½H₂O + 1½H₂O
Explain why anhydrous copper sulphate is used to detect the presence of moisture in liquids.
Answer: Anhydrous CuSO₄ is white. When exposed to moisture, it gets hydrated to CuSO₄·5H₂O and turns blue. Hence, change of colour indicates the presence of moisture.
A sample of bleaching powder exposed to air loses its chlorine smell after some time. Why?
Answer: Because it reacts with carbon dioxide in air to form CaCO₃ and liberates chlorine gas which escapes.

E. Long Answer Questions (5 marks each)

(a) What is bleaching powder? Write its chemical formula.
(b) How is it prepared? Write the equation.
(c) List three important uses.

Answer:
(a) Bleaching powder is calcium oxychloride, formula CaOCl₂.
(b) Prepared by passing chlorine gas over dry slaked lime:
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
(c) Uses:
- Disinfecting drinking water
- Bleaching textiles and paper pulp
- Manufacture of chloroform

(a) What happens when:
(i) Washing soda is exposed to air
(ii) Baking soda is heated
(iii) Blue copper sulphate crystals are heated strongly

(b) Give one use each of washing soda, baking soda and copper sulphate.

Answer:
(a) (i) Washing soda loses water of crystallisation and becomes white powder (efflorescence).
(ii) 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O
(iii) CuSO₄·5H₂O → CuSO₄ + 5H₂O (blue → white)

(b) Uses:
- Washing soda: removes hardness of water
- Baking soda: used in making cakes, antacid
- Copper sulphate: used in electroplating and as a fungicide