Properties of Matter: Physical vs Chemical & SI Units Guide

Properties of Matter and Their Measurement

Some Basic Concepts of Chemistry

1.3.1 Physical and Chemical Properties

Every substance has unique or characteristic properties. These are broadly classified into two categories:

Physical Properties

• Definition: Can be measured or observed without changing the identity or composition of the substance.
• Examples: Colour, odour, melting point, boiling point, density.

Chemical Properties

• Definition: Observation requires a chemical change to occur.
• Examples: Combustibility, reactivity with acids/bases, acidity/basicity.

Chemists use these properties to describe, interpret, and predict the behavior of substances through careful measurement and experimentation.

1.3.2 Measurement of Physical Properties

Scientific investigation requires quantitative measurement. A quantitative observation consists of two parts:

Number + Unit
Example: 6 m (6 is the number, m is the unit for metre)

Systems of Measurement

• English System: Used historically in different parts of the world.
• Metric System: Originated in France (late 18th century). Preferred because it is based on the decimal system.

1.3.3 The International System of Units (SI)

Established in 1960 by the 11th General Conference on Weights and Measures (CGPM). It is the standard system used by the scientific community worldwide.

Table 1.1: Base Physical Quantities and Their Units

Base Physical Quantity	Symbol for Quantity	Name of SI Unit	Symbol for SI Unit
Length	l	metre	m
Mass	m	kilogram	kg
Time	t	second	s
Electric current	I	ampere	A
Thermodynamic temperature	T	kelvin	K
Amount of substance	n	mole	mol
Luminous intensity	Iv	candela	cd

Definitions of SI Base Units

Metre (m): Defined by the fixed numerical value of the speed of light in vacuum ($c$) to be $299,792,458$ m/s.

Kilogram (kg): Defined by the Planck constant ($h$) to be $6.62607015 \times 10^{-34}$ J s.

Second (s): Defined by the caesium frequency ($\Delta \nu_{Cs}$) of the unperturbed ground-state hyperfine transition of the caesium-133 atom.

Mole (mol): Contains exactly $6.02214076 \times 10^{23}$ elementary entities (Avogadro number).

National Standards in India

In India, the National Physical Laboratory (NPL) in New Delhi is responsible for maintaining the National Standards of Measurement. This ensures that the measurements used in our industries and labs match international standards.

© 2026 Chemistry Student Portal

Table 1.2: Definitions of SI Base Units

Note: Since 2019, all SI units are defined by fixing the numerical value of universal constants. This ensures the units remain the same forever, anywhere in the universe.

Length Metre (m)

Defined by the fixed numerical value of the speed of light in vacuum c = 299,792,458 when expressed in m s⁻¹.

Depends on: Speed of Light ($c$), Second ($s$)

Mass Kilogram (kg)

Defined by the fixed numerical value of the Planck constant h = 6.62607015 × 10⁻³⁴ when expressed in J s (kg m² s⁻¹).

Depends on: Planck constant ($h$), Metre ($m$), Second ($s$)

Time Second (s)

Defined by the fixed numerical value of the caesium frequency Δν_Cs = 9,192,631,770 Hz (s⁻¹). This is the hyperfine transition frequency of the caesium-133 atom.

The "Master Unit": Most other units depend on this definition.

Electric Current Ampere (A)

Defined by the fixed numerical value of the elementary charge e = 1.602176634 × 10⁻¹⁹ when expressed in C (A s).

Depends on: Elementary charge ($e$), Second ($s$)

Thermodynamic Temperature Kelvin (K)

Defined by the fixed numerical value of the Boltzmann constant k = 1.380649 × 10⁻²³ when expressed in J K⁻¹ (kg m² s⁻² K⁻¹).

Depends on: Boltzmann constant ($k$), Kilogram ($kg$), Metre ($m$), Second ($s$)

Amount of Substance Mole (mol)

One mole contains exactly 6.02214076 × 10²³ elementary entities. This is the Avogadro constant ($N_A$).

Independent of other base units.

Luminous Intensity Candela (cd)

Defined by the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency $540 \times 10^{12}$ Hz, K_cd = 683 lm W⁻¹.

Depends on: $K_{cd}$, Kilogram ($kg$), Metre ($m$), Second ($s$)

Question Bank: Units & Measurements

Based on NCERT Section 1.3 | Useful for CBSE, NEET, & JEE

Section A: Multiple Choice Questions (MCQs)

NEET PYQ

Q1. Which of the following is NOT a base SI unit?

• A) Candela (cd)
• B) Mole (mol)
• C) Litre (L)
• D) Ampere (A)

Answer: C (Litre is a derived unit, m³ is the SI unit for volume)

Q2. The value of Planck's constant ($h$) used to define the kilogram is:

• A) $6.022 \times 10^{23}$ J s
• B) $6.626 \times 10^{-34}$ J s
• C) $3.00 \times 10^{8}$ m/s
• D) $1.602 \times 10^{-19}$ C

Answer: B

Section B: Assertion & Reason (CBSE/AIIMS Style)

Directions: A = Assertion, R = Reason. Select (a) if both correct & R is correct explanation, (b) if both correct but R is not explanation, (c) if A correct R wrong, (d) if A wrong R correct.

Q3. Assertion (A): The measurement of chemical properties requires a chemical change to occur.

Reason (R): Chemical properties can be observed by just looking at the color or odor of the substance.

Answer: (c) - Assertion is true, but Reason is false (Color/Odor are physical properties).

Section C: Very Short & Short Answer Questions

Q4. Define "Luminous Intensity" and name its SI unit.

Ans: It is the measure of the wavelength-weighted power emitted by a light source in a particular direction. Unit: Candela (cd).

Q5. Differentiate between the Metric System and the English System of measurement.

Ans: The Metric System (originated in France) is based on the decimal system (multiples of 10), making it more convenient than the English System.

Section D: Long Answer Questions (5 Marks)

Q6. (a) Explain how the definition of the Kilogram has changed in the modern SI system.
(b) List the 7 base physical quantities and their symbols.

Hints for students:

• Part (a): Mention the shift from the physical "Le Grand K" prototype to the fixed value of the Planck Constant ($h$).
• Part (b): Use Table 1.1 (Length, Mass, Time, Electric Current, Temp, Amount of Substance, Luminous Intensity).

Section E: Numerical Context

Q7. A room has a length of 6 meters. Identify the number and the unit in this quantitative observation.

Ans: Number = 6; Unit = meter (m).

Study Tip: Focus on the definitions in Table 1.2 for competitive exams.
© 2026 Educational Resource