Properties of Matter and Their Measurement
Quick Notes:
- Properties of matter can be physical (observed without changing identity) or chemical (observed only during a chemical reaction).
- Physical properties include colour, mass, melting point, boiling point, etc.
- Measurement of physical quantities requires a defined unit and a measuring instrument.
- The International System of Units (SI) is the globally accepted standard.
- Base SI units: mass (kg), length (m), time (s), temperature (K), amount of substance (mol), electric current (A), luminous intensity (cd).
- Mass is the quantity of matter; weight is the force due to gravity.
- Volume = space occupied and its standard SI unit = m³ (also cm³, dm³, L).
- Density = mass ÷ volume and its standard units = kg m⁻³ or g cm⁻³.
- Temperature measures thermal energy; common units: °C, K, °F.
Full Notes:
1.3.1 Physical and Chemical Properties
Physical Properties:
- Measurable without changing the composition of the substance.
- Include: colour, odour, density, melting point, boiling point, solubility, hardness, etc.
Examples: Water boils at 373 K. Copper is reddish-brown and conducts electricity.
Chemical Properties:
- Observed when a substance undergoes a chemical change.
- Relate to how a substance reacts with other substances.
Examples: Iron reacts with oxygen and moisture to form rust. Sodium reacts vigorously with water.
NCERT Distinction: Physical changes do not produce new substances; chemical changes do.
1.3.2 Measurement of Physical Properties
Measurement requires two essential components:
- Unit – A standard of comparison.
- Instrument – A tool that provides numerical value.
Physical quantities must be measured accurately to ensure consistent scientific communication.
Example: Length can be measured in metres using a ruler or tape.
1.3.3 The International System of Units (SI)
The SI system is the modern form of the metric system and is used worldwide in science and engineering.
Seven Base Units
| Quantity | SI Unit (Name & Symbol) | Definition (Based on constants) |
|---|---|---|
| Length | metre (m) | Defined by fixing the value of the speed of light in vacuum, c, to exactly 299792458 m·s−1. |
| Mass | kilogram (kg) | Defined by fixing the Planck constant, h, to exactly 6.62607015 × 10−34 J·s. |
| Time | second (s) | Defined by fixing the caesium-133 atom transition frequency, ΔνCs, to exactly 9192631770 Hz. |
| Electric current | ampere (A) | Defined by fixing the elementary charge, e, to exactly 1.602176634 × 10−19 C. |
| Thermodynamic temperature | kelvin (K) | Defined by fixing the Boltzmann constant, k, to exactly 1.380649 × 10−23 J·K−1. |
| Amount of substance | mole (mol) | Defined by fixing the Avogadro constant, NA, to exactly 6.02214076 × 1023 mol−1. |
| Luminous intensity | candela (cd) | Defined by fixing the luminous efficacy, Kv, of monochromatic radiation at 540 × 1012 Hz to 683 lm·W−1. |
NCERT Note: These units are based on international standards and are subject to regular review.
1.3.4 Mass and Weight
Mass:
- Amount of matter in an object.
- SI unit: kilogram (kg).
- Remains constant, regardless of location.
- Measured using a balance.
Weight:
- The force exerted by gravity on an object.
- Depends on local gravitational field.
- Measured using a spring balance.
- SI unit: newton (N).
- Weight (W) = Mass (m) × Acceleration due to gravity (g)
- On Earth: g ≈ 9.8 m/s².
Example: Your mass remains the same on Moon and Earth, but your weight changes because gravity is different.
1.3.5 Volume
Volume = space occupied by matter.
- SI unit: cubic metre (m³).
- Common lab units: cm³ or mL (1 cm³ = 1 mL), dm³ or L (1 dm³ = 1 L = 1000 cm³).
- Often measured using a measuring cylinder, burette, or pipette.
Example: A 250 mL beaker holds 250 cm³ of liquid.
1.3.6 Density
Density = mass per unit volume.
Formula: ρ = m / V
- SI unit: kg m⁻³.
- Commonly used unit: g cm⁻³.
- Examples: Water ≈ 1.00 g cm⁻³; Gold ≈ 19.3 g cm⁻³.
Tip: Density remains constant for a pure substance at a given temperature and pressure.
1.3.7 Temperature
Temperature is a measure of the average kinetic energy of particles.
- SI unit: Kelvin (K).
- Other units: Degree Celsius (°C), Fahrenheit (°F).
Conversions:
- K = °C + 273.15
- °C = (°F – 32) × 5/9
NCERT Conversion Highlight:
- 0°C = 273.15 K
- Normal body temperature: 37°C = 310.15 K
Important: In scientific calculations, Kelvin is always used.
Most students confuse mass vs weight and Celsius vs Kelvin in unit conversions. Always double-check your units – these are high-frequency 1-mark questions in board exams and NEET.
Question: Convert 25°C to Kelvin.
Answer: K = 25 + 273.15 = 298.15 K
Summary
- Physical properties can be measured without changing substance composition; chemical properties describe reactions.
- Accurate measurement requires defined units and instruments.
- SI system has seven base units for fundamental quantities.
- Mass is constant; weight depends on gravity.
- Volume and density describe matter’s size and compactness.
- Kelvin is the SI temperature scale used in scientific work.