THERMAL PROPERTIES AND TEMPERATURE (0625 Physics Syllabus) 

Thermal Expansion

When a substance is heated, its particles gain kinetic energy and move faster. This usually causes expansion, depending on the state of matter.

Expansion in Solids:

• Expand the least because particles are tightly bound.

• Examples: gaps in railway tracks, expansion joints in bridges, and power lines sagging in summer.

• Bimetallic strips (iron + brass) bend when heated — used in thermostats.

Expansion in Liquids:

• Expand more than solids but less than gases.

• Used in thermometers (expand uniformly).

• Hot water tanks have expansion spaces to prevent damage.

Expansion in Gases:

• Greatest expansion among all states.

• Example: hot-air balloons rise because heated air becomes less dense; car tyre pressure increases when hot.

Example Questions:

1. Why are gaps left between railway tracks? → To allow expansion in hot weather, without gaps, tracks would buckle.

2. Why do power cables sag more in summer? → Metal expands and lengthens with higher temperatures.

Specific Heat Capacity

Definition: The specific heat capacity (c) of a substance is the energy required to raise the temperature of 1 kg of the substance by 1°C.

Formula: Q = mc∆T

Key Points:

• Water has a high specific heat capacity (4200 J/kg°C) — heats/cools slowly, stabilizes climate.

• Metals (e.g., copper, aluminum) have low specific heat capacity — they heat up quickly, used in pans.

• Large bodies of water absorb much heat with small temperature change → mild coastal climates.

  
  

Worked Example: Find the energy to heat 3 kg of copper (c = 390 J/kg°C) from 20°C to 100°C. Q = mc∆T = 3 × 390 × (100 − 20) = 93,600 J

Experiments:

• Electrical Method: Use heater (E = VIt), measure ∆T, then find c using Q = mc∆T.

• Method of Mixtures: Hot solid placed in water; use heat lost = heat gained.

Example Question:

 A 2 kg aluminum block (c = 900 J/kg°C) is heated by 18,000 J. Find ∆T. ∆T = Q / (mc) = 18,000 / (2 × 900) = 10°C

Internal Energy

The total internal energy is the sum of the kinetic and potential energies of particles.

• Heating increases kinetic energy (temperature rises).

• During melting/boiling, potential energy increases while temperature remains constant.

Example Question: Why does temperature stay constant during boiling? → Energy is used to overcome intermolecular forces instead of increasing kinetic energy.

Change of State

Substances change state when energy is added or removed. Temperature stays constant while potential energy changes.

Melting: Solid → Liquid (e.g., ice at 0°C) Boiling: Liquid → Gas (e.g., water at 100°C) 

Evaporation: Liquid → Gas at surface, below boiling point (cooling effect). 

Condensation: Gas → Liquid (e.g., water vapor on cold glass). 

Sublimation: Solid ↔ Gas directly (e.g., dry ice).

Evaporation vs Boiling

Example Questions

1. State the melting and boiling points of water. → 0°C and 100°C.

2. Why is evaporation a cooling process? → Fast-moving molecules escape first, lowering average kinetic energy.

3. Why is boiling faster than evaporation? → Energy is supplied to all particles, causing rapid change throughout the liquid.

Written by Zaid 

Curated by Yassein Abdoun