Specific and Latent Heat
~8 min read
- Specific heat: Heat per kg per °C. Water 4186 J/kg·K — highest among common substances.
- Latent heat: Heat absorbed/released during phase change at constant temperature.
- Water: Latent heat of fusion = 334 kJ/kg. Latent heat of vaporisation = 2260 kJ/kg.
Heat changes both temperature and state. CDS/OTA tests specific heat capacity, latent heats of fusion and vaporisation, and why water moderates Earth's climate.
Specific Heat Capacity
- Q = mcΔT. Heat needed to raise the temperature of a body by ΔT.
- c = specific heat capacity, in J/kg·K (or J/kg·°C). Property of the material.
- Water: c = 4186 J/kg·K — among the highest of common substances. Hence water heats and cools slowly.
- Land has much lower specific heat than water — coastal areas have moderate climate; inland areas have wider temperature swings.
- Water is used as a coolant in radiators and reactors because of this.
Latent Heat
- Heat absorbed or released during a change of state at constant temperature.
- Q = mL. L is latent heat, in J/kg.
- Latent heat of fusion (ice → water at 0 °C): 334 kJ/kg.
- Latent heat of vaporisation (water → steam at 100 °C): 2260 kJ/kg.
- That is why steam at 100 °C burns far more severely than water at 100 °C — releases extra 2260 kJ/kg as it condenses on skin.
- Sweating cools us — evaporation absorbs latent heat from the body.
Calorimetry and Principle of Mixtures
- When two bodies at different temperatures are mixed in an insulated calorimeter, heat lost by hot body = heat gained by cold body.
- m₁c₁(T₁ - T) = m₂c₂(T - T₂).
- Used to measure specific heats and latent heats experimentally.
- Bomb calorimeter — to measure energy content of fuels and foods.
CDS/OTA PYQ Examples
Q: Substance with the highest specific heat capacity is:
(a) Iron (b) Copper (c) Water (d) Mercury
Answer: (c) Water — 4186 J/kg·K.
Q: Why does steam at 100 °C cause more severe burns than water at 100 °C?
(a) Steam is hotter (b) Steam has high latent heat of vaporisation released on condensation (c) Steam has higher specific heat (d) Skin reacts more to gas
Answer: (b) Latent heat of condensation (~2260 kJ/kg) on skin.
Q: Latent heat of fusion of ice is approximately:
(a) 80 J/g (b) 80 cal/g (c) 540 cal/g (d) 4.18 J/g
Answer: (b) 80 cal/g ≈ 334 J/g.
Q: Sweating cools the body because:
(a) Sweat is cold (b) Evaporation absorbs latent heat from skin (c) Sweat reflects sunlight (d) Skin contracts
Answer: (b) Evaporation requires latent heat, absorbed from the body.
Drill Specific and Latent Heat for CDS/OTA
CDS/OTA-pattern items on Specific and Latent Heat with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does the coast have a milder climate than inland areas?
Water has high specific heat — heats slowly in summer and cools slowly in winter. Sea breezes moderate coastal temperatures.
Why is ice used to cool drinks rather than cold water?
As ice melts at 0 °C, it absorbs latent heat (334 kJ/kg) from the drink without itself warming up — much more cooling than the same mass of 0 °C water.
Why does a desert cooler work?
Hot dry air passes through wet pads; the water evaporates absorbing latent heat from the air, cooling it before it enters the room.