Specific heat capacity

Some materials soak up more heat

The energy needed to heat something depends on its mass, the temperature change and the material. Heating 1 kg of water by 1 °C takes more energy than heating 1 kg of alcohol by 1 °C.

Q = m c ΔT. Specific heat capacity c is the heat needed to raise 1 kg of a substance by 1 °C (or 1 K). Its SI unit is J kg⁻¹ K⁻¹.

Metals such as steel and copper have low c: they heat up and cool down quickly, so they are used for kettles, radiators and cooking items. Water's c is high: 4,200 J kg⁻¹ K⁻¹.

High c means slow to heat and slow to cool. Water stores a lot of heat for a small temperature rise, which is why it cools engines and steadies climates.

Key terms

Specific heat capacity (c)

Heat needed to raise 1 kg of a substance by 1 K.

Heat energy (Q)

Energy given to or taken from a body, in joules.

Radiator

Where a fan cools the hot water from the engine.

Thermostat

Opens to let hot water into the radiator.

Short questions with model answers

1. Q1. The same heat goes into 1 kg of water and 1 kg of dry soil. How many times more does the soil's temperature rise?

   • ΔT = Q ÷ (m c)
   • 4200 ÷ 810 ≈ 5.2

   About five times more, as your book says.

2. Q2. Why is water used to cool a car engine?

   • High c.
   • The cycle.

   It carries engine heat away efficiently and prevents overheating.

Common mistakes

• ✗ A substance with a high specific heat capacity heats up quickly.

  ✓ High c means it needs more heat per degree, so it heats up slowly.

• ✗ Put 600 g straight into Q = m c ΔT.

  ✓ c is per kilogram, so change grams to kilograms first.

• ✗ Metal feels colder than wood because it has a lower specific heat capacity.

  ✓ Your book's box (p. 8): metal is a conductor and draws heat from your hand; wood is an insulator.

MCQs

1. 1. Which property determines how much heat a solid can absorb before its temperature changes significantly?

   1. (a) Density
   2. (b) Specific heat capacity
   3. (c) Colour
   4. (d) Hardness
   Show answer

   (b) MCQ 10.4 (p. 17).

2. 2. The SI unit of specific heat capacity is:

   1. (a) J kg⁻¹ K⁻¹
   2. (b) J K⁻¹
   3. (c) J kg⁻¹
   4. (d) W kg⁻¹
   Show answer

   (a) p. 8.

3. 3. Coastal areas have milder climates than inland areas mainly because water:

   1. (a) Is always cold
   2. (b) Reflects sunlight
   3. (c) Has a lower specific heat capacity than soil
   4. (d) Has a large specific heat capacity
   Show answer

   (d) p. 9: oceans absorb heat by day and release it slowly at night.

4. 4. Steel and copper are used in kettles and radiators because they:

   1. (a) Have a high specific heat capacity
   2. (b) Never get hot
   3. (c) Have a low specific heat capacity and heat up quickly
   4. (d) Are insulators
   Show answer

   (c) p. 8: metals have low specific heat capacities and heat up and cool down quickly.

5. 5. In a car's cooling system, the thermostat:

   1. (a) Burns the fuel
   2. (b) Opens to let hot water into the radiator
   3. (c) Pumps air into the cylinders
   4. (d) Releases excess pressure
   Show answer

   (b) p. 9. The pressure release cap, not the thermostat, releases excess pressure.

Quick revision

• Q = m c ΔT; c is the heat to raise 1 kg by 1 K, in J kg⁻¹ K⁻¹.
• Metals have low c and heat quickly; water has a high c of 4,200 J kg⁻¹ K⁻¹.
• Water's high c gives milder coasts and makes it a good engine coolant.