Energy changes in systems: The specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius.

The idea

Specific heat capacity describes how much energy must be transferred to change the temperature of a known mass. The definition fixes two conditions: the mass is exactly one kilogram, and the temperature rise is exactly one degree Celsius. For example, water has a specific heat capacity of about 4200 J/kg°C, so heating 1 kg of water by 1 °C needs 4200 J. Heating 2 kg by 1 °C needs 8400 J, and heating 1 kg by 10 °C needs 42 000 J. This shows why the energy depends on mass, temperature change and the material. The unit J/kg°C combines joules of energy with kilograms and degrees Celsius. A larger value means more energy is stored for the same temperature rise, so the substance warms or cools slowly.

What earns the marks

• State that specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C.
• Use the unit J/kg°C and recognise that it means joules per kilogram per degree Celsius.
• Explain that energy transferred depends on mass, temperature change and the material, using ΔE = mcΔθ.
• Calculate energy transferred for a stated mass and temperature rise, for example 3 kg of water heated by 5 °C using c = 4200 J/kg°C gives 63 000 J.
• Compare materials: a substance with a higher specific heat capacity needs more energy for the same mass and temperature rise.
• Interpret a temperature change as a rise or fall, noting that cooling releases energy rather than requiring it.

Common mistakes and how to put them right

• Using the mass in grams instead of kilograms.

  Put it right: Convert grams to kilograms by dividing by 1000 before substituting.

• Misusing units; the correct unit is J/kg°C, not J/kg or J/°C.

  Put it right: Ensure you include all parts of the unit when stating specific heat capacity.

• Assuming a larger temperature change always means a larger specific heat capacity.

  Put it right: Specific heat capacity is a property of the material, while energy transferred also depends on mass and temperature change.

Exam tips

• Write the equation ΔE = mcΔθ, then substitute values with units before calculating.
• Check that the temperature change is a difference, not a single thermometer reading, and that it is in °C.
• When comparing materials, refer to the energy needed for the same mass and the same temperature rise, not just to how hot something feels.