Mass, m, in kilograms, kg

The idea

In the equation ∆E = mc∆θ, the symbol m stands for the mass of the material being heated or cooled, measured in kilograms, kg. Mass is the amount of matter in the object and does not change with temperature, unlike volume. The kilogram is the SI base unit of mass, and in calculations you must convert grams to kilograms by dividing by 1000. For example, 500 g of water is 0.5 kg. Mass matters because a larger mass needs more energy for the same temperature rise: heating 2 kg of water by 10 °C transfers twice the energy needed for 1 kg. When reading a question, identify which mass is being heated, and if a container is involved, decide whether its mass should be included.

What earns the marks

• States that m is the mass of the material, measured in kilograms, kg.
• Converts a mass given in grams to kilograms by dividing by 1000.
• Uses mass correctly in the equation ∆E = mc∆θ, substituting the value in kg.
• Explains that a larger mass requires more thermal energy for the same temperature change.
• Distinguishes mass in kg from specific heat capacity in J/kg°C and temperature change in °C.
• Selects the relevant mass when a question describes more than one material or container.

Common mistakes and how to put them right

• Substituting mass in grams directly into ∆E = mc∆θ.

  Put it right by converting grams to kilograms first.

• Confusing mass with weight or with volume.

  Put it right by recalling that mass is measured in kg and is the amount of matter, while weight is a force in N.

• Using the mass of the container instead of the substance, or vice versa.

  Put it right by reading the question carefully to identify which mass is heated.

Exam tips

• Underline the mass value and its unit in the question, then convert to kg before substituting.
• Show the conversion step, for example 250 g = 0.25 kg, so the examiner can award method credit.
• Check that your final answer uses J and that the mass used was in kg, not g.