When 1800 J of energy is supplied to a mass m of liquid in a container, the temperature of the liquid and the container changes by 10 K. When the mass of the liquid is doubled to 2m, 3000 J of energy is required to change the temperature of the liquid and container by 10 K. What is the specific heat capacity of the liquid in ${\text{J}}\,{\text{k}}{{\text{g}}^{ - 1}}{{\text{K}}^{ - 1}}$?

A.     $\frac{{60}}{m}$

B.     $\frac{{120}}{m}$

C.     $\frac{{180}}{m}$

D.     $\frac{{240}}{m}$

B

Think units.

The units of the answer are given as ${\text{J}}\,{\text{k}}{{\text{g}}^{ - 1}}{{\text{K}}^{ - 1}}$, which means that we need to divide energy by temperature (and mass, but that is already present in each response). C was the most popular option, but this is 120 (obtained by dividing 1800 J by 10 K and totally ignoring the container) so it must be incorrect. Thus it would be reasonable to subtract the energies given before dividing by 10 K – giving the correct answer B.

Alternatively the candidate can write down the two relevant heat exchange equations and subtract them, but this takes longer.