We've got two kilograms of water. The specific heat capacity of water is 4,200 joules per kilogram per degree Celsius (you will be given that value in the exam). The temperature change, in this case, is 80 degrees Celsius (final minus initial; 100 − 20). Substituting these values into the equation gives us a change in thermal energy of 672,000 J. We can convert this into kilojoules by dividing it by 1000, giving us an answer of 672 kJ.