This question is about the nutritional content of two foods, X and Y. The diagram shows two burning foods, X and Y, being used to
increase the temperature of water. The temperature of the water
in each test tube is recorded at the start of the experiment, before
the food is set alight. It is recorded again at the end of the
experiment, after the food has stopped burning. The initial mass
of each food and the volume of water should be kept constant in
both experiments. Explain why these should be kept constant.