An instant cold pack contains a solid that dissolves in water. When the pack is squeezed, the solid dissolves and the pack becomes cold. A student measured ...

This question tests energy changes on dissolving and the meaning of a temperature change. The pack cools because dissolving the solid takes in energy.

(a) The temperature change is the final temperature minus the starting temperature. Working with the values given, the fall is \(22.0 - 4.0 = 18.0\ ^{\circ}\text{C}\), so the temperature falls by \(18.0\ ^{\circ}\text{C}\) (written as a change this is \(-18.0\ ^{\circ}\text{C}\) because the temperature has gone down). [1] Keep the sign or the word "fall" so the direction is clear.

(b) Because the surroundings lose heat and get colder, energy is being taken in by the process. A process that takes in energy from its surroundings is endothermic. [1]

(c) As the solid dissolves it absorbs energy from the water and the pack material around it. Removing that energy from the surroundings lowers their temperature, so the pack feels cold to the touch. [1] Note the direction of energy flow: heat moves from your skin and the water into the dissolving solid.

(d) Temperature is measured with a thermometer. [1]

(e) One sensible precaution scores the mark, for example: do not hold the very cold pack against the skin for too long (to avoid a cold burn), or wear eye protection, or do not burst open or swallow the contents. [1]

Exam tip: "feels cold" always signals an endothermic change, because energy is being taken in rather than released.