The reaction of calcium carbonate with dilute hydrochloric acid (a) Calcium carbonate reacts with dilute hydrochloric acid to produce calcium chloride, carbon dioxide and water - Edexcel - GCSE Chemistry - Question 6 - 2018 - Paper 1

The initial temperature of the reaction mixture was 25°C, and the final temperature increased to 27°C. This rise in temperature indicates that the reaction is exothermic.

In an exothermic reaction, heat is released into the surroundings, causing the temperature of the reaction mixture to increase. Therefore, the increase in temperature provides evidence that the reaction between calcium carbonate and dilute hydrochloric acid releases heat.