Explanation:
An acid and a salt
When we add water to acid, it releases some amount of energy. We can feel this energy by touching the surface of the test tube. Sometimes, released heat is too huge, because of this glasses of test tube may break down and acid can split over. So, in practice, you have to take an excessive amount of water and mix acid drop by drop. In this way, heat generated can be cooled down by this water and mistakes can be prevented.