Question

Gerald's science teacher mixed room temperature samples of hydrochloric acid and sodium hydroxide in a large beaker. The solution still looked clear like water, but when the students carefully touched the beaker one at a time, it felt warm to the touch. Why did the beaker most likely feel warm?

A. The two liquids were not soluble in water.
B. The release of a gas heated the solution.
C. The energy of mixing warmed the solution.
D. A chemical reaction was producing a new substance.

Answer 1

Answer: Option (D) is the correct answer.

Explanation:

The chemical reaction equation will be as follows.

      $HCl + NaOH \rightarrow NaCl + H_{2}O$

This means a new substance has been formed and a chemical change has taken place.

A chemical reaction in which heat energy is absorbed by the reactant molecules is known as an endothermic reaction.  

For example, $A + B + Heat \rightarrow AB$

On the other hand, a chemical reaction in which heat energy is released by the reactant molecules is known as an exothermic reaction.  

For example, $C + D \rightarrow CD + Heat$ is an exothermic reaction.

Thus, we can conclude that in the given situation the beaker most likely felt warm because a chemical reaction was producing a new substance.