When magnesium hydroxide reacts with nitric acid, it produces magnesium nitrate and water.

Mg(OH)2+2HNO3→Mg(NO3)2+2H2O

In one to two sentences, explain how this reaction demonstrates that matter is neither created nor destroyed in a chemical reaction.

This reaction demonstrates the law of conservation of mass, as the number of atoms on both sides of the equation is equal, meaning that matter is neither created nor destroyed during the reaction.