Hydrochloric acid is used in the body to help digestion and kill bacteria.

However too much acid can cause indigestion and we use indigestion remedies to neutralise excess acids.

An indigestion remedy contains a base such as magnesium hydroxide, which reacts to form a neutral compound and raises the pH of the stomach.

Metal hydroxides, such as sodium hydroxide, usually dissolve in water to form clear, colourless solutions. When an acid reacts with a metal hydroxide, the only products formed are a salt plus water. Here is the general word equation for the reaction:

acid + metal hydroxide    →    a salt + water

You usually observe these things during the reaction:

• there is a temperature rise
• the pH of the reaction mixture changes