MOM’s Acid-Base Chemistry

Introduction

In this experiment, students react milk of magnesia with vinegar to study acid-base chemistry, chemical equilibrium, solubility, and heat of neutralization. Milk of magnesia (MOM), which still finds wide use as a laxative and antacid, is a suspension of magnesium hydroxide in water. The suspended particles of magnesium hydroxide give MOM its characteristic milky-white appearance. Although the particles are clearly visible, some magnesium hydroxide does dissolve in water, making milk of magnesia basic.

How they will do it

In the experiment, students add acid to MOM and observe the surprising pH changes during the reaction. Universal indicator added to the solution will show the pH changes.

Like any acid-base reaction, a base, in this case magnesium hydroxide, reacts with an acid, HX, to form a salt, MgX₂, and water:

Mg(OH)₂(s) + 2HX (aq) → MgX₂ (aq) + 2H₂O(l)

Students will use vinegar (CH₃CO₂H) as the acid. The balanced chemical equation for this reaction is:

Mg(OH)₂ (s) + 2CH₃CO₂H(aq) → Mg(CH₃CO₂)₂(aq) + 2H₂O(l)

The amount of magnesium hydroxide in solution and therefore available to react is determined by the equilibrium shown below:

Mg(OH)₂(s) = Mg²+(aq) + 2OH-(aq)

Le Châtelier’s principle

This equilibrium lies far to the left, leaving very little magnesium hydroxide in solution. As acid reacts with the hydroxide ions, removing them from solution, more magnesium hydroxide dissolves. This shift is an example of Le Châtelier’s principle. This process continues until all the magnesium hydroxide is neutralized, leaving only soluble magnesium acetate and water. The solution will be clear and will have the color of the universal indicator solution at that pH.

Your students may also notice a change in temperature of the mixture after they add the acid. This change is the heat of neutralization that accompanies acid-base reactions. They can feel the temperature change by touching the outside of the beaker.