Reaction of a Suspension of Magnesium Hydroxide with a Weak Acid (Milk of Magnesia)

Brief Description: 

A solution of milk of magnesia and universal indicator in water is stirred while acetic acid is added. The indicator changes color as the acid is neutralized. As the suspended solid is consumed, the solution becomes clear.

Purpose/Goal: 

Procedure demonstrates the neutralizing effect of magnesium hydroxide on an acid, color change of indicators with pH, and vividly illustrates how a commercial antacid works.

Explanation of Experiment: 

Magnesium hydroxide reacts with acid to form water and magnesium ions: Mg(OH)2 (s) + 2 H3O+ (aq) = 4 H2O (l) + Mg2+ (aq)

Indicators are substances which have distinctly different colors in acidic or basic media. An insoluble base in water dissolves in the presence of acid. Milk of Magnesia has a neutralizing effect on stomach acid – this experiment shows its effects.

Materials Preparation: 
  1. Milk of Magnesia – any brand should work - primary ingredient should be magnesium hydroxide - Mg(0H)2.
  2. Universal Indicator - available from any chemical supplier.
  3. Vinegar (ca. 5% acetic acid)- available from any grocery store.
  4. Magnetic stirring bar and stirring plate, preferably lighted from below.
  5. Beaker – 500 or 1000 mL
Presentation: 

Place about 100 mL of Milk of Magnesia in a 500 mL beaker and dilute with tap water until the beaker is about half full. Add about 10 mL of Universal Indicator. The Universal Indicator will turn red on the far acidic end of the scale and dark blue on the alkaline side. Use the lighted magnetic stirrer to create a steady mix of the liquids. Observe that the solution turns a light blue, indicating that it is slightly basic due to the small amount of dissolved Mg(OH)2. While stirring the solution, add 10-20 mL of vinegar (depending on your setup) and observe the rapid color change. The mixture quickly changes to red because the acid disperses throughout the beaker. The acid neutralizes the small amount of hydroxide ion from the Mg(0H)2 that has dissolved first, then turns the solution acidic. However, as more of the Mg(0H)2 from the suspension gradually dissolves into solution, the acid is neutralized and eventually the solution becomes basic. Add more vinegar and watch as the liquid goes from red to orange to yellow to green and eventually settles at the bluish-purple color. The mixture changes through the entire Universal Indicator color range. In time, all of the vinegar (acid) will react with the magnesium hydroxide and the solution will remain red and clear.

Hazards: 

Vinegar can cause skin irritation. Avoid vigorous solution stirring to prevent spillage. As with all demonstrations, appropriate safety goggles, gloves and lab attire should be used.

Disposal: 

Solutions produced may be flushed down the drain with copious amounts of water.

Video: 

General Concept:

Type of Reaction:

Primary Reference: 
Steve Spangler Science web site – Milk of Magnesia Experiment – Color Changing Liquid Teaches Science Behind Antacids.