Milk of Magnesia Rainbow
Difficulty: Easy | Time: 15 minutes | Visual Impact: Very High
Historical Context
Milk of magnesia was invented in 1829 by Sir James Murray in Ireland, who created the white suspension of magnesium hydroxide as an antacid and laxative. The name comes from its milky appearance. Charles Henry Phillips commercialized it in 1880 as “Phillips’ Milk of Magnesia,” a brand that still exists today.
The chemistry demonstrates a fundamental principle of antacid action: magnesium hydroxide neutralizes stomach acid (hydrochloric acid in the body, but any acid works for demonstration). The reaction converts the insoluble white suspension into a clear, soluble salt solution.
When combined with universal indicator, this experiment creates a stunning “rainbow” effect as the pH gradually changes from basic (blue/green) through neutral to acidic (yellow/orange/red), making the abstract concept of neutralization beautifully visible.
Materials
- Magnesium hydroxide (milk of magnesia, unflavored) - 15mL (1 tablespoon)
- White vinegar - 120mL (1/2 cup)
- Water - 400mL
- Universal indicator solution - 5mL (or red cabbage juice - 50mL)
- Large clear container or beaker (600mL+)
- Stirring rod
Procedure
Basic Demonstration
- Pour 400mL water into the container
- Add 15mL milk of magnesia and stir - solution turns milky white
- Slowly add vinegar while stirring
- Watch the milky suspension gradually clear as the magnesium hydroxide neutralizes
- The solution becomes completely transparent when neutralization is complete
Rainbow Version (Recommended)
- Pour 400mL water into the container
- Add 5mL universal indicator (or 50mL red cabbage juice)
- Add 15mL milk of magnesia and stir
- Solution turns blue/green (basic pH ~10)
- Slowly pour vinegar down the side of the container (don’t stir)
- Watch rainbow layers form as acid meets base:
- Top: Blue/green (basic - unreacted Mg(OH)₂)
- Middle: Green → Yellow (neutral zone)
- Bottom: Orange/Red (acidic - excess vinegar)
- Stir gently to see colors swirl and mix
- Continue adding vinegar until solution turns uniformly yellow/orange
Reaction
\[\ce{Mg(OH)2(s) + 2CH3COOH(aq) -> Mg(CH3COO)2(aq) + 2H2O(l)}\]
Magnesium hydroxide + Acetic acid → Magnesium acetate + Water
The Science
Milk of magnesia is a suspension - tiny particles of solid magnesium hydroxide dispersed in water. It appears white because the particles scatter light. Magnesium hydroxide is a base (releases OH⁻ ions) but is only slightly soluble in water.
When acid is added: 1. H⁺ ions from vinegar react with OH⁻ ions from Mg(OH)₂ 2. This pulls the equilibrium toward dissolution 3. More Mg(OH)₂ dissolves to replace the consumed OH⁻ 4. Eventually all solid dissolves, forming soluble magnesium acetate 5. The solution clears because there are no more particles to scatter light
The indicator shows pH changes in real-time: - Blue/Purple (pH 10+): Excess base - Green (pH 7-8): Near neutral - Yellow (pH 5-6): Slightly acidic - Orange/Red (pH 3-4): Excess acid
Variations
Layered Rainbow: Pour solutions carefully to create distinct pH layers that slowly mix.
Antacid Comparison: Test different antacids (Tums, Rolaids, Alka-Seltzer) to see which neutralizes more acid.
Quantitative Version: Use a measured amount of acid and determine how much antacid is needed to neutralize it.
Tips
- Unflavored milk of magnesia works best (flavored versions may contain dyes)
- Add vinegar slowly for the best visual effect
- Use a white background behind the container to see colors clearly
- Red cabbage juice works well if you don’t have universal indicator
Safety
This experiment uses only food-safe materials. The products (magnesium acetate solution) can be safely poured down the drain.