A CAROLINA ESSENTIALS INVESTIGATION

Equilibrium and Milk of Magnesia Rainbow

Total Time

Chemistry

Grade Level

9-12 High School

Introduction

This demonstration shows a colorful reaction that is a good introduction to Le Châtelier’s principle, solubility, stoichiometry, neutralization reactions, and reaction rates. The demonstration uses readily available chemicals, is easy to set up and perform, and will engage your students with brilliant colors. Each color change is a visual indicator of a shift in equilibrium. This demonstration can also be used as a limiting reactant demonstration and kinetics demonstration. See the product links for instructions.

Essential Question

How are changes in reaction equilibrium demonstrated and explained?

Demonstration Objectives:

Recognize and identify the properties of an equilibrium reaction.
Apply Le Châtelier’s principle to explain the color change.

Next Generation Science Standards* (NGSS)

SCIENCE & ENGINEERING PRACTICES

Constructing Explanations

DISCIPLINARY CORE IDEA

Matter and Its Interactions

CROSSCUTTING CONCEPTS

Cause and Effect

Materials Needed

Recommended Safety

Wear safety goggles and chemical-resistant gloves while performing this demonstration. Universal indicator solution may stain skin and clothing. It contains ethanol, which is a flammable solvent. Keep flames and sources of ignition away from this solution. Sulfuric acid is corrosive to skin and eyes.

Teacher Prep and Disposal

The final solution will be a slightly acidic solution of magnesium sulfate and can be disposed of down a drain with excess water.

Procedures

Fill a large beaker about 2/3 full with tap water. A 1000-mL beaker works best.
Add a stir bar to the beaker and place the beaker on the stir plate.
Turn the stir plate on to a gentle speed to create a small whirlpool in the center of the solution.
Shake the bottle of milk of magnesia well. Measure out about 20 mL of milk of magnesia using a small beaker or measuring cup.
Pour about 40 mL of sulfuric acid into the graduated cylinder.
Hand out the student worksheet.

Student Procedures

Add 2 to 3 mL of universal indicator to the water in the beaker.
Add all of the milk of magnesia to the stirring water.
Wait until the milk of magnesia is evenly distributed and the color is a consistent purple throughout the suspension.
Ask students to fill out the first section of their worksheet and begin to record observations.
Add approximately 5 mL of 1 M sulfuric acid to the milk of magnesia suspension.
When the suspension has returned to a consistent purple color, add another 5 mL of sulfuric acid.
Prompt students to describe the changes and inquire what the color changes may be indicating.
Continue to add sulfuric acid 5 mL at a time until the solution remains a translucent (clear) red.

Teacher Preparation and Tips

Any universal indicator will work.
Show students the color of universal indicator in neutral water.
Explain the difference between clear and colorless to students. Unversal indicator is clear and green. During step 3, ask if the solution is still clear.
When the acid is added,the suspension should turn red, then yellow, green, blue, and finally back to purple. Instruct students to observe carefully.
If students cannot see the color changes, encourage them to get closer. They must wear goggles.
The time can vary to fit your schedule, but allow a minimum of 30 to 45 minutes.

Data and Observations

When the acid is added, the solution changes from purple (pH 10) to red (pH 4). The color of the solution then slowly changes to orange, yellow, green, blue, and finally back to violet as the magnesium hydroxide slowly dissolves. This process continues until all the magnesium hydroxide is neutralized, leaving only a clear solution of magnesium sulfate, which will be red due to the low pH, a result of the excess sulfuric acid. The final solution is also clear, indicating a solution rather than a suspension.

Mg(OH)₂(s) Mg²⁺(aq) + 2OH^–(aq)

Mg(OH)₂(aq) + H₂SO₄(aq) → MgSO₄(aq) + 2H₂O(l)

Analysis and Discussion

Why does the solution change color very quickly when the acid is added? Explain the reactions or processes that are occurring.

The neutralization reaction between the hydroxide ions in solution and the acid is very fast.
Why does the solution slowly return to the original color? Explain the reactions or processes are occurring?

Once all the hydroxide ions in solution are consumed, the magnesium hydroxide slowly dissolves to relieve the disrupted equilibrium as described by Le Châtelier’s principle.
Using Le Châtelier’s principle, explain the color changes observed in the demonstration.

The hydroxide ions (OH^–) in solution are neutralized, causing a disruption of the solubility equilibrium for magnesium hydroxide. This causes more of the solid magnesium hydroxide to dissolve. The stress of disrupted equilibrium is relieved as described by Le Châtelier’s principle, which states that if a stress is placed upon a reaction at equilibrium, the reaction shifts to relieve that stress.

Buy the Kit

Introduce students to the significance of pH indicators with this colorful 2-part activity that explores solubility, equilibrium, Le Châtelier’s principle, and acid-base chemistry.

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