Summary

In this demonstration, students will observe a color change in a milk of magnesia solution as vinegar is added.

Grade Level

High school

AP Chemistry Curriculum Framework

This demonstration supports the following unit, topics and learning objectives:

• Unit 4: Chemical Reactions
  • Topic 4.3: Representations of Reactions
    • TRA-1.C: Represent a given chemical reaction or physical process with a consistent particulate model.
  • Topic 4.5: Stoichiometry
    • SPQ-4.A: Explain changes in the amounts of reactants and products based on the balanced reaction equation for a chemical process.
  • Topic 4.7: Types of Reactions
    • TRA-2.A: Identify a reaction as acid-base, oxidation-reduction, or precipitation.
• Unit 5: Kinetics
  • Topic 5.1: Reaction Rates
    • TRA-3.A: Explain the relationship between the rate of a chemical reaction and experimental parameters.
• Unit 8: Acids and Bases
  • Topic 8.3: Weak Acid and Base Equilibria
    • SAP-9.C: Explain the relationship among pH, pOH, and concentrations of all species in a solution of a monoprotic weak acid or weak base.
  • Topic 8.7: pH and pKa
  • SAP-10.A: Explain the relationship between the predominant form of a weak acid or base in solution at a given pH and the pKa of the conjugate acid or the pKb of the conjugate base.

Objectives

By the end of this demonstration, students should be able to

• Understand that a chemical change is taking place.
• Recognize that an indicator causes the color change.
• Realize that an acid base reaction is taking place.
• Recognize the limiting and excess reactants during the reaction.
• Apply Le Châtelier’s principle to explain the color change

Chemistry Topics

This demonstration supports students’ understanding of

• Indicators
• Acid/base neutralization reaction
• Limiting Reactant
• Le Châtelier’s principle
• Extension: Reaction Rate - Kinetics

Time

Teacher Preparation: 5 minutes

Lesson: 20 minutes

Materials

• Large beaker (at least 1000ml)
• Graduated Cylinder, 50ml
• Milk of Magnesia, 25ml
• Water (tap water can be used), 700ml
• Universal Indicator, 5 – 10 ml
• Stir Plate and Stir Bar
• Household Vinegar, 90ml
• Ice cubes (optional

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• Students should wash their hands thoroughly before leaving the lab.
• Vinegar (SDS)
• Magnesium hydroxide (SDS)
• Universal Indicator (SDS)

Teacher Notes

• Milk of Magnesia (MOM) is a common antacid. You can choose any antacid that is primarily magnesium hydroxide (Mg(OH)₂).
• Magnesium hydroxide is not very soluble in water (K_sp = 1.8 x 10^-11). MOM is a magnesium hydroxide suspension.
• There are 1200 mg of Mg(OH)₂ in 1 tablespoon of MOM.
• Universal Indicator solution is often used in acid/base chemistry. It is composed of many different compounds that change color at specific pH values. The indicator changes color from pH= 1 through 14 and can be used to indicate how acidic or basic a solution is.
• Vinegar is a 5% by mass solution of acetic acid (HC₂H₃O₂). The density is 1.01 g/ml.
• The MOM will react with the vinegar as it is added:

Mg(OH)₂ (s) + 2HC₂H₃O₂ (aq) → 2H₂O (l) + Mg(C₂H₃O₂)₂ (aq)

• The MOM solution will be blue when the Universal Indicator is added. When vinegar is added the solution will briefly turn red, and then change to orange, yellow, green, and back to blue as the vinegar reacts with the magnesium hydroxide.
• The vinegar will be the limiting reactant for the first several additions. As the magnesium hydroxide reacts with each addition of vinegar, the pH reached when equilibrium is reestablished will be slightly lower.
• Once all of the magnesium hydroxide has reacted and excess vinegar is added, the solution will stay red because of the acetic acid that is present in the solution.
• This demonstration can be used to introduce or strengthen student understanding of the concept of Le Châtelier’s principle.
• Links to Le Châtelier’s principle: The MOM solution is in equilibrium in the large beaker. Adding the vinegar “stresses” the system. The acid-base neutralization reaction between the magnesium hydroxide and acetic acid progresses until one runs out (vinegar in all but the last addition). As the vinegar reacts the pH of the solution increases, resulting in the color change. Once all of the vinegar reacts, equilibrium is reestablished.

Instructions

1. Add about 700ml of water to a 1000ml beaker.
2. Add stir bar to beaker and put the beaker on a stir plate. Turn on the stir plate to create a small whirlpool in the middle of the solution.
3. Use a 50ml graduated cylinder to measure 25ml of Milk of Magnesia. Pour into the beaker of water.
4. Rinse graduated cylinder with water and pour into the large beaker of water.
5. Add 5 - 10ml (about two disposable pipettes) of Universal Indicator to the beaker.
6. When the color in the beaker is evenly distributed, measure out 15-20ml of vinegar in a small beaker and add it to the MOM solution.
7. Make observations of the color changes.
8. When the color in the beaker is evenly distributed discuss if the resulting solution is as basic as it was before the addition of the vinegar.
9. Repeat steps 6 – 8 until all of the magnesium hydroxide has reacted and the solution in the large beaker remains red. NOTE: It will take about 82ml of vinegar to neutralize the Mg(OH)₂ in 25ml of MOM. Adjust your additions based on how many times you’d like to show the color change.
10. Prompt students to explain the color change in terms of the reaction that is happening, using the terms limiting/excess reactant, pH, and equilibrium.

• Kinetics Extension
  • Adding ice slows the reaction quite a bit, which makes it easier to see the color changes. This addition would be a good connection between equilibrium and kinetics.