In this demonstration, students will observe a color change in a milk of magnesia solution as vinegar is added.
AP Chemistry Curriculum Framework
This demonstration supports the following learning objectives:
Idea 3: Changes in matter involve the rearrangement and/or
reorganization of atoms and/or the transfer of electrons.
- 3.2 The student can translate an observed chemical change into a balanced chemical equation and justify the choice of equation type (molecular, ionic, or net ionic) in terms of utility for the given circumstances.
- 3.4 The student is able to relate quantities (measured mass of substances, volumes of solutions, or volumes and pressures of gases) to identify stoichiometric relationships for a reaction, including situations involving limiting reactants and situations in which the reaction has not gone to completion.
Idea 4: Rates of chemical reactions are determined by details
of the molecular collisions.
- 4.1 The student is able to design and/or interpret the results of an experiment regarding the factors (i.e., temperature, concentration, surface area) that may influence the rate of a reaction.
Idea 5: The laws of thermodynamics describe the essential role
of energy and explain and predict the direction of changes in matter.
- 5.10 The student can support the claim about whether a process is a chemical or physical change (or may be classified as both) based on whether the process involves changes in intramolecular versus intermolecular interactions.
- 5.16 The student can use Le Chatelier’s principle to make qualitative predictions for systems in which coupled reactions that share a common intermediate drive formation of a product.
Idea 6: Any bond or intermolecular attraction that can be
formed can be broken. These two processes are in a dynamic competition,
sensitive to initial conditions and external perturbations.
- 6.3 The student can connect kinetics to equilibrium by using reasoning about equilibrium, such as Le Chatelier’s principle, to infer the relative rates of the forward and reverse reactions.
- 6.8 The student is able to use Le Chatelier’s principle to predict the direction of the shift resulting from various possible stresses on a system at chemical equilibrium.
- 6.10 The student is able to connect Le Chatelier’s principle to the comparison of Q to K by explaining the effects of the stress on Q and K.
- 6.20 The student can identify a solution as being a buffer solution and explain the buffer mechanism in terms of the reactions that would occur on addition of acid or base.
- 6.23 The student can interpret data regarding the relative solubility of salts in terms of factors (common ions, pH) that influence the solubility.
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
This demonstration supports students’ understanding of
- Acid/base neutralization reaction
- Limiting Reactant
- Le Châtelier’s principle
- Extension: Reaction Rate - Kinetics
Teacher Preparation: 5 minutes
Lesson: 20 minutes
- 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
- 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)
- 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 (Ksp = 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.
- Add about 700ml of water to a 1000ml beaker.
- 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.
- Use a 50ml graduated cylinder to measure 25ml of Milk of Magnesia. Pour into the beaker of water.
- Rinse graduated cylinder with water and pour into the large beaker of water.
- Add 5 - 10ml (about two disposable pipettes) of Universal Indicator to the beaker.
- 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.
- Make observations of the color changes.
- 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.
- 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.
- 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.