Hydrolysis of Salts—Acidic, Basic or Neutral?

Introduction

Show the effects of hydrolysis of salts on the acid–base properties of a solution with this colorful demonstration that can be done on an overhead projector.

Concepts

• Acids and bases
• pH
• Salt hydrolysis

Background

Acidic and basic properties of aqueous solutions depend on the concentrations of hydrogen ions [H⁺] and hydroxide ions [OH^–]. Water (the solvent in an aqueous solution) dissociates to a small extent into hydrogen ions (H⁺) and hydroxide ions (OH^–) according to Equation 1.

When the concentration of H⁺ is equal to the concentration of OH^–, the solution is neutral (pH = 7). When H⁺ ions exceed OH^– ions, the solution is acidic (pH < 7). When OH^– ions exceed H⁺ ions, the solution is basic (pH > 7). For example, an aqueous solution of HCl or H₂SO₄ has a greater concentration of H⁺ ions and is therefore acidic. An aqueous solution of NaOH or NH₄OH has a greater concentration of OH^– ions and is therefore basic.

Salts, on the other hand, may undergo hydrolysis in water to form acidic, basic, or neutral solutions. Hydrolysis of a salt is the reaction of the salt with water or its ions. A salt is an ionic compound containing a cation other than H⁺ and an anion other than OH^– (or O^2–). The broad range of cations and anions that combine to form salts (e.g., NaNO₂, NH₄I, CuSO₄, NaBr) makes it more difficult to predict whether the resulting salt solution will be acidic, basic, or neutral.

In a dilute salt solution, a soluble salt dissociates completely into its ions. Thus, a water solution labeled “NaBr” actually contains Na⁺ ions and Br^– ions (Equation 2). The acid–base properties of a salt, such as NaBr, are determined by the behavior of its ions. To decide whether a water solution of NaBr is acidic, basic or neutral, the effect of the Na⁺ and Br^– ions on the pH of water must be considered. Some ions have no effect on the pH of water, some ions are acidic because they produce H⁺ ions in water, and others are basic because they produce OH^– ions in water. In this demonstration, five salts will be tested. The salts will be dissolved in water, the pH of the resulting solutions will be measured and chemical equations will be written.

Materials

Safety Precautions

Aluminum chloride, ammonium chloride and sodium phosphate are slightly toxic by ingestion and are body tissue irritants. Do not substitute anhydrous aluminum chloride due to its violent reaction with water. Universal indicator solution is an alcohol-based flammable liquid. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Each of the salts may be disposed of down the drain or in the solid waste disposal according to Flinn Suggested Disposal Methods #26a or #26b.

Prelab Preparation

1. Place approximately 150 mL of distilled or deionized water in a beaker.
2. Using a hot plate or Bunsen burner, boil the water for about 10–15 minutes to remove any dissolved carbon dioxide. (Note: The pH of the water should be near 7.) Cover the beaker and allow the water to cool.
3. Rinse five Petri dishes with distilled water to ensure that they are not contaminated.

Procedure

1. Place five Petri dishes on an overhead transparency sheet on the overhead projector (or on the demonstration table). Label the transparency with the formulas of the five salts to be used.
2. Add 15–20 mL of boiled distilled or deionized water to each Petri dish (enough to fill the dishes half way).
3. Add 15 drops of universal indicator solution to each Petri dish to achieve a neutral green color. (Note: If the solution in any of the dishes is not green after adding the indicator, rinse out the dish with DI water and start again as there must have been some contamination.)
4. Using a different spatula for each solid, add about 1 gram of salt to each Petri dish in the following order:

   Petri Dish	Salt	Solution Color	pH
   1	Aluminum chloride	Red	3
   2	Ammonium chloride	Orange-yellow	5
   3	Sodium chloride	Green	7
   4	Sodium bicarbonate	Blue	9
   5	Sodium phosphate	Purple	12

5. Stir to dissolve each solid using a separate wood stirrer for each.
6. Note the pH of each solution by comparing the solution color to the universal indicator color card.

Student Worksheet PDF

11996_Student.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration at least seven times. The quantities provided in the kit are as follows—10 grams (7 g needed) of each of the five salts (aluminum chloride, ammonium chloride, sodium bicarbonate, sodium chloride, and sodium phosphate), 35 mL of universal indicator solution, 5 reusable Petri dishes, 35 wood stirrers and a universal indicator pH card.
• After reading the discussion, decide if you wish to first perform the demonstration and then discuss the observations. Or you may wish to first have students look at and evaluate the cations and anions of the salts and make predictions as to the acidic or basic nature of the salt solutions. Then perform the demonstration to test their predictions.
• An overhead transparency of the universal indicator color card, the Universal Indicator Overhead Color Chart (Flinn Catalog No. AP5367), is available for use on the overhead projector.

Sample Data

Answers to Worksheet Results Table

Petri Dish	Salt	Solution Color	pH	Acid, Base, or Neutral
1	Aluminum chloride	Red	3	Acid
2	Ammonium chloride	Orange-yellow	5	Acid
3	Sodium chloride	Green	7	Neutral
4	Sodium bicarbonate	Blue	9	Base
5	Sodium phosphate	Purple	12	Base

Answers to Questions

1. Explain what happened to the salts in the water and what caused the acid-base properties of the solutions.

   The salts underwent hydrolysis, which is the reaction of a salt with water or its ions. The ions determine the acid–base properties of the resulting solutions. Some ions have no effect on the pH of the water, while some produce H⁺ ions, and others produce OH^– ions.

2. Salt hydrolysis can be described in two chemical equations, the first showing the dissociation of the salt, and the second net equation showing the production of H⁺ or OH^– ions. Write the two equations for each salt in this demonstration. If neither H⁺ nor OH^– ions are produced, write “no reaction” for the second equation.
1. AlCl₃•6H₂O(s) → Al(H₂O)₆³⁺(aq) + 3Cl^–(aq)

   Al(H₂O)₆³⁺ → H⁺(aq) + Al(H₂O)₅(OH)²⁺(aq)
   Note: You may have to tell students that the aluminum forms a complex ion with water.

2. NH₄Cl(s) → NH₄⁺(aq) + Cl^–(aq)

   NH₄⁺(aq) + H₂O(l) → H⁺(aq) + NH₃(g)

3. NaCl(s) → Na⁺(aq) + Cl^–(aq)

   No reaction

4. NaHCO₃(s) → Na⁺(aq) + HCO₃^–(aq)

   HCO₃^–(aq) + H₂O(l) → H₂CO₃(aq) + OH^–(aq)

5. Na₃PO₄(s) → 3Na⁺(aq) + PO₄^3–(aq)

   PO₄^3–(aq) + H₂O(l) → HPO₄^2–(aq) + OH^–(aq)

Discussion

Results from this demonstration show that aluminum chloride and ammonium chloride form acidic solutions in water (pH < 7); sodium chloride forms a neutral solution (pH = 7); sodium bicarbonate and sodium phosphate form basic solutions (pH > 7). Hydrolysis refers to the reaction of a substance with water or its ions. The chemical equations for the reactions are shown. The equation for the dissociation of the salts are shown first followed by the net equations that produce either H⁺ (if acidic), OH^– (if basic) or neither (if neutral). Note: Spectator ions are omitted from the net equations.

Aluminum chloride
AlCl3•6H2O(s) → Al(H2O)63+(aq) + 3Cl–(aq)
Al(H2O)63+(aq) → H+(aq) + Al(H2O)5(OH)2+(aq)	Acidic

Ammonium chloride
NH4Cl(s) → NH4+(aq) + Cl–(aq)
NH4+(aq) → H+(aq) + NH3(g)	Acidic

Sodium chloride
NaCl(s) → Na+(aq) + Cl–(aq)
Na+(aq) + Cl–(aq) → No further reaction	Neutral

Sodium bicarbonate
NaCl(s) → Na+(aq) + Cl–(aq)
HCO3–(aq) + H2O(l) → H2CO3(aq) + OH–(aq)	Basic

Sodium phosphate
Na3PO4(s) → 3Na+(aq) + PO43–(aq)
PO43–(aq) + H2O(l) → HPO42–(aq) + OH–(aq)	Basic

While acidic or basic properties of salt solutions can be measured in the laboratory, the acidic or basic nature of a salt can also be predicted by considering the properties of its ions. In general, as shown in Table 1, neutral anions are those derived from strong acids and neutral cations are those derived from strong bases. Acidic cations include all cations except those of the alkali metals and the heavier alkaline earths. Acidic anions include the HSO₄^– and H₂PO₄^– anions. Basic anions include any anion derived from a weak acid; there are no common basic cations.

The information provided in Table 1 can be used to predict the acidic or basic nature of the salt; this can then be confirmed by experiment. The five salts tested in this demonstration are listed below. The acidic or basic nature of the cation and of the anion are given, together with a prediction of whether the salt solution will be acidic, basic, or neutral.

Salt	Cation	Anion	Solution of Salt
AlCl3•6H2O	Al3+ (acidic)	Cl– (neutral)	Acidic
NH4Cl	NH4+ (acidic)	Cl– (neutral)	Acidic
NaCl	Na+ (neutral)	Cl– (neutral)	Neutral
NaHCO3	Na+ (neutral)	HCO3– (basic)	Basic
Na3PO4•12H2O	Na+ (neutral)	PO43– (basic)	Basic

References

Flinn Scientific would like to thank John Wass, Western Branch H.S., Chesapeake, VA, for bringing this demonstration to our attention to share with other teachers.