Testing Salt Solutions: pH and Parent Compounds
Unveiling the Acidic, Basic, or Neutral Nature of Salts
Salts are compounds formed from the reaction of acids and bases, but not all salts are "neutral" like common table salt. Some can be acidic or basic depending on the strength of their parent acid and base. In this activity, we will explore the pH of various salt solutions to understand their nature and identify the acids and bases from which they are derived.
Aim of the Activity
- To prepare aqueous solutions of various salt samples.
- To test the action of these solutions on litmus paper and determine their pH using pH paper.
- To classify each salt as acidic, basic, or neutral based on its pH.
- To identify the parent acid and parent base that formed each salt.
Materials You'll Need
- Salt samples: Sodium chloride ($NaCl$), Potassium nitrate ($KNO_3$), Aluminium chloride ($AlCl_3$), Zinc sulphate ($ZnSO_4$), Copper sulphate ($CuSO_4$), Sodium acetate ($CH_3COONa$), Sodium carbonate ($Na_2CO_3$), Sodium hydrogencarbonate ($NaHCO_3$) (and any other available salts)
- Distilled water
- Test tubes or small beakers
- Droppers
- Red and Blue Litmus paper strips
- pH paper strips
- Standard pH color chart
- Stirring rod
Procedure
- Prepare solutions: For each salt sample, prepare a dilute aqueous solution by dissolving a small amount (e.g., 0.5-1 g) in about 5-10 mL of distilled water in a separate test tube or beaker. Stir well to dissolve.
- Test with Litmus Paper: For each solution, dip a strip of red litmus paper and a strip of blue litmus paper. Observe and record any color changes.
- Test with pH Paper: For each solution, dip a fresh strip of pH paper. Compare the color change on the pH paper with the standard pH color chart to determine the approximate pH value.
- Identify Nature: Based on the litmus test and pH value, classify each solution as acidic, basic, or neutral.
-
Identify Parent Compounds: For each salt, identify the strong or weak acid and base from which it is formed. Remember:
- Salt of Strong Acid + Strong Base = Neutral (pH $\approx$ 7)
- Salt of Strong Acid + Weak Base = Acidic (pH < 7)
- Salt of Weak Acid + Strong Base = Basic (pH > 7)
- Salt of Weak Acid + Weak Base = Can be acidic, basic, or neutral depending on relative strengths.
- Record Observations: Compile all your observations and identifications in a table similar to the one provided below.
Observation Table: Salt Properties and Parent Compounds
Here's a typical set of observations you might make:
| Salt | Approx. pH | Nature | Parent Acid | Parent Base |
|---|---|---|---|---|
| Sodium Chloride ($NaCl$) | ~7 | Neutral | Hydrochloric Acid ($HCl$) - Strong | Sodium Hydroxide ($NaOH$) - Strong |
| Potassium Nitrate ($KNO_3$) | ~7 | Neutral | Nitric Acid ($HNO_3$) - Strong | Potassium Hydroxide ($KOH$) - Strong |
| Aluminium Chloride ($AlCl_3$) | ~4-5 | Acidic | Hydrochloric Acid ($HCl$) - Strong | Aluminium Hydroxide ($Al(OH)_3$) - Weak |
| Zinc Sulphate ($ZnSO_4$) | ~5-6 | Acidic | Sulphuric Acid ($H_2SO_4$) - Strong | Zinc Hydroxide ($Zn(OH)_2$) - Weak |
| Copper Sulphate ($CuSO_4$) | ~4-5 | Acidic | Sulphuric Acid ($H_2SO_4$) - Strong | Copper(II) Hydroxide ($Cu(OH)_2$) - Weak |
| Sodium Acetate ($CH_3COONa$) | ~8-9 | Basic | Acetic Acid ($CH_3COOH$) - Weak | Sodium Hydroxide ($NaOH$) - Strong |
| Sodium Carbonate ($Na_2CO_3$) | ~10-11 | Basic | Carbonic Acid ($H_2CO_3$) - Weak | Sodium Hydroxide ($NaOH$) - Strong |
| Sodium Hydrogencarbonate ($NaHCO_3$) | ~8-9 | Basic | Carbonic Acid ($H_2CO_3$) - Weak | Sodium Hydroxide ($NaOH$) - Strong |
Explanation: Why Salts Show Different pH Values
The pH of a salt solution depends on the strength of the acid and base from which it is formed. This phenomenon is called salt hydrolysis.
- Salts of Strong Acid + Strong Base: (e.g., $NaCl$, $KNO_3$) These salts do not undergo hydrolysis significantly. Both the cation and anion are weak conjugate acid/base respectively and do not react with water to produce $H^+$ or $OH^-$ ions. Hence, their solutions are neutral (pH $\approx$ 7).
- Salts of Strong Acid + Weak Base: (e.g., $AlCl_3$, $ZnSO_4$, $CuSO_4$) The cation from the weak base undergoes hydrolysis, reacting with water to produce $H^+$ ions, making the solution acidic (pH < 7). For example, for $AlCl_3$:
$Al^{3+}(aq) + 3H_2O(l) \rightleftharpoons Al(OH)_3(s) + 3H^+(aq)$
- Salts of Weak Acid + Strong Base: (e.g., $CH_3COONa$, $Na_2CO_3$, $NaHCO_3$) The anion from the weak acid undergoes hydrolysis, reacting with water to produce $OH^-$ ions, making the solution basic (pH > 7). For example, for $CH_3COONa$:
$CH_3COO^-(aq) + H_2O(l) \rightleftharpoons CH_3COOH(aq) + OH^-(aq)$
Conclusion
This activity clearly demonstrates that the pH of a salt solution is not always neutral. It depends on the strengths of the parent acid and base that formed the salt. By testing with litmus and pH paper, we can classify salts as acidic, basic, or neutral, gaining a deeper understanding of their chemical behavior in aqueous solutions.
Multiple Choice Questions
1. A salt formed from a strong acid and a weak base will typically have a pH:
- a) Equal to 7
- b) Less than 7
- c) Greater than 7
- d) Cannot be determined
2. Which of the following salts would produce a basic solution when dissolved in water?
- a) $NaCl$
- b) $KNO_3$
- c) $Na_2CO_3$
- d) $AlCl_3$
3. The parent base of Copper Sulphate ($CuSO_4$) is:
- a) $NaOH$
- b) $Cu(OH)_2$
- c) $KOH$
- d) $H_2SO_4$