Give an example of a salt which, when dissolved in water, produces an aqueous solution with pH less than 7 and therefore behaves as an acidic salt solution.

Introduction / Context:
This question examines knowledge about acidic and basic salts and how they behave in aqueous solution. Some salts, when dissolved in water, give solutions with pH less than 7, while others yield neutral or basic solutions. The correct example here must be a salt that, due to hydrolysis of its ions, produces an acidic solution. This concept is important in general chemistry and environmental chemistry because many natural waters contain such salts.

Given Data / Assumptions:

• We are looking for a salt that gives an aqueous solution with pH less than 7.
• The options include common salts of strong and weak acids and bases.
• Water is assumed to be pure and at standard conditions so that pH 7 is neutral.
• Hydrolysis of ions in solution determines whether the solution is acidic, basic, or neutral.

Concept / Approach:
A salt formed from a strong base and a weak acid will generally give an alkaline solution, because the conjugate base of the weak acid hydrolyses and produces hydroxide ions. A salt formed from a weak base and a strong acid will give an acidic solution, because the conjugate acid of the weak base hydrolyses to produce hydronium ions. A salt formed from a strong acid and a strong base will usually give a neutral solution. Ammonium chloride, NH4Cl, is formed from a weak base (ammonia) and a strong acid (hydrochloric acid), so its solution is acidic.

Step-by-Step Solution:

Verification / Alternative check:
The pH of a typical 0.1 molar ammonium chloride solution is around 5.5, clearly below 7, confirming its acidic nature. In contrast, a similar concentration of sodium carbonate gives a pH around 11, and sodium bicarbonate solutions are slightly basic with pH above 7. Sodium chloride solutions are close to neutral with pH near 7. These approximate values confirm that ammonium chloride is the only option among the given salts that produces an acidic solution.

Why Other Options Are Wrong:

• Sodium carbonate: Salt of a strong base and a weak acid, so it hydrolyses to give an alkaline solution with pH greater than 7.
• Sodium bicarbonate: Also related to carbonic acid and a strong base and generally gives a slightly basic solution.
• Sodium chloride: Formed from a strong acid and a strong base, giving an essentially neutral solution.
• Potassium carbonate: Similar to sodium carbonate, this salt yields a basic solution due to hydrolysis of the carbonate ion.

Common Pitfalls:
Students often think that any salt containing chloride or carbonate must be neutral like common kitchen salt. Others may misinterpret bicarbonate as acidic because it can neutralise acids, forgetting that its solution is actually mildly basic. To avoid confusion, always analyse whether the parent acid and base are strong or weak and then predict the behaviour of the solution based on the hydrolysis of the ions.

Final Answer:
The salt that gives an aqueous solution of pH less than 7 is Ammonium chloride.