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More About Salts – Long Answer Questions
Medium Level (Application & Explanation)
Q1. What is meant by a “family of salts”? Classify the given salts into families and state how many families you can recognize.
Answer:
- A family of salts means salts having the same positive (cation) or the same negative (anion) radical.
- From the list: K₂SO₄, Na₂SO₄, CaSO₄, MgSO₄, CuSO₄, NaCl, NaNO₃, Na₂CO₃, NH₄Cl.
- By anions: Sulphate family (SO₄²⁻): K₂SO₄, Na₂SO₄, CaSO₄, MgSO₄, CuSO₄.
- By anions: Chloride family (Cl⁻): NaCl, NH₄Cl; Nitrate family (NO₃⁻): NaNO₃; Carbonate family (CO₃²⁻): Na₂CO₃.
- So, anion families = 4 (sulphate, chloride, nitrate, carbonate).
- By cations: Sodium family (Na⁺): Na₂SO₄, NaCl, NaNO₃, Na₂CO₃; plus K⁺, Ca²⁺, Mg²⁺, Cu²⁺, NH₄⁺. So, cation families = 6.
Q2. Explain how the pH of a salt solution depends on the strengths of the parent acid and base. Give suitable examples.
Answer:
- Salts form from an acid + base reaction. Their pH depends on acid/base strength.
- Strong acid + strong base → neutral salt. Example: NaCl. pH ≈ 7.
- Strong acid + weak base → acidic salt. Example: NH₄Cl. pH < 7.
- Weak acid + strong base → basic salt. Example: Na₂CO₃. pH > 7.
- Reason: The ions of weak partners undergo hydrolysis in water.
- This hydrolysis shifts the pH away from 7, making it acidic or basic.
Q3. Describe the chlor-alkali process. Write the equation and list the main products with their uses.
Answer:
- The chlor-alkali process uses electrolysis of aqueous NaCl (brine).
- Equation: 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g).
- Main products: Sodium hydroxide (NaOH), Chlorine (Cl₂), Hydrogen (H₂).
- Uses: NaOH in soap, paper, textiles.
- Uses: Cl₂ for disinfection, PVC, and making bleaching powder.
- Uses: H₂ as fuel and in some industrial processes.
Q4. How is bleaching powder prepared from common salt indirectly? State the equation and two major uses.
Answer:
- From brine, we get chlorine gas by the chlor-alkali process.
- Chlorine reacts with dry slaked lime to give bleaching powder.
- Equation: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.
- Bleaching powder is calcium oxychloride.
- Uses: Bleaching fabrics and paper.
- Uses: Disinfecting drinking water to kill germs.
Q5. Explain the preparation and uses of baking soda obtained from common salt. Include key reactions.
Answer:
- Baking soda is sodium hydrogencarbonate (NaHCO₃).
- It is made from NaCl, H₂O, CO₂, NH₃.
- Reaction: NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃.
- On heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ (bubbles).
- Uses: In baking, it makes dough rise (CO₂ gas).
- Uses: As an antacid, it neutralizes excess stomach acid.
High Complexity (Analysis & Scenario-Based)
Q6. A student tests NaCl, NH₄Cl, Na₂CO₃, and KNO₃ with pH paper. Predict the pH range and explain your reasoning using the idea of parent acid and base.
Answer:
- NaCl: From strong acid (HCl) and strong base (NaOH). So neutral. pH ≈ 7.
- NH₄Cl: From strong acid (HCl) and weak base (NH₃). So acidic. pH < 7.
- Na₂CO₃: From weak acid (H₂CO₃) and strong base (NaOH). So basic. pH > 7.
- KNO₃: From strong acid (HNO₃) and strong base (KOH). So neutral. pH ≈ 7.
- Rule: Weak partner’s ion hydrolyses and shifts pH.
- Conclusion: Expect two neutrals (NaCl, KNO₃), one acidic (NH₄Cl), one basic (Na₂CO₃).
Q7. In a lab, blue copper sulphate crystals turn white on heating and become blue again after adding water. Explain this behavior and relate it to water of crystallization in other salts.
Answer:
- Blue CuSO₄·5H₂O has water of crystallization.
- On heating, it loses water and becomes anhydrous CuSO₄ (white).
- Water droplets seen in the tube confirm water loss.
- Adding water again restores blue color, as crystals re-form.
- This shows water of crystallization gives color and form to many salts.
- Example: Gypsum (CaSO₄·2H₂O) and Plaster of Paris (CaSO₄·½H₂O) interconvert with water.
Q8. A village has hard water causing scale in kettles. Recommend a sodium compound to soften it. Explain the chemistry with an equation.
Answer:
- Use washing soda (Na₂CO₃·10H₂O).
- It adds CO₃²⁻ ions to water.
- These react with Ca²⁺ / Mg²⁺ to form insoluble carbonates.
- Example: Ca²⁺ + CO₃²⁻ → CaCO₃(s).
- Thus, hard ions are removed from water.
- Result: Water becomes soft, and scaling reduces.
Q9. During heavy rains, a school needs to disinfect stored water quickly. The lab has bleaching powder and lime. Propose a method, write a key reaction, and state safety points.
Answer:
- Use bleaching powder (CaOCl₂) to disinfect water.
- Add a small, measured amount to the storage tank. Stir well.
- In water, CaOCl₂ releases chlorine, which kills germs.
- Simplified: CaOCl₂ + H₂O → Ca(OH)₂ + Cl₂ → disinfecting species (like HOCl).
- Test for mild chlorine smell; avoid over-chlorination.
- Safety: Keep away from eyes/skin, use in ventilated place, store dry.
Q10. You are given three unlabeled salts: NaCl, NH₄Cl, and Na₂CO₃. Design a simple testing plan using litmus and pH paper. Predict outcomes with reasons.
Answer:
- Step 1: Make aqueous solutions of each salt.
- Step 2: Use blue and red litmus.
- NaCl: No change in either litmus. pH ≈ 7. Neutral salt.
- NH₄Cl: Turns blue litmus red. pH < 7. Acidic due to NH₄⁺ hydrolysis.
- Na₂CO₃: Turns red litmus blue. pH > 7. Basic due to CO₃²⁻ hydrolysis.
- Confirm with pH paper: neutral (NaCl), acidic (NH₄Cl), basic (Na₂CO₃).
