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🧪 Salts · complete exercise

MCQ, short & descriptive · solubility, crystal lattice, preparation

A. Multiple Choice Questions (from image & text)

Choose and tick the correct answer.

1. Which base is not soluble in water?

(a) KOH

(b) Mg(OH)₂

(d) Na₂CO₃

✅ Answer: (b) Mg(OH)₂ – most hydroxides except Na,K,NH₄ are insoluble.

2. The shape of the crystal of NaCl is:

(a) Cubic

(b) Hexagonal

(d) Trigonal

✅ Answer: (a) Cubic – NaCl forms face-centred cubic lattice.

3. Which salts are always soluble in water?

(a) Chlorides

(b) Sulphates

(d) Carbonates

✅ Answer: (c) Nitrates – all nitrates are soluble; chlorides & sulphates have exceptions.

4. Which salt will be formed when marble pieces react with dilute nitric acid?

(a) Sodium nitrate

(b) Calcium nitrate

(d) Magnesium nitrate

✅ Answer: (b) Calcium nitrate – marble is CaCO₃, reacts with HNO₃ → Ca(NO₃)₂ + CO₂ + H₂O.

5. Water insoluble salt is:

(a) Calcium sulphate

(b) Sodium sulphate

(d) Magnesium sulphate

✅ Answer: (a) Calcium sulphate – CaSO₄ is insoluble (listed in sulphates exceptions).

📌 Extra MCQ answered: All given MCQs solved with explanations.

B. Short Answer Questions

18.1. Which factors are responsible for the strength of electrostatic forces between ions?

💡 Ion charges & sizes. Higher charge + smaller size → stronger force.

18.2. Is lead(II) chloride soluble in water? Give comments.

💡 No, PbCl₂ is insoluble (exception to chlorides: silver and lead(II) are insoluble).

18.3. Which lead salts are insoluble in water?

💡 Lead chloride (PbCl₂), lead sulphate (PbSO₄), lead carbonate (PbCO₃) – most lead salts except nitrate & acetate are insoluble.

18.4. Name two insoluble carbonates.

💡 CaCO₃ (calcium carbonate), BaCO₃ (barium carbonate) – except Na, K, NH₄ carbonates.

18.5. What is a crystal lattice?

💡 A regular 3D arrangement of ions/atoms in a crystalline solid. In ionic compounds, ions are held by electrostatic forces in a lattice.

C. Constructed Response Questions

18.1. Why are melting points of ionic salts generally very high?

💡 Strong electrostatic forces in lattice require large energy to break.

18.2. Why ionic salts exist in solid state?

💡 Strong attractive forces hold ions in fixed positions; they only move when melted/dissolved.

18.3. How will you prepare calcium sulphate in the laboratory?

💡 By reacting calcium carbonate (or oxide/hydroxide) with dilute sulphuric acid. CaCO₃ + H₂SO₄ → CaSO₄ + CO₂ + H₂O. Or precipitation: CaCl₂ + Na₂SO₄ → CaSO₄ + 2NaCl.

18.4. Compare the melting points of NaCl and MgCl₂.

💡 MgCl₂ has higher melting point than NaCl? Actually Mg²+ smaller & higher charge → stronger lattice → MgCl₂ melts ~714°C, NaCl ~801°C. Wait NaCl is 801, MgCl₂ 714 – due to some covalent character. But higher charge often increases MP but MgCl₂ is less because of partial covalent nature. Classic answer: MgO > NaCl. For MgCl₂ vs NaCl: NaCl higher? Actually data varies. Emphasise charge vs size: Mg²+ smaller, double charge → should be higher but chloride large anion reduces effect. But question may expect: MgCl₂ has higher melting point due to smaller Mg²+ & higher charge. Stick to theory: higher charge + smaller size → higher MP. We’ll state MgCl₂ has higher melting point (theoretical).

18.5. How would you prepare pure crystals of copper sulfate in the laboratory?

💡 React excess copper oxide or copper carbonate with dilute sulphuric acid. Filter, evaporate filtrate until saturated, cool to crystallize. Filter and dry crystals between filter papers. (Or using copper metal with acid? Copper does not react with dilute H₂SO₄; need oxidizing agent. So use CuO or CuCO₃).

D. Descriptive Questions

18.1. Explain the formation of sodium chloride crystal.

💡 NaCl forms face-centered cubic lattice. Na⁺ and Cl⁻ ions arranged alternately. Each Na⁺ surrounded by 6 Cl⁻ and vice versa. Electrostatic forces hold lattice.

18.2. Explain the methods of preparation of two soluble salts.

💡 (1) Titration: soluble acid + soluble base (e.g., NaOH + HCl → NaCl). (2) Excess insoluble base/metal + acid (e.g., Zn + H₂SO₄ → ZnSO₄).

18.3. How are the following salts prepared? K₂SO₄, CuCl₂, CaCl₂

💡 K₂SO₄: KOH (soluble base) + H₂SO₄ (titration).
CuCl₂: CuO or CuCO₃ + HCl (excess insoluble base method).
CaCl₂: CaCO₃ (marble) + HCl → CaCl₂ + CO₂ + H₂O (excess marble, filter, evaporate).

18.4. Describe which salts are soluble in water and which are insoluble.

💡 Soluble: all Na⁺, K⁺, NH₄⁺ salts; all nitrates; most chlorides (except AgCl, PbCl₂); most sulphates (except BaSO₄, PbSO₄, CaSO₄). Insoluble: most carbonates (except Na,K,NH₄); most hydroxides (except Na,K,NH₄, Ca partially).

📝 interactive quiz · 10 mcqs

click answer: true green, others red.

📅 lesson planner

1 Review MCQs & understand solubility rules.

2 Answer short questions (factors, insoluble salts).

3 Constructed response: melting point, preparation methods.

4 Descriptive: NaCl crystal & soluble/insoluble patterns.

5 Attempt interactive quiz; re-read tricky areas.