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Class 10 Chemistry — Chapter 18: Salts

Quran Education Point
Board: Punjab Board

A. Multiple Choice Questions (MCQs)

1. Which base is not soluble in water?

(a) KOH
(b) Mg(OH)₂
(c) NaOH
(d) Na₂CO₃

Correct Answer: (b) Mg(OH)₂

2. The shape of the crystal of NaCl is:

(a) Cubic
(b) Hexagonal
(c) Rhombic
(d) Trigonal

Correct Answer: (a) Cubic

3. Which salts are always soluble in water?

(a) Chlorides
(b) Sulphates
(c) Nitrates
(d) Carbonates

Correct Answer: (c) Nitrates

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

(a) Sodium nitrate
(b) Calcium nitrate
(c) Potassium nitrate
(d) Magnesium nitrate

Correct Answer: (b) Calcium nitrate

5. Which of the following is a water-insoluble salt?

(a) Calcium sulphate
(b) Sodium sulphate
(c) Potassium sulphate
(d) Magnesium sulphate

Correct Answer: (a) Calcium sulphate

Note: Calcium sulphate is sparingly soluble in water.

B. Short Answer Questions

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

The strength of electrostatic forces between ions depends on the following factors:

1. Charge on the ions: A greater magnitude of ionic charges results in stronger electrostatic attraction.

\[ F \propto \frac{|q_1q_2|}{r^2} \]

2. Size of the ions (ionic radius): Smaller ionic radii decrease the distance between ionic centres, resulting in stronger attractive forces.

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

Lead(II) chloride (PbCl₂) is sparingly soluble in cold water but becomes more soluble in hot water.

18.3 Which lead salts are insoluble in water?

The following lead salts are insoluble or sparingly soluble in water:

Lead(II) chloride (PbCl₂), especially in cold water
Lead(II) sulphate (PbSO₄)
Lead(II) carbonate (PbCO₃)
Lead(II) iodide (PbI₂)

Lead(II) nitrate, Pb(NO₃)₂, is soluble in water.

18.4 Name two insoluble carbonates.

Calcium carbonate (CaCO₃)
Magnesium carbonate (MgCO₃)

18.5 What is a crystal lattice?

A crystal lattice is a highly ordered, three-dimensional arrangement of particles in a crystalline solid. In ionic compounds, it consists of positively charged cations and negatively charged anions held together by electrostatic forces of attraction.

C. Constructed Response Questions

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

Ionic salts consist of positive and negative ions held together by strong electrostatic forces of attraction throughout a three-dimensional crystal lattice. A large amount of thermal energy is required to overcome these forces, so ionic salts generally have high melting points.

18.2 Why do ionic salts exist in the solid state?

The strong electrostatic attraction between oppositely charged ions holds them in fixed positions within a three-dimensional crystal lattice. At room temperature, the ions can only vibrate about their fixed positions, so ionic salts exist as solids.

18.3 How will you prepare calcium sulphate in the laboratory?

Calcium sulphate can be prepared by the precipitation method by mixing two soluble salt solutions, calcium chloride and sodium sulphate.

Chemical equation:

\[ \mathrm{CaCl_2(aq)+Na_2SO_4(aq)\rightarrow CaSO_4(s)\downarrow+2NaCl(aq)} \]

The calcium sulphate precipitate is filtered, washed with distilled water, and dried.

18.4 Compare the melting points of NaCl and MgCl₂.

Magnesium chloride (MgCl₂) has a higher lattice energy than sodium chloride (NaCl) because Mg²⁺ has a higher charge and a smaller ionic radius than Na⁺. Therefore, the electrostatic attraction between ions is stronger in MgCl₂.

However, lattice energy alone does not determine melting point. The actual melting points of NaCl and MgCl₂ depend on their crystal structures and bonding characteristics.

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

Pure copper sulphate crystals can be prepared by crystallization using the following steps:

1. Reaction: Add excess copper(II) oxide (CuO) or copper(II) carbonate (CuCO₃) to dilute sulphuric acid (H₂SO₄) while heating until no more solid dissolves.

2. Filtration: Filter the mixture to remove the unreacted excess solid. A blue copper sulphate solution is obtained.

3. Crystallization: Heat the filtrate to evaporate some water until the solution approaches saturation. Allow it to cool slowly to form blue crystals of hydrated copper sulphate (CuSO₄·5H₂O).

4. Drying: Filter the crystals and dry them between sheets of filter paper.

D. Descriptive Questions

18.1 Explain the formation of a sodium chloride crystal.

Sodium chloride (NaCl) forms when a sodium atom transfers its one valence electron to a chlorine atom.

Sodium loses one electron to form a positively charged sodium ion (Na⁺).
Chlorine gains one electron to form a negatively charged chloride ion (Cl⁻).
The oppositely charged ions attract each other through electrostatic forces.
These ions arrange themselves in a three-dimensional crystal lattice.

In the NaCl crystal structure, each Na⁺ ion is surrounded by six Cl⁻ ions, and each Cl⁻ ion is surrounded by six Na⁺ ions. This arrangement provides strong electrostatic attraction throughout the crystal.

18.2 Explain the methods of preparation of two soluble salts.

1. Neutralization of an acid by an alkali (titration)

A soluble salt can be prepared by reacting an acid with an alkali.

Example: Preparation of sodium chloride.

\[ \mathrm{HCl(aq)+NaOH(aq)\rightarrow NaCl(aq)+H_2O(l)} \]

The acid and alkali are mixed in suitable proportions. The resulting solution contains sodium chloride, which can be obtained by evaporation and crystallization.

2. Action of an acid on an insoluble metal oxide

A soluble salt can also be prepared by reacting an acid with an insoluble metal oxide.

Example: Preparation of copper sulphate.

\[ \mathrm{CuO(s)+H_2SO_4(aq)\rightarrow CuSO_4(aq)+H_2O(l)} \]

Excess copper oxide is added to dilute sulphuric acid. The mixture is filtered to remove the excess solid, and the filtrate is concentrated and cooled to obtain copper sulphate crystals.

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

1. Preparation of potassium sulphate (K₂SO₄)

Potassium sulphate is prepared by titrating potassium hydroxide solution with dilute sulphuric acid.

\[ \mathrm{2KOH(aq)+H_2SO_4(aq)\rightarrow K_2SO_4(aq)+2H_2O(l)} \]

The solution is concentrated and allowed to cool to obtain crystals.

2. Preparation of copper(II) chloride (CuCl₂)

Copper(II) chloride is prepared by reacting an insoluble copper compound with dilute hydrochloric acid.

Using copper(II) carbonate:

\[ \mathrm{CuCO_3(s)+2HCl(aq)\rightarrow CuCl_2(aq)+CO_2(g)+H_2O(l)} \]

The mixture is filtered, and the resulting solution is concentrated to obtain the salt.

3. Preparation of calcium chloride (CaCl₂)

Calcium chloride is prepared by reacting calcium carbonate (marble chips) with dilute hydrochloric acid.

\[ \mathrm{CaCO_3(s)+2HCl(aq)\rightarrow CaCl_2(aq)+CO_2(g)+H_2O(l)} \]

The reaction mixture is filtered to remove any excess calcium carbonate. The filtrate is concentrated to obtain calcium chloride.

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

Soluble salts

All sodium (Na⁺), potassium (K⁺), and ammonium (NH₄⁺) salts are soluble.
All nitrates (NO₃⁻) and acetates (CH₃COO⁻) are soluble.
Most chlorides (Cl⁻) are soluble, except certain salts such as silver chloride (AgCl) and lead(II) chloride (PbCl₂), which is sparingly soluble in cold water.
Most sulphates (SO₄²⁻) are soluble. Barium sulphate (BaSO₄) and lead(II) sulphate (PbSO₄) are insoluble, while calcium sulphate (CaSO₄) is sparingly soluble.

Insoluble salts

Most carbonates (CO₃²⁻) are insoluble, except those of Group 1 metals and ammonium.
Most phosphates (PO₄³⁻) are insoluble, except those of Group 1 metals and ammonium.
Most hydroxides (OH⁻) are insoluble, except those of Group 1 metals. Some other hydroxides, such as calcium hydroxide, are slightly soluble.

Notes by Sir Dilawar

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