Q1: What are ionic compounds?

Answer: Ionic compounds are formed by the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions). They are typically formed between metals and non-metals. When dissolved in water, many ionic compounds dissociate into their constituent ions.

Q2: What is dissociation?

Answer: Dissociation is the process where an ionic compound, when dissolved in water, separates into its constituent positively and negatively charged ions. For example, NaCl dissociates into Na⁺ and Cl⁻ ions in water.

Q3: According to Arrhenius theory, what is an acid?

Answer: According to Svante Arrhenius, an acid is a substance which, when dissolved in water, gives hydrogen ions (H⁺) as the only cation. Example: HCl(aq) → H⁺(aq) + Cl⁻(aq).

Q4: According to Arrhenius theory, what is a base?

Answer: According to Svante Arrhenius, a base is a substance which, when dissolved in water, gives hydroxide ions (OH⁻) as the only anion. Example: NaOH(aq) → Na⁺(aq) + OH⁻(aq).

Q5: Why is H⁺ ion always found in hydrated form (H₃O⁺) in aqueous solutions?

Answer: The hydrogen ion (H⁺) is simply a proton and is highly reactive. It cannot exist independently in water and immediately combines with a water molecule (H₂O) to form the hydronium ion (H₃O⁺). H⁺ + H₂O → H₃O⁺.

Q6: How are acids and bases classified based on their strength?

Answer:

• Strong Acids/Bases: Dissociate almost completely in water, producing a high concentration of H⁺ or OH⁻ ions (e.g., HCl, H₂SO₄, NaOH, KOH).
• Weak Acids/Bases: Dissociate only partially in water, resulting in an equilibrium between the undissociated molecule and its ions, producing a low concentration of H⁺ or OH⁻ ions (e.g., CH₃COOH (acetic acid), H₂CO₃ (carbonic acid), NH₄OH (ammonium hydroxide)).
• Alkalis: Bases that are highly soluble in water are called alkalis (e.g., NaOH, KOH).

Q7: What is meant by the basicity of an acid?

Answer: The basicity of an acid is the number of H⁺ ions that can be obtained from one molecule of the acid upon complete dissociation in water. Examples: HCl (monobasic, 1 H⁺), H₂SO₄ (dibasic, 2 H⁺), H₃PO₄ (tribasic, 3 H⁺).

Q8: What is meant by the acidity of a base?

Answer: The acidity of a base is the number of OH⁻ ions that can be obtained from one molecule of the base upon complete dissociation in water. Examples: NaOH (monoacidic, 1 OH⁻), Ca(OH)₂ (diacidic, 2 OH⁻), Al(OH)₃ (triacidic, 3 OH⁻).

Q9: How is the concentration of an acid or base expressed?

Answer: Concentration indicates the amount of solute (acid or base) dissolved in a given amount of solvent (usually water). It can be expressed in:

• Grams per litre (g/L): Mass of solute in grams dissolved in one litre of solution.
• Molarity (Moles per litre, M): Number of moles of solute dissolved in one litre of solution.

Q10: What are indicators?

Answer: Indicators are substances that change their colour in the presence of an acid or a base, allowing us to visually distinguish between acidic, basic, and neutral solutions. They are usually weak organic acids or bases.

Q11: Name some common indicators and their colour changes in acidic and basic solutions.

Answer:

• Litmus (paper or solution): Acidic: Red, Basic: Blue, Neutral: Purple (solution) or no change.
• Phenolphthalein: Acidic: Colourless, Basic: Pink, Neutral: Colourless.
• Methyl Orange: Acidic: Red/Pink, Basic: Yellow, Neutral: Orange.
• Universal Indicator: Shows a range of colours corresponding to different pH values.

Q12: What is the pH scale?

Answer: The pH scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It ranges from 0 to 14. It measures the concentration of hydrogen ions (H⁺) or hydronium ions (H₃O⁺).

Q13: How is pH related to H⁺ ion concentration?

Answer: pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration ([H⁺]) in moles per litre. pH = -log₁₀[H⁺]. A lower pH indicates a higher [H⁺] (more acidic), and a higher pH indicates a lower [H⁺] (more basic).

Q14: What do different pH values indicate?

Answer:

• pH < 7: Acidic solution ([H⁺] > [OH⁻])
• pH = 7: Neutral solution ([H⁺] = [OH⁻], e.g., pure water)
• pH > 7: Basic (alkaline) solution ([H⁺] < [OH⁻])

Q15: What is a neutralization reaction?

Answer: A neutralization reaction is a reaction between an acid and a base, which results in the formation of salt and water. The acidic properties of the acid and the basic properties of the base are destroyed (neutralized).
Example: HCl (acid) + NaOH (base) → NaCl (salt) + H₂O (water)
Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)

Q16: What is a salt?

Answer: A salt is an ionic compound formed when the cation from a base and the anion from an acid combine during a neutralization reaction. It consists of a positive ion (other than H⁺) and a negative ion (other than OH⁻ or O²⁻).

Q17: How are salts classified based on their nature?

Answer: Salts can be classified based on the strength of the acid and base from which they are formed:

• Neutral Salts: Formed from a strong acid and a strong base (e.g., NaCl from HCl + NaOH). pH ≈ 7.
• Acidic Salts: Formed from a strong acid and a weak base (e.g., NH₄Cl from HCl + NH₄OH). pH < 7 (due to hydrolysis producing H⁺).
• Basic Salts: Formed from a weak acid and a strong base (e.g., CH₃COONa from CH₃COOH + NaOH). pH > 7 (due to hydrolysis producing OH⁻).

(Note: Salts formed from weak acid and weak base can be acidic, basic, or neutral depending on the relative strengths, e.g., NH₄CH₃COO).

Q18: Give examples of salts and their parent acid and base.

Answer:

• NaCl (Sodium Chloride): Parent Acid: HCl (Strong), Parent Base: NaOH (Strong) -> Neutral Salt.
• KNO₃ (Potassium Nitrate): Parent Acid: HNO₃ (Strong), Parent Base: KOH (Strong) -> Neutral Salt.
• (NH₄)₂SO₄ (Ammonium Sulphate): Parent Acid: H₂SO₄ (Strong), Parent Base: NH₄OH (Weak) -> Acidic Salt.
• Na₂CO₃ (Sodium Carbonate): Parent Acid: H₂CO₃ (Weak), Parent Base: NaOH (Strong) -> Basic Salt.
• CuSO₄ (Copper Sulphate): Parent Acid: H₂SO₄ (Strong), Parent Base: Cu(OH)₂ (Weak) -> Acidic Salt.

Q19: What is water of crystallization?

Answer: Water of crystallization refers to the fixed number of water molecules that are chemically bound within the crystal structure of some salts when they crystallize from an aqueous solution. This water is essential for the shape and sometimes the colour of the crystal.
Example: Copper Sulphate Pentahydrate (CuSO₄·5H₂O) - blue crystals. Anhydrous CuSO₄ is white.

Q20: What happens when a salt containing water of crystallization is heated?

Answer: When heated, the salt loses its water of crystallization, often changing its colour and crystal structure, becoming an anhydrous salt (without water). Example: Heating blue CuSO₄·5H₂O yields white anhydrous CuSO₄.

Q21: What are electrolytes?

Answer: Electrolytes are substances (acids, bases, or salts) that conduct electricity when dissolved in water or in the molten state because they dissociate or ionize to produce mobile ions.

Q22: What are non-electrolytes?

Answer: Non-electrolytes are substances that do not conduct electricity when dissolved in water or molten because they do not produce ions (e.g., sugar, urea, alcohol).

Q23: Differentiate between strong and weak electrolytes.

Answer:

• Strong Electrolytes: Dissociate almost completely into ions in solution, resulting in high electrical conductivity (e.g., strong acids, strong bases, most soluble salts like NaCl, HCl, NaOH).
• Weak Electrolytes: Dissociate only partially into ions in solution, resulting in low electrical conductivity (e.g., weak acids, weak bases like CH₃COOH, NH₄OH).

Q24: What is electrolysis?

Answer: Electrolysis is the process of decomposing an electrolyte (in molten state or aqueous solution) by passing a direct electric current through it. The electric current causes chemical reactions (oxidation and reduction) at the electrodes.

Q25: Describe the electrolysis of water.

Answer: When pure water (a very weak electrolyte) has a small amount of acid (like H₂SO₄) added to make it conductive, and electric current is passed through it using inert electrodes (like platinum or graphite):

• Water decomposes: 2H₂O(l) → 2H₂(g) + O₂(g)
• At the Cathode (-): Reduction occurs. H⁺ ions (or water) gain electrons to form Hydrogen gas. (2H⁺ + 2e⁻ → H₂ or 2H₂O + 2e⁻ → H₂ + 2OH⁻)
• At the Anode (+): Oxidation occurs. OH⁻ ions (or water) lose electrons to form Oxygen gas. (4OH⁻ → O₂ + 2H₂O + 4e⁻ or 2H₂O → O₂ + 4H⁺ + 4e⁻)
• Result: Hydrogen gas collects at the cathode, Oxygen gas collects at the anode (Volume ratio H₂:O₂ ≈ 2:1).

Q26: Classify the following into acids and bases: HCl, NaOH, KOH, H₂SO₄, Ca(OH)₂, CH₃COOH, NH₄OH.

Answer:

• Acids: HCl, H₂SO₄, CH₃COOH
• Bases: NaOH, KOH, Ca(OH)₂, NH₄OH

Q27: What is the concentration of H⁺ ions in pure water at 25°C?

Answer: In pure water at 25°C, the concentration of H⁺ ions is equal to the concentration of OH⁻ ions, which is 1.0 × 10⁻⁷ moles per litre.

Q28: A solution turns red litmus blue. What is its likely pH? Is it acidic or basic?

Answer: If a solution turns red litmus blue, it is basic. Its pH is likely greater than 7.

Q29: What happens to the pH of an acidic solution when it is diluted?

Answer: When an acidic solution is diluted with water, the concentration of H⁺ ions decreases. Since pH = -log₁₀[H⁺], a decrease in [H⁺] leads to an increase in pH (moving towards 7, but remaining less than 7).

Q30: Give two uses of neutralization reactions in everyday life.

Answer:

1. Treating Indigestion: Antacids (mild bases like Mg(OH)₂) neutralize excess stomach acid (HCl).
2. Treating Soil: Farmers add bases (like slaked lime) to neutralize acidic soil for better crop growth.
3. Treating Insect Stings: Neutralizing acidic bee stings with baking soda (base) or alkaline wasp stings with vinegar (acid).