Acids, Bases and Salts

Correct Option: (d) 10

Explanation:
• Red litmus turns blue in basic solutions.
• pH scale: 0-6.9 (acidic), 7 (neutral), 7.1-14 (basic).
• Among the options, only pH 10 is basic (>7).
• pH 1, 4, 5 are acidic and would turn blue litmus red.

Correct Option: (b) HCl

Explanation:
• Egg-shells contain calcium carbonate (CaCO₃).
• Acids react with carbonates to produce CO₂ gas.
• 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
• CO₂ turns lime-water (Ca(OH)₂) milky due to formation of insoluble calcium carbonate.
• Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O

Correct Option: (d) 16 mL

Explanation:
This is a neutralisation reaction: NaOH + HCl → NaCl + H₂O
• 10 mL NaOH requires 8 mL HCl.
• Moles of acid/base are proportional to volume in neutralisation.
• 20 mL NaOH is double the original amount (10 mL).
• So, required HCl volume = 2 × 8 mL = 16 mL.

Correct Option: (c) Antacid

Explanation:
• Indigestion is caused by excess hydrochloric acid in the stomach.
• Antacids are mild bases (like Mg(OH)₂, NaHCO₃) that neutralize excess stomach acid.
• Example reaction: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
• Antibiotics fight bacteria, analgesics relieve pain, antiseptics kill microbes on skin.

(a) Word Equation: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
Balanced: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

(b) Word Equation: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
Balanced: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

(c) Word Equation: Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen
Balanced: 2Al(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂(g)

(d) Word Equation: Iron + Hydrochloric acid → Iron(II) chloride + Hydrogen
Balanced: Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

Note: All are metal-acid displacement reactions producing hydrogen gas.

Activity (Electrical Conductivity Test):
1. Set up a circuit with a battery, bulb, and two electrodes.
2. Take solutions of glucose, alcohol, and dilute hydrochloric acid in separate beakers.
3. Dip the electrodes in each solution one by one and observe the bulb.
Observations:
• The bulb glows brightly for HCl solution (acid).
• The bulb does not glow for glucose and alcohol solutions.
Conclusion:
• Acids conduct electricity due to presence of H⁺ ions.
• Glucose and alcohol do not dissociate into H⁺ ions in aqueous solution, hence are not acids despite containing hydrogen.

• Distilled water is pure H₂O with no ions. It does not conduct electricity as conduction requires mobile ions.
• Rain water dissolves atmospheric gases (CO₂, SO₂, NO₂) forming weak acids (carbonic acid, etc.) that dissociate into ions.
• Example: CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
• These ions (H⁺, HCO₃⁻, etc.) allow rain water to conduct electricity.

• Acids show acidic behaviour due to H⁺ (or H₃O⁺) ions.
• In absence of water, acids (like dry HCl gas) cannot dissociate into ions.
• Water helps in dissociation: HCl + H₂O → H₃O⁺ + Cl⁻
• Dry HCl gas does not change the colour of dry litmus paper because no H⁺ ions are available.
• Thus, water is essential for acids to exhibit acidic properties.

(a) Neutral: Solution D (pH = 7)
(b) Strongly Alkaline: Solution C (pH = 11)
(c) Strongly Acidic: Solution B (pH = 1)
(d) Weakly Acidic: Solution A (pH = 4)
(e) Weakly Alkaline: Solution E (pH = 9)

Increasing order of H⁺ concentration:
• Higher pH means lower H⁺ concentration.
• pH values: 1 (B) < 4 (A) < 7 (D) < 9 (E) < 11 (C)
• But H⁺ concentration order is opposite: B > A > D > E > C

Fizzing occurs more vigorously in test tube A (with HCl).

Reason:
• HCl is a strong acid that completely dissociates: HCl → H⁺ + Cl⁻
• Acetic acid is a weak acid that partially dissociates: CH₃COOH ⇌ H⁺ + CH₃COO⁻
• Reaction: Mg + 2H⁺ → Mg²⁺ + H₂(g)
• Since HCl provides more H⁺ ions per unit volume, the reaction is faster and hydrogen gas (fizzing) is produced more vigorously.

The pH will decrease (become less than 6, more acidic).

Explanation:
• Fresh milk pH ≈ 6 (slightly acidic).
• Curd formation involves lactic acid fermentation by bacteria (Lactobacillus).
• Bacteria convert lactose (milk sugar) into lactic acid.
• Increase in lactic acid concentration increases H⁺ ions, thus lowering the pH.
• Curd typically has pH around 4.5–5.5.

(a) Baking soda (sodium hydrogencarbonate, NaHCO₃) is a mild base. It neutralizes the slight acidity of fresh milk, raising the pH to slightly alkaline. This prevents immediate souring and extends shelf life.

(b) Curd formation requires an acidic medium for bacteria (Lactobacillus) to grow and produce lactic acid. The alkaline pH due to baking soda inhibits bacterial growth, slowing down the fermentation process, so milk takes longer to set as curd.

• Plaster of Paris is calcium sulphate hemihydrate: CaSO₄·½H₂O
• It readily absorbs moisture from air and reacts to form gypsum:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (Gypsum)
• This reaction causes it to harden and become unusable.
• Storing in a moisture-proof container prevents contact with atmospheric moisture, preserving its properties.

Neutralisation Reaction: A reaction between an acid and a base to form salt and water, with H⁺ and OH⁻ ions combining to form H₂O.

Example 1 (Strong acid + Strong base):
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Example 2 (Treatment of antacid):
Mg(OH)₂(s) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)
Example 3 (In soil treatment):
Ca(OH)₂(s) + H₂SO₄(aq) → CaSO₄(s) + 2H₂O(l)

Washing Soda (Na₂CO₃·10H₂O):
1. Cleaning agent: Used as a household cleaner for clothes and utensils.
2. Water softener: Removes permanent hardness of water by precipitating calcium and magnesium ions.
3. Industrial uses: Manufacture of glass, paper, and borax.

Baking Soda (NaHCO₃):
1. Antacid: Neutralizes excess stomach acid (indigestion).
2. Baking powder: Used in baking to make cakes and bread soft and fluffy (releases CO₂).
3. Fire extinguisher: Component of soda-acid fire extinguishers.

• Solution A (pH = 6) has higher H⁺ concentration than Solution B (pH = 8).
• Lower pH means higher H⁺ concentration.
• Solution A (pH 6) is acidic (pH < 7).
• Solution B (pH 8) is basic (pH > 7).

Note: pH is a logarithmic scale. Solution A with pH 6 has 100 times more H⁺ ions than neutral water (pH 7), while Solution B with pH 8 has 10 times less H⁺ ions than neutral water.

• The concentration of H⁺ ions determines whether a solution is acidic, basic, or neutral.
• High H⁺ concentration → Acidic solution (pH < 7).
• Low H⁺ concentration → Basic solution (pH > 7).
• Equal to OH⁻ concentration → Neutral solution (pH = 7).
• The strength of acidity/basicity depends on the extent of H⁺ concentration.

Yes, basic solutions also have H⁺ ions.

• In any aqueous solution, the ionic product of water is constant:
[H⁺][OH⁻] = 10⁻¹⁴ at 25°C
• In basic solutions, [OH⁻] > [H⁺].
• For example, in NaOH solution (pH = 10):
[H⁺] = 10⁻¹⁰ M, [OH⁻] = 10⁻⁴ M.
• The solution is basic because OH⁻ concentration exceeds H⁺ concentration, not because H⁺ ions are absent.

A farmer would add these substances when the soil is too acidic (low pH).

Reason:
• Excessive use of chemical fertilizers makes soil acidic.
• Acidic soil affects plant growth and nutrient availability.
• Quick lime (CaO), slaked lime (Ca(OH)₂), or chalk (CaCO₃) are bases that neutralize soil acidity:
Ca(OH)₂ + 2H⁺ (from soil) → Ca²⁺ + 2H₂O
• This raises soil pH to optimal level (6–7.5) for healthy plant growth.

Common name: Bleaching powder

Chemical name: Calcium oxychloride
Preparation: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Uses:
1. Bleaching cotton, linen, and wood pulp.
2. Disinfecting drinking water.
3. Oxidizing agent in chemical industries.

Dry slaked lime [Calcium hydroxide, Ca(OH)₂]

Reaction:
Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l)
• The reaction is carried out at about 40°C.
• Bleaching powder is produced on an industrial scale by this method.

Washing soda (Sodium carbonate decahydrate, Na₂CO₃·10H₂O)

How it works:
• Hard water contains calcium and magnesium salts.
• Washing soda reacts with these to form insoluble carbonates:
CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaCl
MgCl₂ + Na₂CO₃ → MgCO₃↓ + 2NaCl
• Precipitates can be removed, softening the water.

On heating, sodium hydrogencarbonate (baking soda) decomposes to sodium carbonate, carbon dioxide, and water.

Reaction:
2NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(g)

Observations:
• CO₂ gas evolves (can be tested with lime-water).
• Water vapor forms.
• White residue of sodium carbonate remains.
• This reaction is used in baking to make bread porous.

Reaction:
CaSO₄·½H₂O(s) + 1½H₂O(l) → CaSO₄·2H₂O(s)

Explanation:
• Plaster of Paris (calcium sulphate hemihydrate) reacts with water to form gypsum (calcium sulphate dihydrate).
• The mixture sets into a hard mass due to this hydration reaction.
• Used for making casts, molds, and in building construction.