Acids, Bases, and Salts
1. Introduction & Definitions
Acid: A substance which in aqueous solution tastes sour, turns blue litmus red, and releases H⁺ ions.
Base: A substance which in aqueous solution tastes bitter, feels soapy, turns red litmus blue, and releases OH⁻ ions.
Alkali: A base that is soluble in water (e.g., NaOH, KOH).
Salt: An ionic compound formed by the neutralization of an acid and a base.
2. Properties of Acids & Bases
Properties of Acids
- Sour taste
- Blue litmus → Red
- Conduct electricity (aqueous)
- React with metals → H₂ gas
- React with carbonates → CO₂ gas
Properties of Bases
- Bitter taste and soapy touch
- Red litmus → Blue
- Conduct electricity (aqueous)
- React with non-metallic oxides → Salt + Water
- React with acids → Neutralization
3. Chemical Reactions
A. Acid + Metal
Acid + Metal → Salt + Hydrogen gas
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) ↑
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) ↑
B. Acid + Metal Carbonates/Bicarbonates
2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂ ↑
HCl + NaHCO₃ → NaCl + H₂O + CO₂ ↑
C. Neutralization Reaction
Acid + Base → Salt + Water (Exothermic)
HCl + NaOH → NaCl + H₂O
HCl + NaOH → NaCl + H₂O
4. pH Scale
The pH scale measures the acidity or basicity of a solution from 0 to 14.
0 (Acidic) <---------- 7 (Neutral) ----------> 14 (Basic)
| Substance | pH Value | Nature |
|---|---|---|
| Gastric juice | ~1.2 | Highly acidic |
| Lemon juice | ~2.2 | Acidic |
| Pure water | 7.0 | Neutral |
| Blood | 7.4 | Slightly basic |
| Milk of Magnesia | 10.5 | Basic |
Importance of pH:
- Tooth Decay: Starts if mouth pH is below 5.5.
- Stomach: Antacids (bases) neutralize excess HCl.
- Plants: Require specific soil pH for healthy growth.
5. Important Salts
| Common Name | Chemical Formula | Preparation | Key Uses |
|---|---|---|---|
| Caustic Soda | NaOH | Chlor-alkali process | Soaps, paper, degreasing metals |
| Bleaching Powder | CaOCl₂ | Cl₂ + Dry slaked lime | Disinfectant, bleaching textiles |
| Baking Soda | NaHCO₃ | Solvay process | Baking, antacid, fire extinguishers |
| Washing Soda | Na₂CO₃·10H₂O | Recrystallization of Na₂CO₃ | Removing permanent hardness of water |
| Plaster of Paris | CaSO₄·½H₂O | Heating Gypsum at 373 K | Fractured bones, toys, decoration |
Water of Crystallization
The fixed number of water molecules present in one formula unit of a salt. For example, Blue Vitriol (CuSO₄·5H₂O) contains 5 molecules of water of crystallization.
Exam Tip: Be careful with Plaster of Paris preparation. If gypsum is heated above 373 K, it forms "Dead Burnt Plaster" (CaSO₄), which loses its setting property.
MCQs: Acids, Bases, and Salts
1. Which of the following is NOT a characteristic of acids?
- A) Turn blue litmus red
- B) Have sour taste
- C) Turn red litmus blue
- D) Produce H⁺ ions in aqueous solution
Answer: C
Explanation: Turning red litmus blue is a property of bases, not acids.
Explanation: Turning red litmus blue is a property of bases, not acids.
2. The reaction between an acid and a base to form salt and water is called:
- A) Esterification
- B) Neutralization
- C) Saponification
- D) Hydrogenation
Answer: B
Explanation: Neutralization is the reaction between acid and base to produce salt + water.
Explanation: Neutralization is the reaction between acid and base to produce salt + water.
3. Which gas is evolved when dilute HCl reacts with zinc metal?
- A) Oxygen
- B) Carbon dioxide
- C) Hydrogen
- D) Chlorine
Answer: C
Explanation: Zn + 2HCl → ZnCl₂ + H₂↑ (Hydrogen gas is liberated).
Explanation: Zn + 2HCl → ZnCl₂ + H₂↑ (Hydrogen gas is liberated).
4. The pH of pure water at room temperature is:
- A) 0
- B) 7
- C) 14
- D) 1
Answer: B
Explanation: Pure water is neutral, so pH = 7.
Explanation: Pure water is neutral, so pH = 7.
5. Which of the following has the highest pH value?
- A) Lemon juice
- B) Gastric juice
- C) Milk of magnesia
- D) Vinegar
Answer: C
Explanation: Milk of magnesia (Mg(OH)₂) is a mild base, pH ≈ 10 (highest among the options).
Explanation: Milk of magnesia (Mg(OH)₂) is a mild base, pH ≈ 10 (highest among the options).
6. Tooth decay starts when the pH of the mouth is:
- A) Above 7
- B) Below 5.5
- C) Equal to 7
- D) Above 8
Answer: B
Explanation: Enamel starts corroding when pH falls below 5.5 due to acid produced by bacteria.
Explanation: Enamel starts corroding when pH falls below 5.5 due to acid produced by bacteria.
7. Which salt does NOT contain water of crystallization?
- A) Blue vitriol (CuSO₄·5H₂O)
- B) Washing soda (Na₂CO₃·10H₂O)
- C) Baking soda (NaHCO₃)
- D) Gypsum (CaSO₄·2H₂O)
Answer: C
Explanation: Baking soda (NaHCO₃) does not have water of crystallization in its formula.
Explanation: Baking soda (NaHCO₃) does not have water of crystallization in its formula.
8. Plaster of Paris is prepared by heating gypsum at:
- A) 100°C
- B) 373 K
- C) 273 K
- D) 473 K
Answer: B
Explanation: Gypsum is heated at 373 K (100°C) to form Plaster of Paris (CaSO₄·½H₂O).
Explanation: Gypsum is heated at 373 K (100°C) to form Plaster of Paris (CaSO₄·½H₂O).
9. The chemical formula of bleaching powder is:
- A) Ca(OH)₂
- B) CaOCl₂
- C) NaOH
- D) Na₂CO₃
Answer: B
Explanation: Bleaching powder is calcium oxychloride, CaOCl₂.
Explanation: Bleaching powder is calcium oxychloride, CaOCl₂.
10. Which of the following is used as an antacid to relieve acidity?
- A) NaCl
- B) NaHCO₃
- C) CaOCl₂
- D) CaSO₄·½H₂O
Answer: B
Explanation: Baking soda (NaHCO₃) is a mild base used to neutralize excess acid in the stomach.
Explanation: Baking soda (NaHCO₃) is a mild base used to neutralize excess acid in the stomach.
11. When CO₂ is passed through lime water:
- A) It remains clear
- B) It turns milky due to CaCO₃ formation
- C) It becomes blue
- D) It produces H₂ gas
Answer: B
Explanation: Ca(OH)₂ + CO₂ → CaCO₃↓ (white precipitate) + H₂O → turns milky.
Explanation: Ca(OH)₂ + CO₂ → CaCO₃↓ (white precipitate) + H₂O → turns milky.
12. The chemical name of caustic soda is:
- A) Sodium carbonate
- B) Sodium hydroxide
- C) Sodium bicarbonate
- D) Calcium hydroxide
Answer: B
Explanation: Caustic soda is NaOH (sodium hydroxide).
Explanation: Caustic soda is NaOH (sodium hydroxide).
13. Which indicator turns pink in basic solution?
- A) Litmus
- B) Methyl orange
- C) Phenolphthalein
- D) Turmeric
Answer: C
Explanation: Phenolphthalein is colourless in acid/neutral and turns pink in basic medium.
Explanation: Phenolphthalein is colourless in acid/neutral and turns pink in basic medium.
14. Which of the following is a weak acid?
- A) HCl
- B) H₂SO₄
- C) CH₃COOH
- D) HNO₃
Answer: C
Explanation: Acetic acid (CH₃COOH) is a weak organic acid; others are strong mineral acids.
Explanation: Acetic acid (CH₃COOH) is a weak organic acid; others are strong mineral acids.
15. The reaction: NaOH + HCl → NaCl + H₂O is an example of:
- A) Combination reaction
- B) Neutralization reaction
- C) Displacement reaction
- D) Double displacement reaction
Answer: B
Explanation: It is a classic acid-base neutralization reaction.
Explanation: It is a classic acid-base neutralization reaction.
16. Which salt is used in the manufacture of glass?
- A) NaCl
- B) Na₂CO₃
- C) NaHCO₃
- D) CaSO₄
Answer: B
Explanation: Washing soda (Na₂CO₃) is used in glass manufacturing.
Explanation: Washing soda (Na₂CO₃) is used in glass manufacturing.
17. Baking soda on heating gives:
- A) Na₂CO₃ + CO₂ + H₂O
- B) NaOH + CO₂
- C) NaCl + CO₂
- D) CaO + CO₂
Answer: A
Explanation: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O (on heating).
Explanation: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O (on heating).
18. The pH of blood is approximately:
- A) 5.5
- B) 7.4
- C) 9.0
- D) 1.2
Answer: B
Explanation: Blood is slightly alkaline with pH around 7.4.
Explanation: Blood is slightly alkaline with pH around 7.4.
19. Which of the following is an alkali?
- A) Cu(OH)₂
- B) Fe(OH)₃
- C) NaOH
- D) Al(OH)₃
Answer: C
Explanation: Alkali is a water-soluble base. NaOH is soluble, others are insoluble.
Explanation: Alkali is a water-soluble base. NaOH is soluble, others are insoluble.
20. The chemical formula of washing soda is:
- A) NaHCO₃
- B) Na₂CO₃·10H₂O
- C) Na₂CO₃
- D) CaOCl₂
Answer: B
Explanation: Washing soda is hydrated sodium carbonate, Na₂CO₃·10H₂O.
Explanation: Washing soda is hydrated sodium carbonate, Na₂CO₃·10H₂O.
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