How to Use This Page
These are complete exercise solutions for Samacheer Kalvi 9th Science Chapter 14 — Acids, Bases and Salts. Solve each question yourself first, then verify your method and answer below.
Key Concepts (Quick Recap)
- Acids: Substances that release H⁺ ions in aqueous solution. Taste sour, turn blue litmus red, pH < 7.
- Mineral acids: HCl, H₂SO₄, HNO₃
- Organic acids: Acetic acid (CH₃COOH), Citric acid, Tartaric acid
- Bases: Substances that release OH⁻ ions in aqueous solution. Taste bitter, feel soapy, turn red litmus blue, pH > 7.
- Strong bases: NaOH, KOH, Ca(OH)₂
- Weak bases: NH₄OH, Al(OH)₃
- pH scale: 0–14; pH < 7 acidic, pH = 7 neutral, pH > 7 basic.
- Indicators: Litmus, phenolphthalein, methyl orange — change colour to identify acid/base.
- Neutralisation: Acid + Base → Salt + Water
- Types of salts: Normal salts, acidic salts, basic salts
- Important reactions:
- Metal + Acid → Salt + Hydrogen gas
- Metal oxide + Acid → Salt + Water
- Base + Acid → Salt + Water
- Metal carbonate + Acid → Salt + Water + CO₂
I. Choose the Correct Answer
Q1. The pH of a neutral solution is: (a) 0 (b) 7 ✓ (c) 14 (d) 1
Answer(b) ✓ — A neutral solution has pH = 7, indicating equal concentrations of H⁺ and OH⁻ ions.
Q2. Which of the following is a weak acid? (a) HCl (b) H₂SO₄ (c) HNO₃ (d) CH₃COOH ✓
Answer(d) ✓ — Acetic acid (CH₃COOH) is a weak organic acid that partially dissociates in water.
Q3. Blue litmus turns red in: (a) NaOH solution (b) HCl solution ✓ (c) NaCl solution (d) Distilled water
Answer(b) ✓ — HCl is an acid; blue litmus turns red in acidic solutions.
Q4. Which is a strong base? (a) NH₄OH (b) Al(OH)₃ (c) NaOH ✓ (d) Mg(OH)₂
Answer(c) ✓ — NaOH (sodium hydroxide / caustic soda) is a strong base that completely dissociates in water.
Q5. The product of neutralisation is always: (a) Salt and water ✓ (b) Salt and gas (c) Water and gas (d) Salt only
Answer(a) ✓ — Neutralisation: Acid + Base → Salt + Water.
Q6. Phenolphthalein is colourless in: (a) Basic solution (b) Neutral solution (c) Acidic solution ✓ (d) Both acidic and basic
Answer(c) ✓ — Phenolphthalein is colourless in acidic solutions (pH < 8.2) and pink in basic solutions.
Q7. When an acid reacts with a metal carbonate, the gas evolved is: (a) Hydrogen (b) Oxygen (c) Carbon dioxide ✓ (d) Nitrogen
Answer(c) ✓ — Metal carbonate + Acid → Salt + Water + CO₂. Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
Q8. Which of the following is a normal salt? (a) NaHCO₃ (b) NaHSO₄ (c) NaCl ✓ (d) Cu(OH)Cl
Answer(c) ✓ — NaCl is a normal salt (all replaceable H atoms of the acid are replaced by the metal). NaHCO₃ is an acid salt; Cu(OH)Cl is a basic salt.
II. Fill in the Blanks
1. Acids turn blue litmus to red. 2. Bases turn red litmus to blue. 3. The pH of an acidic solution is less than 7. 4. The reaction between an acid and a base to form salt and water is called neutralisation. 5. The gas evolved when acid reacts with metal carbonate is carbon dioxide (CO₂). 6. Lemon juice contains citric acid. 7. The chemical name of baking soda is sodium hydrogen carbonate (NaHCO₃). 8. The pH of pure water is 7.
III. True or False
1. All acids contain hydrogen.
AnswerTrue — Acids release H⁺ ions in aqueous solution; all acids contain at least one hydrogen atom.
2. Bases are bitter in taste and soapy to touch.
AnswerTrue — Bases taste bitter and feel soapy/slippery.
3. pH of gastric juice is around 7.
AnswerFalse — Correction: Gastric juice is acidic; its pH is around 1.5–2.
4. Phenolphthalein turns pink in acidic solutions.
AnswerFalse — Correction: Phenolphthalein turns pink in basic solutions and remains colourless in acidic solutions.
5. Neutralisation reaction produces salt and water.
AnswerTrue — Acid + Base → Salt + Water.
6. Sodium hydroxide is a weak base.
AnswerFalse — Correction: NaOH is a strong base; it completely dissociates in water.
7. Acids conduct electricity in aqueous solution.
AnswerTrue — Acids release H⁺ ions which carry electric current.
8. Quicklime (CaO) is an acidic oxide.
AnswerFalse — Correction: CaO is a basic oxide; it reacts with water to form Ca(OH)₂, a base.
IV. Match the Following
| Column I | Column II |
|---|
| 1. Hydrochloric acid | (c) HCl |
| 2. Sodium hydroxide | (d) NaOH |
| 3. Acetic acid | (a) CH₃COOH |
| 4. Calcium carbonate | (e) CaCO₃ |
| 5. Sulphuric acid | (b) H₂SO₄ |
V. Very Short Answers
Q1. Define acid.
Answer
An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water. Acids taste sour, turn blue litmus red, and have pH less than 7. Examples: HCl, H₂SO₄, HNO₃.
Q2. Define base.
Answer
A base is a substance that releases hydroxide ions (OH⁻) when dissolved in water. Bases taste bitter, feel soapy, turn red litmus blue, and have pH greater than 7. Examples: NaOH, KOH, Ca(OH)₂.
Q3. What is pH scale?
Answer
The pH scale is a 0–14 scale used to measure the acidity or alkalinity of a solution. pH < 7 indicates acidic, pH = 7 indicates neutral, and pH > 7 indicates basic solution. pH = −log[H⁺].
Q4. What are indicators? Name any three.
Answer
Indicators are substances that show different colours in acidic and basic solutions, used to identify the nature of a solution. 1. Litmus — red in acid, blue in base 2. Phenolphthalein — colourless in acid, pink in base 3. Methyl orange — red in acid, yellow in base
Q5. What is neutralisation? Give one example.
Answer
Neutralisation is the reaction between an acid and a base to form salt and water. Example: HCl + NaOH → NaCl + H₂O
Q6. What is the chemical name and formula of baking soda?
Answer
Chemical name: Sodium hydrogen carbonate (also called sodium bicarbonate) Formula: NaHCO₃
Q7. Why do acids not show acidic behaviour in the absence of water?
Answer
Acids produce H⁺ ions only in aqueous solution. In the absence of water, H⁺ ions are not released, so acids do not exhibit acidic properties (no sour taste, no litmus change, no reaction with metals).
VI. Short Answers
Q1. Differentiate between acids and bases.
Answer
| Property | Acids | Bases |
|---|
| Definition | Release H⁺ ions in water | Release OH⁻ ions in water |
| Taste | Sour | Bitter |
| Touch | Non-soapy | Soapy/slippery |
| Litmus test | Blue → Red | Red → Blue |
| pH | Less than 7 | Greater than 7 |
| Examples | HCl, H₂SO₄, HNO₃ | NaOH, KOH, Ca(OH)₂ |
| Reaction with metals | Produce H₂ gas | No H₂ gas |
| Reaction with carbonates | Produce CO₂ gas | No CO₂ gas |
Q2. What are the types of salts? Give examples.
Answer
1. Normal salts: Formed by complete neutralisation. All H atoms of the acid are replaced by metal. Example: NaCl, K₂SO₄. 2. Acidic salts: Formed by partial neutralisation of a dibasic/tribasic acid. Still contain replaceable H. Example: NaHSO₄, NaHCO₃. 3. Basic salts: Formed by partial neutralisation of a diacidic/triacidic base. Still contain replaceable OH. Example: Cu(OH)Cl, Pb(OH)NO₃.
Q3. Explain the importance of pH in everyday life.
Answer
1. Digestion: Stomach has pH ~1.5–2 (HCl) for digestion; antacids (basic) neutralise excess acid. 2. Soil: Plants grow best at specific pH; soil too acidic is treated with quicklime (CaO). 3. Tooth decay: Bacteria produce acids (pH < 5.5) that dissolve tooth enamel; toothpastes are basic to neutralise acid. 4. Self-defence: Bee stings are acidic (methanoic acid); rubbing baking soda (basic) gives relief. 5. Rainwater: Normal rain pH ~5.6; acid rain pH < 5.6 damages buildings and crops.
Q4. Write the reactions of acids with: (a) metals (b) metal carbonates (c) metal oxides
Answer
(a) Metal + Acid → Salt + Hydrogen gas Zn + H₂SO₄ → ZnSO₄ + H₂↑ (Hydrogen gas burns with a pop sound.)
(b) Metal carbonate + Acid → Salt + Water + Carbon dioxide CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ (CO₂ turns lime water milky.)
(c) Metal oxide + Acid → Salt + Water CuO + 2HCl → CuCl₂ + H₂O (Metal oxides are basic in nature.)
VII. Detailed Answers
Q1. Describe the preparation and properties of acids.
Answer
Preparation of acids: 1. Dissolving non-metal oxides in water: SO₃ + H₂O → H₂SO₄ (Sulphuric acid) CO₂ + H₂O → H₂CO₃ (Carbonic acid) 2. Reaction of metal with acid (laboratory method): 2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl↑ 3. Oxidation of non-metals: S + O₂ → SO₂; then SO₃ + H₂O → H₂SO₄
Properties of acids: 1. Taste sour. 2. Turn blue litmus red. 3. Conduct electricity in aqueous solution (release H⁺ ions). 4. React with metals to produce hydrogen gas: Zn + 2HCl → ZnCl₂ + H₂↑ 5. React with metal carbonates to produce CO₂: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑ 6. React with metal oxides to produce salt and water: CuO + H₂SO₄ → CuSO₄ + H₂O 7. React with bases (neutralisation): HCl + NaOH → NaCl + H₂O
Q2. Describe the preparation and properties of bases.
Answer
Preparation of bases: 1. Dissolving metal oxides in water: Na₂O + H₂O → 2NaOH (Sodium hydroxide) CaO + H₂O → Ca(OH)₂ (Calcium hydroxide) 2. Reaction of metal with water: 2Na + 2H₂O → 2NaOH + H₂↑ 3. Dissolving ammonia in water: NH₃ + H₂O → NH₄OH (Ammonium hydroxide)
Properties of bases: 1. Taste bitter and feel soapy/slippery. 2. Turn red litmus blue. 3. Conduct electricity in aqueous solution (release OH⁻ ions). 4. React with acids to form salt and water (neutralisation). 5. Bases react with non-metal oxides: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O 6. Strong bases are corrosive and can burn skin.
Q3. Explain the pH scale and its applications.
Answer
The pH scale measures the hydrogen ion concentration in a solution:
- pH = −log[H⁺]
- pH ranges from 0 to 14
- pH < 7: Acidic (lower pH = stronger acid)
- pH = 7: Neutral (pure water)
- pH > 7: Basic/Alkaline (higher pH = stronger base)
Applications: 1. Agriculture: Soil pH affects plant growth. Most plants prefer pH 6–7.5. Acidic soil is treated with lime (CaO). 2. Medicine: Antacids (basic substances) neutralise excess stomach acid (pH ~1.5–2). 3. Industry: Monitoring pH is crucial in chemical manufacturing, food processing, and water treatment. 4. Biology: Blood pH must be maintained at 7.35–7.45; slight deviations can be fatal. 5. Daily life: Shampoos are slightly acidic (pH ~5.5) to match scalp pH; cleaning products are basic.
Common Mistakes
1. Confusing acid and base litmus tests — Acids turn blue litmus red; bases turn red litmus blue. Remember: "Blue to Red = Acid." 2. Saying all H-containing compounds are acids — Water (H₂O) and glucose (C₆H₁₂O₆) contain hydrogen but are not acids; acids must release H⁺ in water. 3. Forgetting CO₂ test — CO₂ turns lime water milky (not hydrogen). The pop sound test is for H₂ gas. 4. Wrong pH interpretation — Lower pH = stronger acid (more H⁺), higher pH = stronger base (more OH⁻). Don't reverse this. 5. Confusing strong/weak with concentrated/dilute — Strong/weak refers to degree of dissociation; concentrated/dilute refers to amount of acid per unit volume. 6. Incomplete neutralisation equations — Always write the balanced equation: Acid + Base → Salt + Water, with correct state symbols. 7. Confusing acid salts and normal salts — NaHCO₃ is an acid salt (one H remains); NaCl is a normal salt (all H replaced by metal).
Exam Checklist
- [ ] Definitions of acid, base, salt, pH, indicator, neutralisation memorized
- [ ] pH scale values and their significance (0–14, acidic/neutral/basic)
- [ ] Types of indicators and their colour changes (litmus, phenolphthalein, methyl orange)
- [ ] Chemical equations: metal + acid, metal carbonate + acid, metal oxide + acid, neutralisation
- [ ] Types of salts: normal, acidic, basic — with examples
- [ ] Everyday applications of pH (digestion, soil, tooth decay, self-defence)
- [ ] Difference tables: acids vs bases, strong vs weak, concentrated vs dilute
- [ ] Common chemical names and formulas (baking soda, washing soda, quicklime, slaked lime)
Internal Links
- [Chapter 14 Guide — Acids, Bases and Salts](/school/tamil-nadu-state-board/class/9/science/acids-bases-and-salts)
- [All Class 9 Science Solutions](/school/tamil-nadu-state-board/class/9/science/solutions)
- [Class 9 Science Subject Guide](/school/tamil-nadu-state-board/class/9/science)
