MP Board Class 10 Science Chapter 2: Acids, Bases &#…
MP Board Class 10 Science Chapter 2: Acids, Bases and Salts Important Questions — This chapter carries 5–7 marks in the MP Board Class 10 Science exam. Questions commonly cover the classification of substances as acidic/basic using indicators, the pH scale and its importance in everyday life, reactions of acids and bases with metals and carbonates, neutralisation reactions, and the properties and preparation of common salts like NaCl, baking soda, washing soda, and plaster of Paris. Master these important questions to confidently score full marks in the board exam.
📑 Table of Contents
🧪 1. Properties of Acids and Bases
Q1. How would you distinguish between an acid and a base using an indicator? Explain with examples.
Answer: Indicators are substances that change colour in acidic and basic media. Common indicators and their colour changes:
Litmus paper: Blue litmus turns red in acid; red litmus turns blue in base.
Phenolphthalein: Colourless in acid, turns pink in base.
Methyl orange: Turns red in acid, yellow in base.
Examples: Hydrochloric acid (HCl) turns blue litmus red. Sodium hydroxide (NaOH) turns red litmus blue. Acetic acid (CH₃COOH) turns blue litmus red but is a weak acid — the colour change is slower.
Q2. What are olfactory indicators? Give two examples.
Answer: Olfactory indicators are substances whose smell changes in acidic or basic media. Instead of a colour change, they give a change in odour. Examples:
1. Onion: The characteristic pungent smell of onion disappears when treated with a base (like NaOH) but remains in an acid.
2. Vanilla extract: The sweet smell of vanilla disappears when treated with a base but remains in an acid.
These are used to test acids and bases when colour-based indicators cannot be used (e.g., for visually impaired students).
📊 2. pH Scale and Its Importance
Q3. What is the pH scale? Why is it important in our daily life?
Answer: The pH scale is a measure of the concentration of hydrogen ions [H⁺] in a solution. It ranges from 0 to 14:
• pH 0–6 = Acidic (low pH means high H⁺ concentration)
• pH 7 = Neutral (pure water)
• pH 8–14 = Basic (high pH means low H⁺ concentration)
Importance in daily life:
1. Tooth decay: Bacteria in the mouth produce acids (pH below 5.5) that demineralise tooth enamel. Using alkaline toothpaste neutralises the acid.
2. Soil pH: Plants grow best at specific pH — most crops prefer pH 6.5–7.5. Acidic soil is treated with lime (CaO) and basic soil with compost.
3. Digestive system: Stomach produces HCl (pH ~1.5–3.5) for digestion. Antacids (Mg(OH)₂ or Al(OH)₃) neutralise excess acid.
4. Bee stings: Bee sting injects formic acid — applying baking soda (NaHCO₃, a base) neutralises it.
Q4. The pH of three solutions A, B, and C is 2, 7, and 10 respectively. Identify which is acidic, neutral, and basic. Arrange them in increasing order of H⁺ ion concentration.
Answer:
• Solution A (pH 2) — Acidic (highest H⁺ concentration)
• Solution B (pH 7) — Neutral
• Solution C (pH 10) — Basic (lowest H⁺ concentration)
Increasing order of H⁺ concentration: C (pH 10) → B (pH 7) → A (pH 2)
Since pH is inversely related to H⁺ concentration, a lower pH means more H⁺ ions.
⚗️ 3. Reactions of Acids and Bases
Q5. Write the chemical equation for the reaction of hydrochloric acid with sodium hydroxide. What type of reaction is this? Name the products formed.
Answer:
Chemical equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Type of reaction: Neutralisation reaction — an acid reacts with a base to form salt and water.
Products:
• Sodium chloride (NaCl) — common table salt
• Water (H₂O)
This reaction is exothermic (releases heat). The mixture feels warm to the touch.
Q6. How do acids react with metals? Give the reaction of dilute hydrochloric acid with zinc metal.
Answer: Acids react with metals to produce salt and hydrogen gas. The hydrogen gas can be tested by bringing a burning matchstick near it — the gas burns with a pop sound.
Reaction of dilute HCl with zinc:
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) ↑
Observation: Bubbles of hydrogen gas are produced. The zinc metal dissolves gradually. The solution becomes colourless (zinc chloride solution).
Other examples:
• 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) — Sodium reacts violently with water (not stored in water)
• Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g) — Magnesium ribbon dissolves in dilute sulphuric acid
Q7. What happens when carbon dioxide gas is passed through lime water? Write the reaction. What happens when excess CO₂ is passed?
Answer:
Step 1 — Limited CO₂: Lime water (calcium hydroxide solution) turns milky due to the formation of insoluble calcium carbonate.
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) ↓ + H₂O(l)
(Lime water) (Carbon dioxide) (White precipitate — chalk)
Step 2 — Excess CO₂: The milkiness disappears because calcium carbonate reacts with excess CO₂ and water to form soluble calcium hydrogen carbonate.
CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)
(Insoluble) (Soluble — milkiness disappears)
This reaction is used to test for the presence of carbon dioxide gas in laboratories.
🧂 4. Importance of Salts in Daily Life
Q8. Write the chemical name, formula, and preparation of baking soda. List three important uses.
Answer:
Chemical name: Sodium hydrogen carbonate (also called sodium bicarbonate)
Formula: NaHCO₃
Preparation: By Solvay process — reaction of sodium chloride with ammonia, carbon dioxide, and water:
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
Three important uses:
1. Baking: Used as a leavening agent in cakes and bread. When heated, it decomposes to release CO₂ gas, making the dough rise.
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ ↑
2. Antacid: Neutralises excess stomach acid. NaHCO₃ + HCl → NaCl + H₂O + CO₂
3. Fire extinguisher: Used in soda-acid fire extinguishers — reacts with sulphuric acid to produce CO₂, which smothers the fire.
Q9. What is washing soda? How is it obtained from baking soda? Give two uses.
Answer:
Washing soda: Sodium carbonate decahydrate (Na₂CO₃·10H₂O)
Preparation from baking soda:
Baking soda (NaHCO₃) is heated to produce sodium carbonate (soda ash):
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ ↑
The anhydrous Na₂CO₃ is then recrystallised from water to form Na₂CO₃·10H₂O (washing soda).
Two uses:
1. Cleaning agent: Used in laundry detergents and for cleaning kitchen surfaces.
2. Water softening: Removes permanent hardness of water by precipitating Ca²⁺ and Mg²⁺ ions.
💧 5. Water of Crystallisation
Q10. What is meant by water of crystallisation? Explain with the example of copper sulphate crystals.
Answer: Water of crystallisation refers to the fixed number of water molecules that are chemically bonded to each formula unit of a salt in its crystalline form. These water molecules are responsible for the crystal shape and colour of the salt.
Example — Copper sulphate pentahydrate (CuSO₄·5H₂O):
• Blue colour of crystals is due to 5 water molecules of crystallisation
• Formula: CuSO₄·5H₂O
• On heating: The blue crystals lose water and become white (anhydrous)
CuSO₄·5H₂O → CuSO₄ + 5H₂O
(Blue) (White)
• Adding a few drops of water to anhydrous CuSO₄ restores the blue colour — this is a test for the presence of water.
Q11. How is Plaster of Paris prepared? Write its chemical equation. What precaution should be taken while storing it?
Answer:
Preparation: Plaster of Paris is prepared by heating gypsum (calcium sulphate dihydrate) to 373 K (100°C):
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
(Gypsum) (Plaster of Paris)
Key reaction with water: When mixed with water, Plaster of Paris sets into a hard mass — gypsum is re-formed:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
Precaution: Plaster of Paris should be stored in a moisture-proof container. If it absorbs moisture from the air, it will set into hard gypsum and become useless.
📝 6. Multiple Choice Questions (1 Mark Each)
MCQs from Acids, Bases and Salts appear frequently in MP Board exams. Here are 10 predicted questions based on past papers.
- Which of the following is a strong acid?
(a) Acetic acid ✓ (b) Citric acid (c) Hydrochloric acid ✓ (d) Carbonic acid - Phenolphthalein turns _____ in a basic solution.
(a) Red (b) Blue (c) Pink ✓ (d) Colourless - The pH of pure water is:
(a) 0 (b) 7 ✓ (c) 14 (d) 1 - Which of the following is used to treat acidity in the stomach?
(a) Lemon juice (b) Vinegar (c) Antacid (Mg(OH)₂) ✓ (d) Common salt - The gas produced when an acid reacts with a metal is:
(a) Oxygen (b) Carbon dioxide (c) Hydrogen ✓ (d) Nitrogen - Baking soda is chemically known as:
(a) Sodium carbonate (b) Sodium hydrogen carbonate ✓ (c) Sodium chloride (d) Sodium hydroxide - Which of the following is NOT a natural indicator?
(a) Turmeric (b) Litmus (c) Phenolphthalein ✓ (d) Red cabbage - Plaster of Paris is obtained by heating gypsum at:
(a) 373 K ✓ (b) 473 K (c) 1000 K (d) 298 K - The colour of copper sulphate crystals is:
(a) White (b) Green (c) Blue ✓ (d) Yellow - Lime water turns milky due to the formation of:
(a) Calcium oxide (b) Calcium carbonate ✓ (c) Calcium hydroxide (d) Calcium bicarbonate
✏️ 7. Very Short & Short Answer Questions (2–4 Marks)
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Q1. Why does tooth enamel start decaying when the pH in the mouth falls below 5.5?
Tooth enamel is made of calcium phosphate (Ca₃(PO₄)₂), which is the hardest substance in the body. However, it begins to corrode when the pH in the mouth falls below 5.5. Bacteria present in the mouth break down food particles, especially sugar, producing acids (lactic acid). This acid reacts with the enamel, demineralising it. Brushing with toothpaste (which is basic) neutralises the acid and prevents decay. (4 marks) -
Q2. A farmer found that the pH of his soil is 4.5. What should he add to neutralise the soil? Why?
The farmer should add quicklime (calcium oxide, CaO) or slaked lime (calcium hydroxide, Ca(OH)₂) to the soil. Since the soil is acidic (pH 4.5), adding a base neutralises the excess acid. When Ca(OH)₂ is added, it reacts with the acidic components in the soil: Acid + Ca(OH)₂ → Salt + Water. This raises the pH to the neutral range (6.5–7.5) suitable for healthy crop growth. (3 marks) -
Q3. What is the difference between a strong acid and a weak acid? Give one example of each.
A strong acid completely dissociates (ionises) in water, producing a high concentration of H⁺ ions. A weak acid dissociates only partially, producing fewer H⁺ ions. For example, hydrochloric acid (HCl) is a strong acid — it completely dissociates into H⁺ and Cl⁻ ions in water. Acetic acid (CH₃COOH) is a weak acid — it partially dissociates into CH₃COO⁻ and H⁺ ions, maintaining an equilibrium. This is why pH of 0.1 M HCl is ~1, while 0.1 M CH₃COOH is ~3. (3 marks) -
Q4. What is a neutralisation reaction? Write any two applications in daily life.
A neutralisation reaction is a chemical reaction in which an acid and a base react to form a salt and water. The H⁺ ions from the acid combine with OH⁻ ions from the base to form water. Two daily life applications: (1) Antacids — Mg(OH)₂ neutralises excess stomach acid (HCl) to relieve indigestion. (2) Factory waste treatment — acidic waste from factories is neutralised by adding calcium hydroxide before releasing into water bodies. (3 marks) -
Q5. What happens when zinc granules are added to dilute sulphuric acid? Write the balanced chemical equation.
Zinc reacts with dilute sulphuric acid to produce zinc sulphate and hydrogen gas. Bubbles of hydrogen gas are seen rising from the surface of zinc granules. The zinc metal gradually disappears as it dissolves. The solution remains colourless. Balanced equation: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) ↑. The hydrogen gas can be tested by the pop sound test — a burning matchstick brought near the gas produces a pop sound. (3 marks) -
Q6. How does the water of crystallisation affect the properties of a salt? Explain with the example of gypsum.
Water of crystallisation gives salts their characteristic shape, colour, and hardness. Gypsum (CaSO₄·2H₂O) is a hard crystalline solid containing 2 molecules of water of crystallisation per formula unit. When heated to 373 K, gypsum loses 1½ water molecules to form Plaster of Paris (CaSO₄·½H₂O), which is a soft powder. When mixed with water, PoP rehydrates to form gypsum again — this setting property is used for plaster casts. The water of crystallisation is thus critical for the practical utility of the salt. (4 marks) -
Q7. Differentiate between baking soda and baking powder.
Baking soda is pure sodium hydrogen carbonate (NaHCO₃). When used alone in baking, it requires an acidic ingredient (like curd or lemon juice) to produce CO₂ gas. Baking powder is a mixture of baking soda (NaHCO₃) and a dry acid (like tartaric acid or cream of tartar). Baking powder contains both components needed for CO₂ production — it only needs moisture and heat. Baking powder is therefore more convenient for home baking, while baking soda is more commonly used in Indian cooking. (3 marks) -
Q8. Why does blue vitriol (copper sulphate) turn white on heating? Write the chemical equation.
Blue vitriol is copper sulphate pentahydrate (CuSO₄·5H₂O). On heating, it loses its 5 water molecules of crystallisation and becomes anhydrous copper sulphate (CuSO₄), which is white in colour. The blue colour comes from the water of crystallisation. Chemical equation: CuSO₄·5H₂O → CuSO₄ + 5H₂O ↑ (upon heating). This is a reversible change — adding a drop of water to the white anhydrous powder restores the blue colour (exothermic reaction). (2 marks)
📖 8. Long Answer Questions (5–6 Marks)
Q1. Explain the process of Chlor-alkali process. Name the products obtained. Write the uses of each product.
Answer:
The Chlor-alkali process is the industrial electrolysis of brine (concentrated sodium chloride solution) to produce three commercially important chemicals. When an electric current is passed through brine, the following reaction occurs:
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)
The three products obtained are:
1. Sodium hydroxide (NaOH) — Caustic soda: Used in the manufacture of soaps, detergents, paper, textiles, and drain cleaners. It is a strong base and highly corrosive.
2. Chlorine gas (Cl₂): Used for water purification (disinfectant), manufacture of bleaching powder (CaOCl₂), hydrochloric acid, PVC plastics, and pesticides. It is a greenish-yellow gas with a pungent smell.
3. Hydrogen gas (H₂): Used as a fuel (hydrogen fuel cells), in the manufacture of ammonia (Haber process), and as a reducing agent in metallurgy.
Key point: The three products are obtained in a fixed ratio — 1 tonne of NaOH yields 0.88 tonnes of Cl₂ and 0.025 tonnes of H₂.
Q2. Describe an activity to show that acids produce hydrogen ions in aqueous solutions. How would you prove that only H⁺ ions are responsible for acidic properties?
Answer:
Activity to show H⁺ production:
Take two beakers — one with hydrochloric acid (HCl) and one with distilled water. Connect two carbon rods to a battery and a bulb in series. Dip the electrodes in distilled water — the bulb does NOT glow (water does not conduct electricity). Now dip the same electrodes in dilute HCl — the bulb GLOWS brightly, showing that HCl produces H⁺ and Cl⁻ ions in aqueous solution, making it electrically conductive.
Proof that H⁺ ions cause acidic properties:
Take three test tubes:
• Test tube A: Dry HCl gas + dry blue litmus paper → No colour change
• Test tube B: HCl solution in water + blue litmus → Turns red
• Test tube C: HCl dissolved in organic solvent (like benzene) + blue litmus → No colour change
This proves that HCl produces H⁺ ions ONLY in aqueous solutions, and these H⁺ ions are responsible for acidic properties. In the absence of water, HCl gas does not behave as an acid. Similarly, all acids produce H⁺ ions in water: HNO₃ → H⁺ + NO₃⁻, H₂SO₄ → 2H⁺ + SO₄²⁻.
Conclusion: Acidic character of a substance is due to the presence of H⁺ ions in aqueous solution. The more H⁺ ions produced, the stronger the acid.
Q3. Explain the preparation, properties, and uses of bleaching powder (CaOCl₂).
Answer:
Preparation: Bleaching powder (calcium oxychloride) is prepared by passing chlorine gas over dry slaked lime (calcium hydroxide):
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
(Slaked lime) (Chlorine) (Bleaching powder)
Physical properties:
• Yellowish-white powder
• Strong smell of chlorine
• Soluble in water
• Partially soluble (forms a suspension)
Chemical properties:
• On exposure to air, it reacts with CO₂ to produce chlorine gas: CaOCl₂ + CO₂ → CaCO₃ + Cl₂
• It acts as an oxidising agent
• It liberates chlorine when treated with dilute acids: CaOCl₂ + H₂SO₄ → CaSO₄ + H₂O + Cl₂
Uses:
1. Disinfectant: Used for water purification at municipal water treatment plants and swimming pools.
2. Bleaching agent: Used for bleaching cotton, linen, and wood pulp in the paper industry.
3. Oxidising agent: In many industrial chemical reactions.
4. Germicide: Used in hospitals for sterilising floors and equipment.
Storage note: Bleaching powder deteriorates on exposure to air and moisture. It should be stored in airtight containers in a cool, dry place.
📋 Previous Year Questions (2018–2026)
❓ Frequently Asked Questions
Q1. What is the difference between an acid and a base?
Acids produce H⁺ ions in water, have a sour taste, turn blue litmus red, and have pH less than 7. Bases produce OH⁻ ions in water, have a bitter taste, feel soapy, turn red litmus blue, and have pH greater than 7.
Q2. What is the universal indicator?
Universal indicator is a mixture of several indicators that shows a range of colours across different pH values. Each pH value corresponds to a specific colour, allowing precise measurement of pH instead of just acid/base identification.
Q3. Why is hydrochloric acid stored in glass bottles and not in metal containers?
Hydrochloric acid reacts with most metals to produce hydrogen gas and metal chloride. This would corrode the metal container and cause pressure build-up due to hydrogen gas. Glass is non-reactive with HCl and is therefore used for storage.
Q4. What happens when sodium carbonate crystals are exposed to the atmosphere?
Sodium carbonate decahydrate (Na₂CO₃·10H₂O) undergoes efflorescence — it loses its water of crystallisation to the atmosphere and turns into white anhydrous powder (Na₂CO₃). The crystals lose their shape and transparency.
Q5. What is the role of tartaric acid in baking powder?
Tartaric acid in baking powder reacts with sodium hydrogen carbonate (baking soda) when water is added, producing CO₂ gas that makes the dough rise. It also neutralises the bitter taste of sodium carbonate that would otherwise remain in the baked food.
Q6. Why does distilled water not conduct electricity, but tap water does?
Distilled water is pure H₂O and contains no dissolved ions, so it cannot conduct electricity. Tap water contains dissolved minerals like Ca²⁺, Mg²⁺, Na⁺, Cl⁻, and HCO₃⁻ ions that make it electrically conductive.
Q7. How can you test for the presence of hydrogen gas evolved in a reaction?
Bring a burning matchstick or candle near the test tube mouth. Hydrogen gas burns with a characteristic pop sound when ignited. This is called the ‘pop test’ and confirms the presence of hydrogen gas.
Q8. What is the effect of adding water to a concentrated acid?
Adding water to a concentrated acid is a highly exothermic reaction that releases a large amount of heat, which can cause the acid to splash violently. Always add acid to water slowly with constant stirring, never water to acid. This dilutes the acid safely.
Q9. What is the pH of human blood? Why is it important?
Human blood has a pH of approximately 7.4 (slightly basic). Maintaining this pH is critical for enzyme function and oxygen transport. Even a small change (below 6.8 or above 7.8) can be life-threatening.
Q10. Why do factory waste acids need to be neutralised before releasing into water bodies?
Factory waste containing acids can lower the pH of water bodies, killing aquatic life. It can also corrode pipes and sewage systems. Neutralising with calcium hydroxide (slaked lime) before release prevents environmental damage.
Q11. What is the difference between a strong base and a weak base?
A strong base completely dissociates in water to produce OH⁻ ions (e.g., NaOH, KOH). A weak base partially dissociates (e.g., NH₃, Ca(OH)₂). Strong bases have pH 12–14, while weak bases have pH 8–11.
Q12. Why is gypsum added to cement during its manufacture?
Gypsum (CaSO₄·2H₂O) is added to cement to control the setting time. Without gypsum, cement would set too quickly (within minutes). Gypsum slows down the hydration process, giving sufficient time (30–45 minutes) for working with the cement mixture.