⚗️ Acids, Bases & Salts — 80 Marks Paper
Class X · Science (Chemistry) · Chapter 2
Acids, Bases and Salts
Annual Examination — Theory Paper
General Instructions
- All questions are compulsory.
- Section A: Q1–20 (1 mark each) = 20 marks.
- Section B: Q21–30 (2 marks each) = 20 marks.
- Section C: Q31–38 (3 marks each) = 24 marks.
- Section D: Q39–42 (4 marks each) = 16 marks.
- Draw diagrams wherever necessary.
- Write chemical equations in balanced form only.
- Click "Show / Hide Answers" to reveal the answer key.
A
Section A — Multiple Choice & Very Short Answer
Questions 1–20 | 1 mark each | Total: 20 marks
1.
A solution turns red litmus blue. Its pH is most likely to be:
1 Mark
✅ Answer
(d) 10 — A solution that turns red litmus blue is basic. Basic solutions have pH > 7. Among the options, only 10 is greater than 7.
2.
Which of the following is a natural indicator?
1 Mark
✅ Answer
(c) Litmus — Litmus is a natural purple dye extracted from lichen (a plant belonging to Thallophyta). Phenolphthalein and methyl orange are synthetic indicators.
3.
The gas evolved when zinc reacts with dilute sulphuric acid is:
1 Mark
✅ Answer
(c) Hydrogen — Zn + H2SO4 → ZnSO4 + H2↑. The hydrogen gas produced burns with a "pop" sound when a burning candle is brought near.
4.
The colour of phenolphthalein in a basic solution is:
1 Mark
✅ Answer
(d) Pink — Phenolphthalein is colourless in acidic or neutral solutions, but turns pink/magenta in basic (alkaline) solutions.
5.
What is the pH of a neutral solution?
1 Mark
✅ Answer
(b) 7 — Pure water is neutral with pH = 7. A neutral solution has equal concentrations of H+ and OH− ions.
6.
Which medicine is used to treat indigestion caused by excess acid in the stomach?
1 Mark
✅ Answer
(c) Antacid — Antacids are mild bases (e.g., Milk of Magnesia — Mg(OH)2) that neutralise excess HCl in the stomach.
7.
The chemical formula of Plaster of Paris is:
1 Mark
✅ Answer
(b) CaSO4·½H2O — Plaster of Paris is calcium sulphate hemihydrate. It is made by heating gypsum (CaSO4·2H2O) at 373 K.
8.
Acid rain is said to occur when the pH of rain water falls below:
1 Mark
✅ Answer
(c) 5.6 — Normal rain water has pH ≈ 5.6 due to dissolved CO2. When pH drops below 5.6 due to pollutants like SO2 and NOx, it is called acid rain.
9.
The process of electrolysis of brine is called:
1 Mark
✅ Answer
(c) Chlor-alkali process — Brine (NaCl solution) on electrolysis gives chlorine gas at anode, hydrogen gas at cathode, and NaOH solution near cathode.
10.
A solution has pH = 1. Which of the following correctly describes it?
1 Mark
✅ Answer
(c) Strongly acidic — Lower the pH below 7, stronger the acid. pH = 1 indicates very high H+ concentration, hence strongly acidic.
11.
The colour of methyl orange in an acidic solution is:
1 Mark
✅ Answer
(d) Red — Methyl orange turns red in acid and yellow in base.
12.
The substance that turns milk of magnesia into a salt and water is:
1 Mark
✅ Answer
(b) HCl — Milk of magnesia [Mg(OH)2] is a base. Reacting with HCl (an acid) gives MgCl2 (salt) + H2O (water) — a neutralisation reaction.
13.
Which of the following is an olfactory indicator?
1 Mark
✅ Answer
(c) Onion — Olfactory indicators change their odour in acidic or basic media. Onion, vanilla, and clove are common examples. Litmus, turmeric, and red cabbage change colour.
14.
The common name of NaHCO3 is:
1 Mark
✅ Answer
(b) Baking soda — NaHCO3 = Sodium hydrogencarbonate = Baking soda. Na2CO3·10H2O is washing soda.
15.
Tooth decay begins when the pH in the mouth falls below:
1 Mark
✅ Answer
(d) 5.5 — Tooth enamel (calcium hydroxyapatite) is corroded when mouth pH drops below 5.5. Bacteria break down sugars to produce this acid.
16.
Name the acid present in ant sting.
1 Mark
✅ Answer
Methanoic acid (formic acid) — Ant sting injects methanoic acid (HCOOH), causing burning pain. The remedy is applying a mild base.
17.
What does the "p" in pH stand for?
1 Mark
✅ Answer
Potenz — The "p" stands for potenz, a German word meaning "power." pH measures the power (concentration) of hydrogen ions.
18.
What is the chemical name of bleaching powder?
1 Mark
✅ Answer
Calcium oxychloride / Calcium hypochlorite — Formula: Ca(ClO)2 (sometimes written CaOCl2). It is made by passing Cl2 through dry slaked lime.
19.
State True or False: Dry HCl gas turns moist blue litmus paper red.
1 Mark
✅ Answer
False — Dry HCl gas does NOT change dry litmus. It requires water to ionise (HCl + H2O → H3O+ + Cl−). Moist blue litmus turns red.
20.
Name the gas that turns lime water milky.
1 Mark
✅ Answer
Carbon dioxide (CO2) — CO2 + Ca(OH)2 → CaCO3↓ + H2O. The white precipitate of CaCO3 makes lime water milky.
B
Section B — Short Answer Questions
Questions 21–30 | 2 marks each | Total: 20 marks
21.
Why do HCl and H2SO4 show acidic behaviour in aqueous solution, while glucose and alcohol do not?
2 Marks
✅ Answer
HCl and H2SO4 ionise in water to produce H+ (hydronium) ions, which are responsible for acidic behaviour.Glucose and alcohol contain hydrogen atoms, but they do not ionise in water — they do not release H+ ions. Acidic behaviour requires the production of H+(aq) ions, not merely the presence of hydrogen.
22.
Write the balanced chemical equation for the reaction between sodium carbonate and dilute hydrochloric acid. What gas is evolved? How is it tested?
2 Marks
✅ Answer
Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + CO2(g)Gas evolved: Carbon dioxide (CO2).
Test: Pass the gas through lime water [Ca(OH)2]. It turns milky due to formation of white CaCO3 precipitate.
23.
While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
2 Marks
✅ Answer
Mixing acid with water is a highly exothermic process.If water is added to concentrated acid, the heat generated is so intense that it can cause the mixture to splash out violently, causing serious corrosive burns and may crack the glass container.
Adding acid slowly to water allows the large volume of water to absorb the heat safely.
24.
What is water of crystallisation? Give one example with chemical formula.
2 Marks
✅ Answer
Water of crystallisation is the fixed number of water molecules present in one formula unit of a crystalline salt. These water molecules are part of the salt's crystal structure.Example: Blue copper sulphate — CuSO4·5H2O. When heated, it loses its 5 water molecules and turns white (anhydrous).
25.
A milkman adds a small amount of baking soda to fresh milk. (a) Why does he make the milk slightly alkaline? (b) Why does the milk take longer to set as curd?
2 Marks
✅ Answer
(a) He makes the milk slightly alkaline to prevent it from turning sour quickly. Milk turns sour due to production of lactic acid by bacteria. The baking soda neutralises this acid, extending shelf life.(b) Curd is set when the pH of milk drops sufficiently. Since the milk is now alkaline, the bacteria need more time to produce enough acid to lower the pH to curd-setting level — so it takes longer to set.
26.
Why should Plaster of Paris be stored in a moisture-proof container?
2 Marks
✅ Answer
Plaster of Paris (CaSO4·½H2O) reacts with water to form gypsum (CaSO4·2H2O), a hard solid:CaSO4·½H2O + 1½H2O → CaSO4·2H2O
If stored in the open, it absorbs moisture from the air and sets hard, making it useless for its intended purpose. Hence a moisture-proof container is essential.
27.
Why do acids not show acidic properties in the absence of water?
2 Marks
✅ Answer
Acidic properties are due to the production of H+ (hydronium) ions. The ionisation of acid molecules to release H+ can only occur in the presence of water.For example: HCl + H2O → H3O+ + Cl−.
In the absence of water, HCl remains as molecular HCl gas and cannot release ions — hence it shows no acidic properties (e.g., dry HCl gas does not change dry litmus paper).
28.
Fresh milk has a pH of 6. How will the pH change as it turns into curd? Explain.
2 Marks
✅ Answer
As milk turns into curd, the pH will decrease (become more acidic).Bacteria present in milk ferment lactose (milk sugar) to produce lactic acid. The accumulation of lactic acid increases the H+ ion concentration, lowering the pH of the milk from 6 to even lower values (around 4–5), resulting in the sour taste of curd.
29.
Why does distilled water not conduct electricity, whereas rain water does?
2 Marks
✅ Answer
Electrical conduction in water requires ions.Distilled water is pure H2O with virtually no ions — it does not conduct electricity.
Rain water dissolves CO2 and other gases from the atmosphere, forming carbonic acid (H2CO3) and other electrolytes, which ionise to produce H+ and HCO3− ions. These ions carry the electric current.
30.
Write two uses of (a) baking soda and (b) washing soda.
2 Marks
✅ Answer
Baking soda (NaHCO3): (i) Used in baking powder to make cakes/bread soft and spongy (releases CO2). (ii) Used as an ingredient in antacids to neutralise excess stomach acid. (iii) Used in soda-acid fire extinguishers.Washing soda (Na2CO3·10H2O): (i) Used as a cleaning agent for domestic purposes. (ii) Used in glass, soap, and paper industries. (iii) Used to remove permanent hardness of water.
C
Section C — Long Answer Questions
Questions 31–38 | 3 marks each | Total: 24 marks
31.
Explain the chlor-alkali process with a balanced chemical equation. Name the products formed at the anode and cathode.
3 Marks
✅ Answer
The chlor-alkali process is the electrolysis of an aqueous solution of sodium chloride (brine).Equation: 2NaCl(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)
At the anode (+): Chlorine gas (Cl2) is evolved — used for making bleaching powder, PVC, pesticides, disinfectants.
At the cathode (−): Hydrogen gas (H2) is evolved — used as fuel, in margarine production, and for making ammonia.
Near cathode: Sodium hydroxide (NaOH) solution is formed — used in soaps, detergents, paper making, artificial fibres.
32.
Write balanced chemical equations for the following reactions: (a) Dilute sulphuric acid reacts with zinc granules. (b) Dilute hydrochloric acid reacts with sodium hydroxide. (c) Dilute hydrochloric acid reacts with iron filings.
3 Marks
✅ Answer
(a) Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)(b) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) (Neutralisation reaction)
(c) Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
33.
Describe an activity to show that acids produce H+ ions in water and these ions are responsible for acidic properties. (Activity to test electrical conductivity of acids and non-acids)
3 Marks
✅ Answer
Activity: Fix two nails on a cork and place it in a 100 mL beaker. Connect the nails to a 6-volt battery through a bulb and switch.Observation 1: Pour dilute HCl — the bulb glows. Repeat with dilute H2SO4 — the bulb glows again.
Observation 2: Pour glucose solution — bulb does not glow. Repeat with alcohol — bulb does not glow.
Inference: HCl and H2SO4 ionise in water to produce H+ and anion ions, which carry electric current. Glucose and alcohol do not ionise — they have no free ions — so the bulb does not glow. This proves acidic behaviour is due to H+(aq) ions.
34.
Explain the importance of pH in everyday life with three examples.
3 Marks
✅ Answer
1. Human Body: Our body functions best in a narrow pH range of 7.0–7.8. Blood pH is maintained around 7.4. Significant deviation leads to serious health issues.2. Digestive System: The stomach secretes HCl (pH ~1.2) for digestion. Excess acid causes indigestion; antacids (mild bases) neutralise it.
3. Tooth Decay: Bacteria ferment sugars producing acid. When mouth pH drops below 5.5, tooth enamel (calcium hydroxyapatite) dissolves, causing decay. Toothpaste (basic) neutralises this acid.
Other valid examples: Acid rain (pH < 5.6), soil pH for plants, bee sting remedies.
35.
How is bleaching powder prepared? Write the chemical equation and give three uses of bleaching powder.
3 Marks
✅ Answer
Preparation: Bleaching powder is produced by passing chlorine gas over dry slaked lime [Ca(OH)2].Equation: 2Ca(OH)2 + 2Cl2 → Ca(ClO)2 + CaCl2 + 2H2O
Uses:
(i) For bleaching cotton/linen in textile industry and wood pulp in paper factories.
(ii) To disinfect drinking water and make it germ-free.
(iii) Used as an oxidising agent in various chemical industries.
36.
Explain what happens when (a) copper oxide is added to dilute HCl, and (b) carbon dioxide is passed through lime water. Write the type of reaction in each case.
3 Marks
✅ Answer
(a) CuO + HCl:CuO(s) + 2HCl(aq) → CuCl2(aq) + H2O(l)
The solution turns blue-green due to formation of copper(II) chloride.
Type: Metal oxide + Acid → Salt + Water (acid-base type; metallic oxides are basic oxides).
(b) CO2 + lime water:
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Lime water turns milky due to white CaCO3 precipitate.
Type: Non-metallic oxide + Base → Salt + Water (non-metallic oxides are acidic in nature).
37.
Differentiate between a strong acid and a weak acid. Give one example of each. In which test tube will fizzing be more vigorous — HCl or CH₃COOH added to magnesium ribbon (same amount and concentration)? Why?
3 Marks
✅ Answer
Strong acid: Completely ionises in water to produce a large number of H+ ions. Example: HCl (hydrochloric acid).Weak acid: Partially ionises in water to produce fewer H+ ions. Example: CH3COOH (acetic acid).
Fizzing comparison: Fizzing (H2 gas evolution) will be more vigorous in the test tube with HCl.
Reason: HCl is a strong acid and produces more H+ ions than CH3COOH (weak acid) at the same concentration. More H+ ions react faster with magnesium, releasing more hydrogen gas more rapidly.
38.
Five solutions A, B, C, D and E show pH values of 4, 1, 11, 7 and 9 respectively. (a) Which is neutral? (b) Which is strongly alkaline? (c) Strongly acidic? (d) Weakly acidic? (e) Weakly alkaline? Arrange all in increasing order of hydrogen ion concentration.
3 Marks
✅ Answer
(a) Neutral → D (pH 7)(b) Strongly alkaline → C (pH 11)
(c) Strongly acidic → B (pH 1)
(d) Weakly acidic → A (pH 4)
(e) Weakly alkaline → E (pH 9)
Increasing H+ concentration (more H+ = lower pH):
C (pH 11) < E (pH 9) < D (pH 7) < A (pH 4) < B (pH 1)
D
Section D — Extended / Case-Based Questions
Questions 39–42 | 4 marks each | Total: 16 marks
39.
Case Study: Riya is making a chemistry project on "Salts from Common Salt." She lists four products — NaOH, Ca(ClO)2, NaHCO3, and Na2CO3·10H2O. Answer the following:
(a) Name the common name of NaHCO3 and Na2CO3·10H2O.
(b) Which of these is formed by the chlor-alkali process?
(c) Write the reaction that produces NaHCO3 from NaCl.
(d) What happens to NaHCO3 when heated? Write the equation.
4 Marks
(a) Name the common name of NaHCO3 and Na2CO3·10H2O.
(b) Which of these is formed by the chlor-alkali process?
(c) Write the reaction that produces NaHCO3 from NaCl.
(d) What happens to NaHCO3 when heated? Write the equation.
✅ Answer
(a) NaHCO3 = Baking soda (sodium hydrogencarbonate); Na2CO3·10H2O = Washing soda.(b) NaOH and Ca(ClO)2 (bleaching powder — uses Cl2 from the chlor-alkali process) are products of the chlor-alkali process.
(c) NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3
(d) On heating, NaHCO3 decomposes:
2NaHCO3 →(heat)→ Na2CO3 + H2O + CO2
This reaction is used in baking — CO2 makes cakes rise and become spongy.
40.
(a) What is a neutralisation reaction? Write the general equation.
(b) A farmer finds his soil is too acidic. Which substance — quick lime (CaO), slaked lime [Ca(OH)2], or chalk (CaCO3) — can he use to treat the soil? Why?
(c) Write any two examples of neutralisation reactions that occur in everyday life.
(d) Why are both H+ and OH− present in all aqueous solutions, including basic ones?
4 Marks
(b) A farmer finds his soil is too acidic. Which substance — quick lime (CaO), slaked lime [Ca(OH)2], or chalk (CaCO3) — can he use to treat the soil? Why?
(c) Write any two examples of neutralisation reactions that occur in everyday life.
(d) Why are both H+ and OH− present in all aqueous solutions, including basic ones?
✅ Answer
(a) A reaction in which an acid and a base react to form a salt and water is called a neutralisation reaction.Acid + Base → Salt + Water; at ionic level: H+(aq) + OH−(aq) → H2O(l)
(b) The farmer can use any of these (all are bases/alkaline). Most commonly slaked lime [Ca(OH)2] or chalk (CaCO3) are used as they are safe, cheap, and raise soil pH by neutralising excess acid. Quick lime is used when faster action is needed.
(c) (i) Bee sting: Formic acid (acid) + baking soda (base) → neutralised, giving relief.
(ii) Indigestion: HCl in stomach (acid) + Mg(OH)2 antacid (base) → MgCl2 + H2O.
(d) Water itself undergoes self-ionisation: H2O ⇌ H+ + OH−. Every aqueous solution contains both ions. In basic solutions, [OH−] > [H+], but both are still present. The solution is basic because OH− dominates.
41.
(a) Define "water of crystallisation." How many water molecules are present in one formula unit of: (i) washing soda, (ii) gypsum?
(b) Describe an activity to show that blue copper sulphate crystals contain water of crystallisation. What change do you observe on heating and then adding water again?
(c) How is Plaster of Paris made from gypsum? Write the temperature and equation. Why is it stored in moisture-proof containers?
4 Marks
(b) Describe an activity to show that blue copper sulphate crystals contain water of crystallisation. What change do you observe on heating and then adding water again?
(c) How is Plaster of Paris made from gypsum? Write the temperature and equation. Why is it stored in moisture-proof containers?
✅ Answer
(a) Water of crystallisation: Fixed number of water molecules present in one formula unit of a crystalline salt, forming part of its crystal structure.(i) Washing soda (Na2CO3·10H2O) — 10 water molecules
(ii) Gypsum (CaSO4·2H2O) — 2 water molecules
(b) Activity: Heat a few crystals of blue copper sulphate (CuSO4·5H2O) in a dry test tube.
On heating: the blue crystals turn white (anhydrous CuSO4); water droplets form on cooler parts of the tube.
On adding water: the blue colour is restored, showing the water of crystallisation is re-absorbed.
(c) Plaster of Paris preparation: Gypsum is heated at 373 K (100°C):
CaSO4·2H2O →(373K)→ CaSO4·½H2O + 1½H2O
Stored in moisture-proof containers because it readily absorbs moisture to revert to hard gypsum, becoming useless.
42.
Case Study: Ankur during a hike accidentally touches a nettle plant and gets a burning sting. His friend suggests rubbing the affected area with a dock plant leaf. Later, Ankur learns from his chemistry teacher about this and acid rain.
(a) Name the acid injected by the nettle plant.
(b) What is the nature (acidic/basic/neutral) of the dock plant? Why does it give relief?
(c) Explain how acid rain is formed and what harm it causes to aquatic life.
(d) Compounds like alcohols and glucose contain hydrogen. Why are they not classified as acids? Describe a test to distinguish acids from these compounds.
4 Marks
(a) Name the acid injected by the nettle plant.
(b) What is the nature (acidic/basic/neutral) of the dock plant? Why does it give relief?
(c) Explain how acid rain is formed and what harm it causes to aquatic life.
(d) Compounds like alcohols and glucose contain hydrogen. Why are they not classified as acids? Describe a test to distinguish acids from these compounds.
✅ Answer
(a) Methanoic acid (formic acid, HCOOH) — injected by stinging hair of nettle leaves.(b) The dock plant leaf is basic in nature. When rubbed on the affected area, the base in the dock leaf neutralises the methanoic acid, reducing the burning sensation (neutralisation reaction).
(c) Acid rain: Oxides of sulphur (SO2, SO3) and nitrogen (NOx) released from burning fossil fuels dissolve in rain water to form sulphuric acid and nitric acid. When the pH of rain drops below 5.6, it is called acid rain.
Harm: Acid rain flows into rivers, lowering their pH. Most aquatic organisms (fish, amphibians, plankton) cannot survive in low pH conditions, leading to death of aquatic life.
(d) Glucose and alcohol do not ionise in water — they do not release H+ ions. Acidic behaviour requires H+(aq) ions, not just hydrogen atoms in the molecule.
Test (electrical conductivity): Set up a circuit with two electrodes in a beaker connected to a bulb and battery. Pour the solution. Acids (HCl) make the bulb glow (ions present); glucose and alcohol solutions do not make the bulb glow (no ions — no acidic character).
Section A
20
Q1–20 · 1 mark each
Section B
20
Q21–30 · 2 marks each
Section C
24
Q31–38 · 3 marks each
Grand Total
80
Section D = 16 marks
