Exercise — Acids, Bases and Salts

15 questions✓ Free · step-by-step

11 markNCERT Cl-10 Science, Exercise Q1
A solution turns red litmus blue, its pH is likely to be (a) 1 (b) 4 (c) 5 (d) 10
Hint. Turning red litmus blue only happens on one side of pH 7.
Step 1 — Interpret the litmus result. Red litmus turning blue means the solution is basic.

Step 2 — Match to a pH value. Basic solutions have pH greater than 7, since higher pH means lower H⁺ concentration and more OH⁻. Among the given options, only 10 is above 7.

✦ Answer: (d) 10.
Where students slip. Picking (b) 4 or (c) 5 because they're 'in the middle' — both are still below 7 and therefore acidic, not basic.
21 markNCERT Cl-10 Science, Exercise Q2
A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains (a) NaCl (b) HCl (c) LiCl (d) KCl
Hint. Egg-shells are calcium carbonate — only one of the four options is actually an acid.
Step 1 — Identify the gas. A gas that turns limewater milky is carbon dioxide, CO₂.

Step 2 — Work out what reaction produces it. Egg-shells are made of calcium carbonate, and metal carbonates release CO₂ only when they react with an acid.

Step 3 — Check the options. NaCl, LiCl and KCl are all neutral salts, not acids, so they won't react with the egg-shell at all. HCl is the only acid among the four.

✦ Answer: (b) HCl.
Where students slip. Picking any of the chloride salts because they 'contain Cl like an acid might' — NaCl, LiCl and KCl are neutral salts and don't react with carbonates; only an actual acid does.
31 markNCERT Cl-10 Science, Exercise Q3
10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralise it will be (a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL
Hint. Neutralisation volumes scale in direct proportion — double the base needs double the acid.
Step 1 — Set up the known ratio. 10 mL NaOH needs 8 mL HCl to neutralise it completely.

Step 2 — Scale up proportionally. 20 mL of NaOH is exactly double 10 mL, so it needs double the HCl: 2 × 8 mL = 16 mL.

✦ Answer: (d) 16 mL.
Where students slip. Adding 8 mL instead of doubling it (giving 16 mL) — the relationship is a direct proportion, so doubling the base volume doubles the acid volume needed, not just adds a fixed amount.
41 markNCERT Cl-10 Science, Exercise Q4
Which one of the following types of medicines is used for treating indigestion? (a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic
Hint. Indigestion here means excess stomach acid — which of these four words literally means 'against acid'?
Step 1 — Recall the cause of indigestion. Indigestion from overeating is often caused by excess acid production in the stomach.

Step 2 — Match to the right medicine type. An antacid is a base that neutralises this excess acid, relieving the discomfort, since neutralisation is exactly what removes the extra H⁺ — this is the milk-of-magnesia example from the chapter.

✦ Answer: (c) Antacid.
Where students slip. Picking Analgesic — that treats pain generally, not the excess-acid cause of indigestion specifically; Antacid is the term built for this exact purpose.
54 marksNCERT Cl-10 Science, Exercise Q5
Write word equations and then balanced equations for the reaction taking place when – (a) dilute sulphuric acid reacts with zinc granules. (b) dilute hydrochloric acid reacts with magnesium ribbon. (c) dilute sulphuric acid reacts with aluminium powder. (d) dilute hydrochloric acid reacts with iron filings.
Hint. Every one of these follows the same pattern: metal + acid → salt + hydrogen.
Step 1 — (a) Zinc + Sulphuric acid → Zinc sulphate + Hydrogen. Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g).

Step 2 — (b) Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).

Step 3 — (c) Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen. Aluminium is +3, so Al₂(SO₄)₃ needs 2 Al and 3 SO₄, balancing to: 2Al(s) + 3H₂SO₄(aq) → Al₂(SO₄)₃(aq) + 3H₂(g).

Step 4 — (d) Iron + Hydrochloric acid → Iron(II) chloride + Hydrogen. Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g).

✦ Answer: (a) Zn + H₂SO₄ → ZnSO₄ + H₂ (b) Mg + 2HCl → MgCl₂ + H₂ (c) 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂ (d) Fe + 2HCl → FeCl₂ + H₂
Where students slip. In (c), forgetting that aluminium sulphate needs the coefficient 2 on aluminium and 3 on both the acid and the hydrogen — the +3 charge on aluminium forces every other coefficient in this particular equation.
62 marksNCERT Cl-10 Science, Exercise Q6
Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Hint. This is Activity 2.8 from the chapter — a circuit with a bulb is the giveaway.
Step 1 — Set up the activity. Fix two nails on a cork in a beaker, connect them through a bulb and switch to a 6 volt battery (as in Activity 2.8).

Step 2 — Run the comparison. Pour dilute HCl (or dilute H₂SO₄) into the beaker and switch on the current — the bulb glows brightly, since the acid ionises to release H⁺ and Cl⁻ ions that conduct the current. Repeat separately with glucose solution and with alcohol solution — the bulb does not glow in either case.

Step 3 — Draw the conclusion. Since the bulb only glows when ions are present to carry current, and it doesn't glow for glucose or alcohol, these compounds are not releasing H⁺ ions in solution — so despite containing hydrogen atoms, they show no acidic character.

✦ Answer: Set up a circuit with two electrodes, a bulb and a battery dipped in the test solution (Activity 2.8). The bulb glows for dilute HCl/H₂SO₄ (proving ions, and hence acidic behaviour, are present) but does not glow for glucose or alcohol solutions, showing that these compounds don't ionise to release H⁺, even though they contain hydrogen.
Where students slip. Describing only the acid part of the activity and forgetting to run the glucose/alcohol comparison — the proof specifically rests on contrasting the two outcomes, not just showing the acid conducts.
72 marksNCERT Cl-10 Science, Exercise Q7
Why does distilled water not conduct electricity, whereas rain water does?
Hint. One of these two kinds of water has picked something up on its way to the ground.
Step 1 — Consider distilled water. Distilled water is pure H₂O with no dissolved ions, so it has no charge carriers and cannot conduct electricity.

Step 2 — Consider rain water. As rain falls through the atmosphere, it dissolves gases such as CO₂ (forming small amounts of carbonic acid) and picks up other dissolved impurities, giving it some ionic content.

Step 3 — Connect this to conduction. Those dissolved ions in rain water are able to carry an electric current, unlike the ion-free distilled water.

✦ Answer: Distilled water has no dissolved ions and so cannot conduct electricity, while rain water picks up dissolved gases (like CO₂) and impurities as it falls, giving it ions that do conduct electricity.
Where students slip. Saying rain water conducts 'because it's dirty' without naming the actual mechanism — it's specifically the dissolved ionic content that enables conduction, not dirtiness in general.
82 marksNCERT Cl-10 Science, Exercise Q8
Why does dry HCl gas not show acidic behaviour in the absence of water?
Hint. Same idea as the in-text question on this exact topic — check what HCl needs before it can release H⁺.
Step 1 — Recall how HCl releases H⁺ ions. HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq) — this ionisation needs water; H⁺ cannot separate from HCl on its own.

Step 2 — Apply this to the dry case. With no water present, dry HCl gas cannot ionise, so no H⁺ ions are produced, and with no H⁺ ions there is no acidic behaviour to show.

✦ Answer: Dry HCl gas shows no acidic behaviour because it only ionises into H⁺ and Cl⁻ ions in the presence of water — without water, there are no H⁺ ions to produce acidic properties.
Where students slip. Saying HCl 'needs to dissolve to become an acid' as if it changes identity — it's the same compound; what changes is whether it can ionise to release H⁺, which strictly requires water.
94 marksNCERT Cl-10 Science, Exercise Q9
Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is (a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline? Arrange the pH in increasing order of hydrogen-ion concentration.
Hint. Match each solution to its number first, then remember pH and H⁺ concentration move in opposite directions.
Step 1 — List the pairs. A = 4, B = 1, C = 11, D = 7, E = 9.

Step 2 — Classify each by pH value. (a) Neutral (pH = 7): D. (b) Strongly alkaline (highest pH): C (11). (c) Strongly acidic (lowest pH): B (1). (d) Weakly acidic (just below 7): A (4). (e) Weakly alkaline (just above 7): E (9).

Step 3 — Order by increasing H⁺ concentration. Since H⁺ concentration increases as pH decreases, increasing H⁺ concentration means decreasing pH: C (11) < E (9) < D (7) < A (4) < B (1).

✦ Answer: (a) D (b) C (c) B (d) A (e) E. Increasing H⁺ concentration order: C < E < D < A < B.
Where students slip. Arranging the pH values in increasing numerical order (1, 4, 7, 9, 11) when asked for increasing H⁺ concentration — that's the order of increasing pH, which is actually decreasing H⁺ concentration; the two orders are exact opposites.
102 marksNCERT Cl-10 Science, Exercise Q10
Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH₃COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?
Hint. Same concentration doesn't mean the same number of free H⁺ ions — one of these acids is strong, the other weak.
Step 1 — Classify the two acids. HCl is a strong acid, which ionises almost completely in water. Acetic acid is a weak acid, which only partially ionises.

Step 2 — Compare the H⁺ availability. At the same overall concentration, HCl produces far more free H⁺ ions in solution than acetic acid does, since most of the acetic acid molecules remain unionised.

Step 3 — Connect this to reaction rate. More available H⁺ ions means a faster reaction with the magnesium ribbon, producing hydrogen gas more quickly.

✦ Answer: Test tube A (HCl) fizzes more vigorously, since HCl is a strong acid that ionises almost completely to give a much higher concentration of H⁺ ions than the weak, only partially ionised acetic acid in test tube B.
Where students slip. Assuming 'same concentration' means 'same reactivity' — concentration measures the total acid dissolved, not how much of it has actually ionised into H⁺ ions, which is what differs between a strong and a weak acid.
112 marksNCERT Cl-10 Science, Exercise Q11
Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Hint. Think about what curdling actually is at the level of bacteria and the acid they produce.
Step 1 — Recall what causes milk to turn into curd. Bacteria in the milk ferment it, and this fermentation produces lactic acid.

Step 2 — Track the effect on pH. As lactic acid accumulates during fermentation, the solution becomes more acidic, so its pH decreases, falling below the starting value of 6.

✦ Answer: The pH decreases (drops below 6) as milk turns into curd, since bacterial fermentation produces lactic acid, making the milk progressively more acidic.
Where students slip. Saying the pH increases — curdling is caused by acid build-up, which lowers pH, not raises it.
123 marksNCERT Cl-10 Science, Exercise Q12
A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd?
Hint. Baking soda is a base — think about what it does to milk's existing mild acidity, and what has to happen again before curdling can start.
Step 1 — (a) What baking soda does to the pH. Baking soda (sodium hydrogencarbonate) is basic. Adding a small amount to milk neutralises its natural mild acidity (pH 6) and shifts the pH slightly alkaline (above 7), which helps keep the milk from souring too quickly during storage or transport.

Step 2 — (b) Why curdling is delayed. Curdling needs enough lactic acid to build up and lower the pH to the point where milk proteins coagulate. Since the milk now starts out alkaline instead of at its natural pH 6, the bacterial fermentation first has to neutralise this extra alkalinity before enough net acid can accumulate to bring the pH down far enough — so setting as curd takes noticeably longer.

✦ Answer: (a) Baking soda is basic and neutralises milk's natural mild acidity, shifting its pH from 6 to slightly alkaline, which helps preserve the milk longer. (b) Because the milk starts more alkaline than usual, the fermentation that produces lactic acid has to first cancel out that extra alkalinity before enough acid builds up to curdle the milk, delaying the setting time.
Where students slip. Treating (a) and (b) as unrelated facts to memorise separately — (b) is a direct consequence of (a): the same alkalinity that preserves the milk is exactly what slows down its eventual curdling.
132 marksNCERT Cl-10 Science, Exercise Q13
Plaster of Paris should be stored in a moisture-proof container. Explain why?
Hint. Recall the equation for what Plaster of Paris does when it meets water — moisture in the air is a slow version of the same thing.
Step 1 — Recall the reaction. CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O — Plaster of Paris reacts readily with water to reform gypsum, setting into a hard solid mass.

Step 2 — Apply this to storage. Ordinary moisture in the air is enough water to trigger this same reaction slowly, over time, inside an unsealed container.

Step 3 — State the consequence. Since this reaction sets into a hard mass, if it happens before the Plaster of Paris is actually used, it hardens inside its own container and becomes useless.

✦ Answer: Plaster of Paris reacts with even small amounts of moisture to reform gypsum and set into a hard mass. Storing it in a moisture-proof container prevents atmospheric moisture from triggering this reaction early, which would otherwise ruin it before use.
Where students slip. Saying moisture just 'makes it damp' — the real issue is a genuine chemical reaction (recombining with water to reform gypsum), which sets the material hard, not just wets it.
142 marksNCERT Cl-10 Science, Exercise Q14
What is a neutralisation reaction? Give two examples.
Hint. The general pattern is Acid + Base → Salt + Water — just supply two specific instances of it.
Step 1 — Define it. A neutralisation reaction is a reaction between an acid and a base, giving a salt and water: Acid + Base → Salt + Water.

Step 2 — Give two examples. HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l), since the H⁺ from the acid and OH⁻ from the base combine to form water while Na⁺ and Cl⁻ form the salt. Also: 2HCl(aq) + Ca(OH)₂(aq) → CaCl₂(aq) + 2H₂O(l).

✦ Answer: A neutralisation reaction is Acid + Base → Salt + Water, e.g. HCl + NaOH → NaCl + H₂O, and 2HCl + Ca(OH)₂ → CaCl₂ + 2H₂O.
Where students slip. Giving two examples that are really the same reaction written with different acids or bases but no actual variety — pick genuinely distinct acid-base pairs, as above.
152 marksNCERT Cl-10 Science, Exercise Q15
Give two important uses of washing soda and baking soda.
Hint. One of these two is mostly an industrial/cleaning chemical; the other shows up in your kitchen and medicine cabinet.
Step 1 — Washing soda (Na₂CO₃·10H₂O). Used in the glass, soap and paper industries, and also as a cleaning agent for removing the permanent hardness of water.

Step 2 — Baking soda (NaHCO₃). Used in cooking, since it releases CO₂ on heating to make bread, cakes and pakoras soft and spongy, and as a mild antacid to relieve indigestion by neutralising excess stomach acid.

✦ Answer: Washing soda — used in glass/soap/paper manufacture, and for softening hard water. Baking soda — used in cooking (releases CO₂ to make food rise/turn spongy), and as a mild antacid for indigestion.
Where students slip. Swapping the two — washing soda is the industrial cleaning/glass-making compound, while baking soda is the kitchen and antacid one; mixing up which does which is a common slip.

Solutions written by the tuition.in editorial team and checked against the NCERT Class 10 Science textbook, Reprint 2026-27 (jesc102.pdf) — five in-text question sets (19 questions total) plus one end-of-chapter Exercise (15 questions, not 20 as some older manifests claim). Bleaching powder is written as Ca(ClO)₂ in the current text, matching the chapter's own in-text question.. Questions are referenced from the NCERT textbook for identification.

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