NCERT Solutions Class 10 Science Chapter 2 — Acids, Bases and Salts

NCERT Class 10 Science — Acids, Bases and Salts, Chapter 2 Exercises. All 15 questions solved.

Also here. Solutions to the in-text questions for this chapter and the activities for this chapter are on their own pages.

The exercise runs along the chapter’s own line of argument. Questions 1 to 4 and 9 are about the pH scale and reading it. Questions 5 to 8 and 10 are about what acids actually are — substances that give hydrogen ions in water, which is why dry hydrogen chloride is not acidic and why a strong acid fizzes harder than a weak one. Questions 11 to 15 are about salts and everyday chemistry. Four things carry all of it:

• pH measures hydrogen-ion concentration, on a scale from 0 to 14. Below 7 is acidic, exactly 7 neutral, above 7 basic — and the lower the pH, the higher the hydrogen-ion concentration.
• Acid + metal → salt + hydrogen. The gas that pops with a burning splint is hydrogen.
• Acid + base → salt + water. That is neutralisation, and it is the whole of Question 14.
• An acid shows its acidic behaviour only in water, because only then does it release $\text{H}^+$ ions.

Key insight. Nearly every “why” on this page reduces to one question: how many hydrogen ions are there? A strong acid gives more of them than a weak one at the same concentration, which is Question 10. Dry hydrogen chloride gives none at all, which is Question 8. Distilled water has almost none and no other ions either, which is Question 7. And pH is nothing but a measure of that same quantity, which is Questions 1, 9 and 11.

Question 1

A solution turns red litmus blue, its pH is likely to be

Solution. Red litmus turns blue only in a basic solution — that is the standard test for a base. Acids leave red litmus red, and turn blue litmus red instead.

A basic solution has a pH greater than 7. Of the four options, 1, 4 and 5 are all below 7 and so acidic; only 10 is above 7.

Question 2

A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains

Solution. Two clues, and they point the same way. Egg-shells are made of calcium carbonate, and a carbonate gives off a gas when it meets an acid. The gas turning lime-water milky identifies it as carbon dioxide, since the milkiness is insoluble calcium carbonate forming:

$$\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \longrightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$$

So the solution must be an acid. Of the four options, NaCl, LiCl and KCl are all neutral salts and would do nothing to an egg-shell. Only HCl is an acid:

$$\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \longrightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$$

Question 3

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 HCl solution (the same solution as before) required to neutralise it will be

Solution. Neither solution has been changed — only the amount of the alkali has. Twice as much sodium hydroxide solution contains twice as much sodium hydroxide, so it needs twice as much of the same acid to neutralise it.

$$10\ \text{mL NaOH} \equiv 8\ \text{mL HCl} \quad\Longrightarrow\quad 20\ \text{mL NaOH} \equiv \frac{20}{10} \times 8 = 16\ \text{mL HCl}$$

The ratio is what matters, and it is fixed as long as both solutions stay the same.

Question 4

Which one of the following types of medicines is used for treating indigestion?

Solution. Indigestion is caused by the stomach producing too much hydrochloric acid. The cure is to remove some of that excess acid, and the way to remove an acid is to neutralise it with a mild base.

That is exactly what an antacid is — a mild base such as magnesium hydroxide (milk of magnesia) or sodium hydrogencarbonate, which neutralises the surplus acid and relieves the pain.

The others treat different things: an antibiotic kills bacteria, an analgesic relieves pain of other kinds, and an antiseptic is applied to wounds to stop infection.

Question 5

Write word equations and then balanced equations for the reaction taking place when —

Solution. All four are the same reaction in different clothes: acid + metal → salt + hydrogen. Writing the word equation first makes the formula equation much easier, because it fixes which salt is formed.

(a) Dilute sulphuric acid reacts with zinc granules.

Zinc + sulphuric acid → zinc sulphate + hydrogen

$$\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(dil.)} \longrightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}$$

(b) Dilute hydrochloric acid reacts with magnesium ribbon.

Magnesium + hydrochloric acid → magnesium chloride + hydrogen

$$\text{Mg(s)} + 2\text{HCl(dil.)} \longrightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$

(c) Dilute sulphuric acid reacts with aluminium powder.

Aluminium + sulphuric acid → aluminium sulphate + hydrogen

Aluminium needs three sulphate groups per two atoms, so three acid molecules are needed and three molecules of hydrogen come off:

$$2\text{Al(s)} + 3\text{H}_2\text{SO}_4\text{(dil.)} \longrightarrow \text{Al}_2(\text{SO}_4)_3\text{(aq)} + 3\text{H}_2\text{(g)}$$

(d) Dilute hydrochloric acid reacts with iron filings.

Iron + hydrochloric acid → iron(II) chloride + hydrogen

$$\text{Fe(s)} + 2\text{HCl(dil.)} \longrightarrow \text{FeCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$

Question 6

Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.

Solution. The point to prove is that containing hydrogen is not enough: an acid must release hydrogen ions in water. Free ions are what let a solution conduct electricity, so conductivity is the test.

The activity. Take a beaker of dilute hydrochloric acid. Fit two iron nails through a rubber cork and stand them in the acid so they do not touch. Connect the nails through a switch to a bulb and a battery, so the solution completes the circuit.

Switch on. The bulb glows, because the hydrochloric acid has released $\text{H}^+$ ions into the water and the ions carry the current.

Now repeat the whole thing with glucose solution, and again with alcohol, keeping everything else the same. The bulb does not glow in either case.

The conclusion. Glucose and alcohol certainly contain hydrogen — their formulae, $\text{C}_6\text{H}_{12}\text{O}_6$ and $\text{C}_2\text{H}_5\text{OH}$, say so. But they do not release that hydrogen as $\text{H}^+$ ions in water, so their solutions carry no current and they are not acids. Only compounds that give hydrogen ions in solution are acids.

Question 7

Why does distilled water not conduct electricity, whereas rain water does?

Solution. A liquid conducts electricity only if it contains free ions to carry the charge.

Distilled water has been purified by distillation, so the dissolved salts and gases have been removed. Pure water ionises only to an extremely small extent, so there are almost no ions in it and it does not conduct.

Rain water is not pure. As it falls it dissolves gases from the air — carbon dioxide above all — and carbon dioxide reacts with water to form a weak acid:

$$\text{CO}_2\text{(g)} + \text{H}_2\text{O(l)} \longrightarrow \text{H}_2\text{CO}_3\text{(aq)}$$

Carbonic acid releases $\text{H}^+$ ions in the water, and those ions conduct the current. This is also why rain water is slightly acidic, with a pH a little below 7.

Question 8

Why does dry HCl gas not show acidic behaviour in the absence of water?

Solution. Acidic behaviour — turning blue litmus red, reacting with metals to give hydrogen, neutralising bases — is not a property of the molecule $\text{HCl}$ itself. It is a property of the hydrogen ions it produces.

Hydrogen chloride only releases those ions when it dissolves in water:

$$\text{HCl(g)} + \text{H}_2\text{O(l)} \longrightarrow \text{H}_3\text{O}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$$

In the dry state there is no water, so the molecule does not split up and no $\text{H}^+$ ions exist. With no hydrogen ions, there is nothing to show acidic behaviour — which is why dry hydrogen chloride gas leaves dry blue litmus paper unchanged, while damp litmus paper turns red at once.

Question 9

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.

Solution. Set the values out first, then read them against the scale. Below 7 is acidic, 7 is neutral, above 7 is alkaline — and the further from 7, the stronger.

Solution	A	B	C	D	E
pH	4	1	11	7	9

(a) Neutral — pH exactly 7: D.

(b) Strongly alkaline — the highest pH, furthest above 7: C (pH 11).

(c) Strongly acidic — the lowest pH, furthest below 7: B (pH 1).

(d) Weakly acidic — below 7 but not far: A (pH 4).

(e) Weakly alkaline — above 7 but not far: E (pH 9).

Increasing order of hydrogen-ion concentration. This is where the scale runs backwards, and it is the part most often got wrong. pH is a measure of hydrogen-ion concentration, but a low pH means a high concentration. So increasing hydrogen-ion concentration means decreasing pH:

$$11 < 9 < 7 < 4 < 1 \quad \text{in } [\text{H}^+], \qquad \text{that is} \quad \text{C} < \text{E} < \text{D} < \text{A} < \text{B}$$

Question 10

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?

Solution. The fizzing is hydrogen gas, released as the acid attacks the magnesium:

$$\text{Mg(s)} + 2\text{HCl(aq)} \longrightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$$

How fast that happens depends on how many hydrogen ions are available, and that is exactly where the two acids differ. Everything else in the experiment has been kept the same deliberately — same length of ribbon, same amount of acid, same concentration — so the only variable left is the acid itself.

Hydrochloric acid is a strong acid: it ionises almost completely in water, so nearly every molecule contributes an $\text{H}^+$ ion. Acetic acid is a weak acid: only a small fraction of its molecules ionise, so at the same concentration it supplies far fewer hydrogen ions.

More hydrogen ions means a faster reaction, so the fizzing is more vigorous in test tube A, the one with hydrochloric acid.

Question 11

Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Solution. Fresh milk at pH 6 is already very slightly acidic. When it is set into curd, bacteria ferment the lactose in it and produce lactic acid.

Adding acid raises the hydrogen-ion concentration, and a higher hydrogen-ion concentration means a lower pH. So the pH falls below 6 as the curd forms, and the curd tastes distinctly sour — which is the lactic acid you are tasting.

Question 12

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?

Solution. Baking soda, sodium hydrogencarbonate, is a mild base, so adding a little of it raises the pH from 6 to just above 7.

(a) Milk goes sour because bacteria turn its lactose into lactic acid. Making the milk slightly alkaline first means that any acid formed is neutralised as it appears, so the milk does not become sour quickly. This is simply a way of keeping fresh milk from spoiling for longer in warm weather.

(b) For milk to set as curd, it must become acidic — the lactic acid has to build up enough to bring the pH down. In milk that has been made alkaline, the lactic acid produced by the bacteria is spent first on neutralising the added baking soda. Only after that is used up can the acid begin to lower the pH. So the curd takes much longer to set.

Question 13

Plaster of Paris should be stored in a moisture-proof container. Explain why?

Solution. Plaster of Paris is calcium sulphate hemihydrate, $\text{CaSO}_4 \cdot \tfrac12\text{H}_2\text{O}$ — calcium sulphate holding half a molecule of water for each formula unit. Its usefulness comes from what it does with more water.

On contact with water it takes up the rest and sets into a hard solid mass of gypsum:

$$\text{CaSO}_4 \cdot \tfrac12\text{H}_2\text{O} + 1\tfrac12\text{H}_2\text{O} \longrightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$$

That is exactly what makes it useful for plaster casts and mouldings — and exactly why it must be kept dry. Moisture in the air is enough to start the reaction, and plaster that has already set in its bag is a hard useless lump. So it is stored in a moisture-proof container.

Question 14

What is a neutralisation reaction? Give two examples.

Solution. A neutralisation reaction is the reaction between an acid and a base, in which they cancel each other’s properties and give a salt and water:

$$\text{acid} + \text{base} \longrightarrow \text{salt} + \text{water}$$

What actually happens is that the $\text{H}^+$ ions from the acid combine with the $\text{OH}^-$ ions from the base to make water, which is why neither acidic nor basic behaviour survives.

Example 1 — hydrochloric acid and sodium hydroxide, giving common salt:

$$\text{NaOH(aq)} + \text{HCl(aq)} \longrightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}$$

Example 2 — sulphuric acid and sodium hydroxide:

$$2\text{NaOH(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \longrightarrow \text{Na}_2\text{SO}_4\text{(aq)} + 2\text{H}_2\text{O(l)}$$

Taking an antacid for indigestion, and rubbing baking soda on a bee sting, are both this reaction put to use.

Question 15

Give two important uses of washing soda and baking soda.

Solution. Both are sodium salts of carbonic acid, and both are mildly basic — which is what most of their uses depend on.

Washing soda, $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$:

• It is used in the glass, soap and paper industries, and in making borax and other sodium compounds.
• It removes the permanent hardness of water, by precipitating the calcium and magnesium salts that cause it — which is where the name comes from.

Baking soda, $\text{NaHCO}_3$:

• It is used in baking, as one part of baking powder. On heating it gives off carbon dioxide, and the bubbles of gas make the cake or bread rise.
• It is a mild base, so it is used as an antacid to neutralise excess acid in the stomach, and in soda–acid fire extinguishers.

Common mistakes

• Reading the pH scale the right way round but the hydrogen-ion scale the same way in Question 9. They run oppositely: the lowest pH has the highest hydrogen-ion concentration, so increasing $[\text{H}^+]$ means going 11, 9, 7, 4, 1.
• Saying acetic acid fizzes more in Question 10 because it is an acid too. Both are acids; what differs is how completely they ionise. The strong acid supplies more $\text{H}^+$ ions and so reacts faster.
• Answering Question 8 with “because it is a gas”. Dissolved hydrogen chloride is acidic and dry hydrogen chloride is not, and the state is not the reason — the absence of water is, because without water there are no $\text{H}^+$ ions.
• Saying the pH of milk rises as it turns to curd in Question 11. Curd is sour because acid is being produced, and more acid always means a lower pH.
• Confusing washing soda with baking soda in Question 15. Washing soda is $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}$ and is industrial; baking soda is $\text{NaHCO}_3$ and is the one used in food and as an antacid.
• Forgetting that aluminium needs three sulphate groups in Question 5(c). $\text{Al}_2(\text{SO}_4)_3$ requires 3 molecules of acid and releases 3 of hydrogen, not one of each.

Practise next

• The in-text questions of this chapter, nineteen across five boxes, which test the same pH reasoning and the same reactions of acids and bases in shorter form.
• Chapter 1, Chemical Reactions and Equations, where neutralisation first appeared as an example of a double displacement reaction.