NCERT Exemplar Solutions Class 10 Science Acids, Bases and Salts — Multiple Choice Questions Q1–30

NCERT Exemplar Problems, Class 10 Science — Acids, Bases and Salts, Multiple Choice Questions. All 30 questions solved.

Thirty questions is the longest multiple-choice run in the Class 10 Science exemplar, and they divide into three groups that are worth recognising on sight, because each wants a different kind of thinking.

• Definitions and properties — what an acid does to litmus, what pH means, which substances are acidic. Questions 1, 2, 8, 14, 15, 19, 22, 23, 28 and 29. These are fast if the definitions are exact and slow if they are approximate.
• Named chemicals and their formulae — baking soda, washing soda, bleaching powder, plaster of Paris, gypsum. Questions 4, 11, 12, 18, 21, 25 and 30. There is no reasoning here; it is recall, and the exemplar tests it hard.
• Applied judgement — a laboratory observation, a safety decision, an apparatus. Questions 3, 5, 6, 7, 9, 10, 13, 16, 17, 20, 24, 26 and 27. These are the interesting ones.

Key insight. Almost every question here is settled by asking where the hydrogen ion is. An acid releases $\mathrm{H^+}$ in water (as $\mathrm{H_3O^+}$); a base releases $\mathrm{OH^-}$; pH counts how many $\mathrm{H^+}$ there are, so a lower pH means a stronger acid and a higher pH a stronger base. Half the wrong options in this exercise are that sentence with a word reversed.

Choose the correct answer from the given four options in the following questions:

Question 1

What happens when a solution of an acid is mixed with a solution of a base in a test tube?

(i) The temperature of the solution increases (ii) The temperature of the solution decreases (iii) The temperature of the solution remains the same (iv) Salt formation takes place

Solution. Mixing an acid and a base is neutralisation, and two things happen at once:

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

A salt is formed, so statement (iv) is true. And the reaction is exothermic — the hydrogen ions and hydroxide ions combining to make water release energy — so the test tube warms and statement (i) is true. Statements (ii) and (iii) contradict (i) and cannot stand with it.

Option (a) is the trap: it is correct as far as it goes, but it leaves out the salt, and the question asks what happens, not what happens to the temperature. When two statements are both true, the option containing both is the answer.

Question 2

An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change?

Solution. Turning red litmus blue is the test for a base, so the starting solution is basic. To reverse the change — to get the litmus back to red — the solution must be made acidic, and the only way to do that is to add an acid in excess.

Three of the four options are bases or basic solutions. Baking powder contains sodium hydrogencarbonate and is basic; lime is calcium oxide or hydroxide and is basic; ammonium hydroxide is a base by its name. Adding any of them would make the solution more basic, not less.

Hydrochloric acid is the only acid on the list. Added in excess it first neutralises the base and then makes the solution acidic, and the blue litmus turns red again.

Question 3

During the preparation of hydrogen chloride gas on a humid day, the gas is usually passed through the guard tube containing calcium chloride. The role of calcium chloride taken in the guard tube is to

Solution. The phrase on a humid day is the clue, and so is the name guard tube — it guards the collected gas against something.

Anhydrous calcium chloride is a drying agent: it absorbs water vapour readily. Passing the hydrogen chloride through it removes the moisture the humid air has put into it, so that dry hydrogen chloride gas is collected.

This matters because dry hydrogen chloride and moist hydrogen chloride behave completely differently. Dry $\mathrm{HCl}$ gas has no free hydrogen ions and does not turn dry litmus red at all — Question 33 of the short-answer band rests on exactly that. It is only when water is present that the gas ionises and becomes an acid.

Option (a) would defeat the purpose of the preparation, and options (b) and (d) describe things calcium chloride simply does not do.

Question 4

Which of the following salts does not contain water of crystallisation?

Solution. Water of crystallisation is water built into the crystal in a fixed proportion, and it shows up in the formula after a dot. Write the four formulae out and the answer is immediate:

• Blue vitriol — $\mathrm{CuSO_4 \cdot 5H_2O}$, five molecules
• Baking soda — $\mathrm{NaHCO_3}$, none
• Washing soda — $\mathrm{Na_2CO_3 \cdot 10H_2O}$, ten molecules
• Gypsum — $\mathrm{CaSO_4 \cdot 2H_2O}$, two molecules

Baking soda is the only one with no dot in its formula. It is worth noticing how easy the question becomes once the formulae are written and how nearly impossible it is otherwise — which is why the named-chemical formulae in this chapter have to be learnt rather than reasoned out.

Question 5

Sodium carbonate is a basic salt because it is a salt of

Solution. Take the salt apart into the acid and base that would produce it. Sodium carbonate, $\mathrm{Na_2CO_3}$, comes from sodium hydroxide and carbonic acid:

$$2\mathrm{NaOH} + \mathrm{H_2CO_3} \rightarrow \mathrm{Na_2CO_3} + 2\mathrm{H_2O}$$

Sodium hydroxide is a strong base and carbonic acid is a weak acid. That is option (d).

The rule behind it is worth stating, because it decides Question 6 as well: the stronger parent wins. A salt of a strong base and a weak acid is basic in solution, a salt of a strong acid and a weak base is acidic, and a salt of two strong parents is neutral. The question even tells you the answer is basic, which fixes the strong parent as the base.

Question 6

Calcium phosphate is present in tooth enamel. Its nature is

Solution. Apply the rule from Question 5. Calcium phosphate, $\mathrm{Ca_3(PO_4)_2}$, is the salt of calcium hydroxide — a strong base — and phosphoric acid, which is a weak acid. The strong parent is the base, so the salt is basic.

This is also the chemistry of tooth decay, which is why the exemplar asks it. Bacteria in the mouth turn leftover sugar into acid, and when the pH falls below about 5.5 the acid attacks the basic calcium phosphate of the enamel and dissolves it. Brushing with a basic toothpaste — Question 13 — neutralises that acid before it can do the damage.

Question 7

A sample of soil is mixed with water and allowed to settle. The clear supernatant solution turns the pH paper yellowish-orange. Which of the following would change the colour of this pH paper to greenish-blue?

Solution. Read the colours off the pH scale. Yellowish-orange sits at the low end, around pH 4 or 5, so the soil solution is acidic. Greenish-blue sits above 7, so the question is asking what would make the solution basic.

Lemon juice contains citric acid and vinegar contains acetic acid; both would push the pH lower, deepening the orange towards red. Common salt is neutral and would leave the colour where it is.

An antacid is by definition a base — that is what “anti-acid” means, and its job is to neutralise stomach acid. Adding enough of it neutralises the soil acid and then takes the solution past neutral into the basic range, where the paper reads greenish-blue.

Question 8

Which of the following gives the correct increasing order of acidic strength?

Solution. Acidic strength means how readily the substance releases hydrogen ions in water, and the three named substances are three well-separated points on that scale.

Water is very nearly neutral — it ionises to a tiny extent, giving pH 7 — so it is the weakest of the three. Acetic acid is a weak acid: it ionises only partly, which is why vinegar is safe to eat. Hydrochloric acid is a strong acid: it ionises completely, and the same concentration gives a far higher hydrogen ion concentration.

So the increasing order is water, then acetic acid, then hydrochloric acid — option (a).

A quick check that costs nothing: any option putting hydrochloric acid below acetic acid, as (b) and (d) do, has the strong acid weaker than the weak one and can be discarded at a glance.

Question 9

If a few drops of a concentrated acid accidentally spills over the hand of a student, what should be done?

Solution. The right procedure has two stages and the order matters. First dilute the acid by washing with a large volume of water, which both removes most of it and reduces the concentration of what remains. Then neutralise the traces with a mild base.

Sodium hydrogencarbonate is exactly the right mild base — it is weak enough to be harmless on skin, which is why it is the standard first-aid measure.

Options (c) and (d) both get the chemistry right and the safety badly wrong. Sodium hydroxide is a strong alkali and is itself corrosive; putting it on burnt skin adds an alkali burn to the acid burn. Neutralisation is also strongly exothermic, so a strong base on a concentrated acid releases heat exactly where the skin is already damaged.

Option (a) is simply useless — saline solution is neutral and does nothing the water has not already done.

Question 10

Sodium hydrogencarbonate when added to acetic acid evolves a gas. Which of the following statements are true about the gas evolved?

(i) It turns lime water milky (ii) It extinguishes a burning splinter (iii) It dissolves in a solution of sodium hydroxide (iv) It has a pungent odour

Solution. Identify the gas first. An acid on a hydrogencarbonate always gives carbon dioxide:

$$\mathrm{NaHCO_3} + \mathrm{CH_3COOH} \rightarrow \mathrm{CH_3COONa} + \mathrm{H_2O} + \mathrm{CO_2}$$

Now test the four statements against carbon dioxide.

(i) is its standard identification — it turns lime water milky by forming insoluble calcium carbonate. True.

(ii) Carbon dioxide does not support combustion and is denser than air, which is why it is used in fire extinguishers. A burning splinter goes out in it. True.

(iii) Carbon dioxide is an acidic oxide, so it reacts with the base sodium hydroxide and dissolves: $2\mathrm{NaOH} + \mathrm{CO_2} \rightarrow \mathrm{Na_2CO_3} + \mathrm{H_2O}$. True.

(iv) Carbon dioxide is odourless. This is the only false statement, and it is what separates the answer from option (c).

Three true statements means option (b).

Question 11

Common salt besides being used in kitchen can also be used as the raw material for making

(i) washing soda (ii) bleaching powder (iii) baking soda (iv) slaked lime

Solution. The reliable test is whether the sodium in the product came from the common salt, and it sorts all four cleanly.

Baking soda is $\mathrm{NaHCO_3}$ and is made directly from brine:

$$\mathrm{NaCl} + \mathrm{H_2O} + \mathrm{CO_2} + \mathrm{NH_3} \rightarrow \mathrm{NH_4Cl} + \mathrm{NaHCO_3}$$

Washing soda is $\mathrm{Na_2CO_3 \cdot 10H_2O}$, made by heating that baking soda and recrystallising. Both carry sodium that arrived as common salt, so (i) and (iii) are in.

Slaked lime is $\mathrm{Ca(OH)_2}$ — calcium, not sodium. It comes from limestone, heated to quicklime and slaked with water, and common salt is nowhere in its manufacture. Statement (iv) is out.

Bleaching powder is the one to think about. It is $\mathrm{Ca(ClO)_2}$, again a calcium compound, and it is made by passing chlorine over slaked lime. Its two raw materials are limestone and chlorine. Common salt is the source of that chlorine in industry, so salt stands one step back in the chain — but it is not a raw material of the bleaching powder itself, and no atom of the finished compound came from the sodium of the salt. On the strict reading the exemplar intends, statement (ii) is out, which gives option (c).

Question 12

One of the constituents of baking powder is sodium hydrogencarbonate, the other constituent is

Solution. This question defines the difference between two things the chapter keeps using, so it is worth stating plainly:

Baking soda is sodium hydrogencarbonate alone. Baking powder is baking soda plus a mild edible acid, and that acid is tartaric acid.

The acid is there to do a job. Heating baking soda alone gives sodium carbonate, which is bitter, and that bitterness would go into the cake. The tartaric acid neutralises the sodium carbonate as it forms, so the cake rises on the carbon dioxide without tasting bitter.

The other three options are all acids, but hydrochloric and sulphuric acids are strong mineral acids and could not be eaten, and acetic acid is volatile and would evaporate away and smell of vinegar. Tartaric acid is a mild, solid, edible acid — exactly what is needed.

Question 13

To protect tooth decay we are advised to brush our teeth regularly. The nature of the tooth paste commonly used is

Solution. Tooth decay is caused by acid. Bacteria in the mouth break down sugar left on the teeth and produce acid, and once the mouth’s pH falls below about 5.5 that acid begins to dissolve the calcium phosphate of the enamel — the substance of Question 6.

A toothpaste that was itself acidic would make matters worse, and a neutral one would do nothing about the acid at all. A basic toothpaste neutralises the acid the bacteria have produced and brings the mouth back above the danger pH, which is why toothpastes are formulated to be mildly alkaline.

Option (d) is not a nature in the acid–base sense at all, and a corrosive substance is the last thing to put in a mouth.

Question 14

Which of the following statements is correct about an aqueous solution of an acid and of a base?

(i) Higher the pH, stronger the acid (ii) Higher the pH, weaker the acid (iii) Lower the pH, stronger the base (iv) Lower the pH, weaker the base

Solution. The whole question is the direction of the pH scale, and it runs the way that feels backwards: pH goes down as acidity goes up. It is a measure of hydrogen ion concentration on a reversed logarithmic scale, so more hydrogen ions gives a smaller number.

From that single fact:

• A high pH means few hydrogen ions, so an acid with a high pH is a weak one. Statement (ii) is true, (i) false.
• A low pH means few hydroxide ions, so a base with a low pH is a weak one. Statement (iv) is true, (iii) false.

The pairing is (ii) and (iv), option (d).

A cleaner way to hold this: strength always sits at the ends of the scale. Strong acids are near 0, strong bases near 14, and everything weak crowds towards 7 from one side or the other.

Question 15

The pH of the gastric juices released during digestion is

Solution. The stomach secretes hydrochloric acid, which activates the enzyme pepsin and kills swallowed bacteria. Any acid has a pH below 7, so option (a) is correct.

Option (d) is the interesting distractor. Gastric juice really is strongly acidic — its pH is about 1.2 to 3 — but pH exactly 0 would mean a hydrogen ion concentration of 1 mole per litre, which is roughly concentrated hydrochloric acid and would destroy the stomach lining. “Less than 7” is the honest general answer; “equal to 0” is a specific claim that is simply false.

This acid is also why antacids exist. When too much is produced the excess is neutralised with a mild base such as magnesium hydroxide, which is the practical use behind Question 7.

Question 16

Which of the following phenomena occur, when a small amount of acid is added to water?

(i) Ionisation (ii) Neutralisation (iii) Dilution (iv) Salt formation

Solution. Nothing basic has been added, so read the four words carefully and two of them cannot apply.

Neutralisation requires an acid and a base. Water is neutral, not basic, so there is nothing for the acid to neutralise. Statement (ii) is out, and with it options (a), (c) and (d).

Salt formation is the product of neutralisation, so it goes for the same reason.

Ionisation does happen: the acid dissolves and splits into ions, which is the very thing that makes it an acid in solution. $\mathrm{HCl} + \mathrm{H_2O} \rightarrow \mathrm{H_3O^+} + \mathrm{Cl^-}$.

Dilution also happens: a small amount of acid in a large amount of water means a low concentration, and the hydrogen ion concentration per unit volume falls.

Both (i) and (iii), which is option (b). Notice that ionisation and dilution pull in opposite directions — ionisation creates hydrogen ions, dilution spreads them out — and both are still going on at once.

Question 17

Which one of the following can be used as an acid–base indicator by a visually impaired student?

Solution. The question is really asking which indicator signals its result through a sense other than sight.

Litmus, turmeric and petunia extract are all colour indicators — they change from red to blue, yellow to red, and so on. None of them is any use to a student who cannot see the change.

Vanilla essence is an olfactory indicator: its characteristic smell is destroyed in a basic solution and survives in an acidic one. A student can therefore tell acid from base by smell alone. Onion extract and clove oil work the same way, and the textbook introduces all three under that heading.

Question 18

Which of the following substance will not give carbon dioxide on treatment with dilute acid?

Solution. Carbon dioxide comes from an acid only when the substance contains a carbonate or hydrogencarbonate group — the acid displaces carbonic acid, which breaks up at once into water and carbon dioxide.

Marble and limestone are both calcium carbonate, $\mathrm{CaCO_3}$, in different physical forms, so both fizz:

$$\mathrm{CaCO_3} + 2\mathrm{HCl} \rightarrow \mathrm{CaCl_2} + \mathrm{H_2O} + \mathrm{CO_2}$$

Baking soda is sodium hydrogencarbonate and fizzes too.

Lime is calcium oxide, $\mathrm{CaO}$, with no carbon in it at all. It reacts with acid readily enough, but it gives only a salt and water:

$$\mathrm{CaO} + 2\mathrm{HCl} \rightarrow \mathrm{CaCl_2} + \mathrm{H_2O}$$

No carbon in the reactant means no carbon dioxide in the products — an argument that takes one glance at the formula.

Question 19

Which of the following is acidic in nature?

Solution. Lime juice contains citric acid and has a pH around 2. It is acidic.

The other three all sit at or above neutral. Human blood is very slightly basic, at pH about 7.4, and the body holds it there tightly. Lime water is a solution of calcium hydroxide and is strongly basic — note that despite the similar name it has nothing to do with the fruit. An antacid is a base by definition.

The pair lime juice and lime water is the whole point of the question, and they are opposites: one is fruit juice and acidic, the other is calcium hydroxide solution and basic.

Question 20

In an attempt to demonstrate electrical conductivity through an electrolyte, the following apparatus (Figure 2.1) was set up.

Which among the following statement(s) is(are) correct?

(i) Bulb will not glow because electrolyte is not acidic (ii) Bulb will glow because NaOH is a strong base and furnishes ions for conduction. (iii) Bulb will not glow because circuit is incomplete (iv) Bulb will not glow because it depends upon the type of electrolytic solution

Solution. A solution conducts electricity if it contains free ions, and it does not matter in the least whether those ions came from an acid or a base.

Sodium hydroxide is a strong base and dissociates completely in water into $\mathrm{Na^+}$ and $\mathrm{OH^-}$. Those ions move to the electrodes and carry the current, so the circuit conducts and the bulb glows. Statement (ii) is correct.

Statement (i) has the misconception the question was written to catch — that only acids conduct. Any strong electrolyte does, acid or base or salt. Statement (iii) is false on the evidence of the diagram: the battery, bulb, switch, both nails and the solution form one unbroken loop. Statement (iv) is self-contradictory as a reason for not glowing; the type of solution does matter, but here the solution is a good conductor, so the conclusion drawn from it is wrong.

Only (ii) survives, which is option (c).

Question 21

Which of the following is used for dissolution of gold?

Solution. Gold is at the very bottom of the reactivity series, which is why it is found as the free metal and why it does not tarnish. No single mineral acid attacks it — not hydrochloric, not sulphuric, not even nitric.

Aqua regia does. It is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3 : 1, and it is a fuming, highly corrosive liquid. Neither acid alone dissolves gold; the mixture works because the nitric acid oxidises the gold while the chloride ions from the hydrochloric acid immediately lock up the resulting gold ions in a complex, so the oxidation is never reversed.

The name is Latin for “royal water”, given because it dissolves the royal metals — gold and platinum.

Question 22

Which of the following is not a mineral acid?

Solution. Acids are sorted into two families by where they come from. Mineral acids are made from mineral sources in a laboratory or factory and are generally strong; hydrochloric, sulphuric and nitric acids are the three standard examples, and they are exactly the three in this list.

Organic acids come from living things — plants and animals — and are generally weak. Citric acid comes from citrus fruit, and it belongs to the same family as the acetic acid of vinegar, the lactic acid of curd and the formic acid of an ant sting.

So citric acid is the odd one out. Notice the overlap with Question 8: the mineral/organic split and the strong/weak split are different classifications, but in the school syllabus they happen to line up almost exactly.

Question 23

Which among the following is not a base?

Solution. Three of these are hydroxides, and a hydroxide releases $\mathrm{OH^-}$ ions in water, which is what makes a base a base. Sodium hydroxide and potassium hydroxide are strong bases; ammonium hydroxide is a weak one, but still a base.

$\mathrm{C_2H_5OH}$ is ethanol, and this question is a trap sprung entirely by how the formula is written. The $\mathrm{-OH}$ in ethanol is a covalently bonded hydroxyl group, part of the molecule, not a hydroxide ion waiting to be released. Ethanol dissolves in water without producing any $\mathrm{OH^-}$ at all, and its solution is neutral.

The lesson generalises: an $\mathrm{OH}$ in a formula means a base only when it is attached to a metal or to ammonium and can leave as an ion.

Question 24

Which of the following statements is not correct?

Solution. Watch the quantifiers — all and some — because they are doing the work.

(a) is true and is a genuine universal: every metal carbonate gives a salt, water and carbon dioxide with an acid.

(c) and (d) both say some, which is a modest claim and easy to satisfy. Some metals do react with acids to give hydrogen, namely those above hydrogen in the reactivity series. Some non-metal oxides do form acids with water, such as $\mathrm{CO_2}$ and $\mathrm{SO_2}$.

(b) is wrong twice over. Most metal oxides do not react with water at all — only the oxides of the most reactive metals do. And those that do react give a base, not “salt and acid”: $\mathrm{CaO} + \mathrm{H_2O} \rightarrow \mathrm{Ca(OH)_2}$. Metal oxides are basic oxides; the acidic ones are the non-metal oxides of statement (d). Statement (b) has the two families the wrong way round.

Question 25

Match the chemical substances given in Column (A) with their appropriate application given in Column (B)

Column (A)	Column (B)
(A) Bleaching powder	(i) Preparation of glass
(B) Baking soda	(ii) Production of $\mathrm{H_2}$ and $\mathrm{Cl_2}$
(C) Washing soda	(iii) Decolourisation
(D) Sodium chloride	(iv) Antacid

Solution. Two of the four matches are almost given away by the words themselves, and they are enough to pick the option.

Bleaching powder → (iii) decolourisation. Bleaching is decolourisation; that is what the name means.

Sodium chloride → (ii) production of $\mathrm{H_2}$ and $\mathrm{Cl_2}$. This is the chlor-alkali process, the electrolysis of brine, which gives hydrogen at the cathode, chlorine at the anode and sodium hydroxide in the solution.

Those two alone leave only option (c), and the remaining pair confirms it. Baking soda → (iv) antacid, because sodium hydrogencarbonate is the mild base taken to neutralise excess stomach acid. Washing soda → (i) preparation of glass, because sodium carbonate is one of the three raw materials of ordinary glass, along with sand and limestone.

In a matching question, always place the matches you are sure of first and then check which single option accommodates them all.

Question 26

Equal volumes of hydrochloric acid and sodium hydroxide solutions of same concentration are mixed and the pH of the resulting solution is checked with a pH paper. What would be the colour obtained? (You may use colour guide given in Figure 2.2)

Solution. Hydrochloric acid and sodium hydroxide are a strong acid and a strong base, and they react in a one-to-one ratio:

$$\mathrm{HCl} + \mathrm{NaOH} \rightarrow \mathrm{NaCl} + \mathrm{H_2O}$$

Equal volumes at the same concentration means equal numbers of moles, so the acid and base exactly neutralise each other. Nothing is left over.

What remains is sodium chloride solution — the salt of a strong acid and a strong base, which by the rule of Question 5 is neutral. So the pH is 7.

Reading 7 off the colour guide gives green, and the closest option is yellowish green. Option (d), blue, would mean an excess of base; options (a) and (b) would mean an excess of acid; neither is possible when the two are matched exactly.

Question 27

Which of the following is(are) true when HCl (g) is passed through water?

(i) It does not ionise in the solution as it is a covalent compound. (ii) It ionises in the solution (iii) It gives both hydrogen and hydroxyl ion in the solution (iv) It forms hydronium ion in the solution due to the combination of hydrogen ion with water molecule

Solution. Hydrogen chloride really is a covalent compound in the gaseous state — it is a molecule with a shared pair of electrons, not an ionic lattice. Statement (i) gets that far and then draws the wrong conclusion.

What happens in water is that the polar water molecules pull the molecule apart, and it ionises completely:

$$\mathrm{HCl}(g) + \mathrm{H_2O}(l) \rightarrow \mathrm{H_3O^+}(aq) + \mathrm{Cl^-}(aq)$$

So statement (ii) is true. Being covalent does not prevent ionisation in water; it only means the compound is not already ionic before the water gets to it.

Statement (iv) adds the detail. A bare hydrogen ion is a lone proton and cannot exist free in solution; it attaches immediately to a water molecule to give the hydronium ion, $\mathrm{H_3O^+}$. That is exactly what (iv) says.

Statement (iii) is false, and importantly so: an acid gives hydrogen ions and the anion of the acid, here chloride. Hydroxyl ions come from bases. A solution containing appreciable amounts of both would not be acidic at all.

Question 28

Which of the following statements is true for acids?

Solution. Two properties have to be right at once, and the four options cover all four combinations, so both must be settled.

Taste. Acids are sour — the word “acid” comes from the Latin for sour, and lemon juice, vinegar and curd are the everyday evidence. Bases are bitter and feel soapy.

Litmus. Acids turn blue litmus red. Bases turn red litmus blue.

Sour and blue-to-red is option (c). A memory hook that survives exam pressure: both of the acid’s properties involve red — sour like a red berry, and it turns litmus red.

Option (a) describes a base completely, and (b) and (d) each mix one property of an acid with one of a base.

Question 29

Which of the following are present in a dilute aqueous solution of hydrochloric acid?

Solution. This is Question 27 asked in the form of a product list, and the equation answers it directly:

$$\mathrm{HCl} + \mathrm{H_2O} \rightarrow \mathrm{H_3O^+} + \mathrm{Cl^-}$$

Hydrochloric acid is a strong acid, so in dilute solution it is essentially completely ionised — which disposes of option (d). What is present is the hydronium ion and the chloride ion, option (a).

Options (b) and (c) both contain the hydroxide ion, and that is the error the question is looking for. Hydroxide is the characteristic ion of a base. Water does contain a vanishingly small amount of it, but it is not something hydrochloric acid produces, and naming it as a constituent of an acid solution misses the definition of an acid entirely.

Question 30

Identify the correct representation of reaction occurring during chloralkali process

Solution. All four options have the same substances and the same coefficients. Only the state symbols differ, so the question is entirely about getting those right, and each wrong option carries exactly one error.

The process is the electrolysis of brine, which is a concentrated aqueous solution of sodium chloride — so $\mathrm{NaCl}(aq)$, not $\mathrm{NaCl}(l)$. That kills (a), which shows molten salt; molten sodium chloride is a different electrolysis with different products, and it also writes sodium hydroxide as a liquid rather than in solution.

Water is the solvent and is a liquid, so $\mathrm{H_2O}(l)$. Option (b) writes $\mathrm{H_2O}(aq)$, which is meaningless — water cannot be dissolved in itself.

Chlorine and hydrogen are both gases at room temperature and bubble off at the electrodes, so both are $(g)$. Option (c) writes them as $(aq)$, which would mean they stayed dissolved, and then there would be nothing to collect.

Option (d) has all four right: aqueous brine, liquid water, aqueous sodium hydroxide, and the two gases.

Common mistakes

• Reversing the pH scale in Question 14. Lower pH means a stronger acid, not a weaker one, because pH counts hydrogen ions on a reversed scale. Statements (i) and (iii) of that question are the reversed version, and they reappear as distractors in Questions 7, 15 and 26.
• Confusing baking soda with baking powder. Baking soda is $\mathrm{NaHCO_3}$ alone; baking powder is that plus tartaric acid. Questions 4, 12 and 25 all depend on the distinction, and so do Questions 37, 38 and 44 in the other band.
• Assuming only acids conduct electricity, in Question 20. Any solution with free ions conducts. Sodium hydroxide is a strong base and a better conductor than many acids.
• Treating the $\mathrm{-OH}$ in ethanol as a hydroxide in Question 23. A hydroxide releases $\mathrm{OH^-}$; a covalently bonded hydroxyl group does not. $\mathrm{C_2H_5OH}$ is neutral.
• Naming hydroxide ions in an acid solution, in Question 29. An acid gives $\mathrm{H_3O^+}$ and the anion of the acid. Option (b) there and statement (iii) in Question 27 are the same mistake twice.
• Putting a strong base on an acid burn, in Question 9. The chemistry is right and the first aid is dangerous. Water first, then a mild base — sodium hydrogencarbonate, never sodium hydroxide.
• Reading past the state symbols in Question 30. When every option has the same equation, the state symbols are the question. Brine is $(aq)$, water is $(l)$, and the two products collected at the electrodes are $(g)$.
• Forgetting that lime is an oxide, in Question 18. Only carbonates and hydrogencarbonates give carbon dioxide with acid. If there is no carbon in the formula there can be no carbon dioxide in the products.

Practise next

• Exemplar Short and Long Answer Questions — the same named chemicals, but now you have to produce formulae and equations rather than recognise them, which is where Questions 4, 11 and 25 above really get tested.
• NCERT textbook Chapter 2 — Acids, Bases and Salts for the end-of-chapter exercise on indicators, pH and the family of salts.
• Textbook in-text questions for the short section checks, which cover the chlor-alkali process of Question 30 and the formulae of Question 4.
• Textbook activities for the conductivity experiment of Question 20 and the olfactory indicators of Question 17, both of which are done there rather than described.