Acids, Bases and Salts — Practice

Question 1

A solution turns red litmus blue. Its pH is likely to be

Accepted Answer

10

Question 2

If a base dissolves in water, by what name is it better known?

Accepted Answer

alkali

Question 3

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

Accepted Answer

HCl

Question 4

Which of the following types of medicine is used for treating indigestion?

Accepted Answer

antacid

Question 5

Distilled water does not conduct electricity. Why?

Accepted Answer

Conduction through a solution requires mobile ions. Distilled water contains no dissolved acid, base or salt, so it has essentially no free H3O+ or OH- ions to carry charge, and the bulb does not glow. Adding even a small amount of an acid or a base supplies ions and the solution then conducts.

Question 6

An acid or a base is mixed with water. Is the process exothermic or endothermic?

Accepted Answer

Exothermic. In Activity 9, adding a few drops of concentrated H2SO4 to water makes the test tube hot, and the same happens with sodium hydroxide pellets. Because so much heat is released, a concentrated acid must always be added slowly to water with constant stirring, never the other way round.

Question 7

Five solutions A, B, C, D and E tested with universal indicator showed pH of 4, 1, 11, 7 and 9 respectively. Classify them as neutral, strongly alkaline, strongly acidic, weakly acidic and weakly alkaline.

Accepted Answer

D with pH 7 is neutral. C with pH 11 is strongly alkaline. B with pH 1 is strongly acidic. A with pH 4 is weakly acidic. E with pH 9 is weakly alkaline.

Question 8

How does baking powder make a cake soft and spongy?

Accepted Answer

Baking powder contains sodium hydrogen carbonate along with Ca(H2PO4)2 and starch. On heating during baking, the NaHCO3 decomposes as 2NaHCO3 -> Na2CO3 + H2O + CO2. The carbon dioxide produced rises through the dough as bubbles, and those trapped bubbles make the cake or bread smooth and spongy.

Question 9

Magnesium ribbons of the same size are taken in test tubes A and B. Hydrochloric acid is added to A and acetic acid to B, both at the same concentration. In which tube will the fizzing occur more vigorously, and why?

Accepted Answer

In test tube A, with hydrochloric acid. At the same concentration HCl is a strong acid and dissociates almost completely, giving a much higher concentration of H3O+ ions than acetic acid, which is weak and dissociates only slightly. Since the fizzing is the evolution of hydrogen from the reaction of H+ with magnesium, more H3O+ means a faster reaction and more vigorous fizzing.

Question 10

Fresh milk has a pH of 6.6. Explain why the pH changes as it turns into curd. A milkman adds a small amount of baking soda to fresh milk — why, and why does that milk take longer to set as curd?

Accepted Answer

As milk turns into curd, bacteria ferment the lactose in it and produce acid. The extra hydrogen ions lower the pH, so the curd is more acidic than the fresh milk and the pH falls below 6.6. The milkman adds baking soda, a mild base, to shift the pH of fresh milk from slightly acidic to slightly alkaline, which keeps the milk from turning sour quickly. Because it now starts from a higher pH, the acid produced during fermentation must first neutralise the added base before the pH falls far enough for curd to set, so setting takes longer.

Question 11

Why should Plaster of Paris be stored in a moisture-proof container?

Accepted Answer

Plaster of Paris is calcium sulphate hemihydrate, CaSO4.1/2H2O, made by heating gypsum at 373 K so that it loses part of its water of crystallisation. On contact with water it takes that water back and sets into a hard solid mass of gypsum, CaSO4.2H2O. If it is stored where moisture can reach it, the atmospheric moisture will set it in the container and it will harden into a useless lump, losing the property that makes it valuable for casts and moulds.

Practice — Acids, Bases and Salts

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