Acids, Bases and Salts — Important Questions

Answer: (c) Baking soda ($NaHCO_3$)

Baking soda has no water molecules attached to its formula unit, so it has no water of crystallisation. The other three salts are hydrated, shown by the $\cdot nH_2O$ in their formulae.

Answer: (d) 10

A solution that turns red litmus blue is basic, so its pH must be greater than 7. Among the options, only pH 10 is basic.

Answer: (a) Gas released when zinc reacts with dilute sulphuric acid

Metal + dilute acid gives hydrogen gas, which burns with a pop sound: $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$. The other options release $CO_2$, which extinguishes a burning matchstick.

Answer: (c) 16 mL

Neutralisation depends on the amount of acid and base. If 10 mL NaOH needs 8 mL HCl, then doubling the base to 20 mL needs double the acid, i.e. $8 \times 2 = 16$ mL of the same HCl.

Answer: (a) Both A and R are true and R is the correct explanation of A

Mixing acid with water releases a large amount of heat (exothermic). If water is added to concentrated acid, the heat produced can make the mixture splash out or crack the container. Adding acid slowly to water lets the heat spread out safely, so R correctly explains A.

The acidic behaviour of an acid is due to the hydrogen ions ($H^+$) it produces, and these ions are formed only in the presence of water.

• Dry HCl gas cannot ionise as there is no water, so no $H^+$ ions are released and dry litmus stays unchanged.
• Hydrochloric acid (HCl dissolved in water) releases $H^+$ (as $H_3O^+$) ions, which turn blue litmus red.

Water of crystallisation is the fixed number of water molecules chemically combined in one formula unit of a salt, which gives the crystal its shape and often its colour.

• Washing soda: $Na_2CO_3 \cdot 10H_2O$ (10 water molecules)
• Blue vitriol (copper sulphate crystals): $CuSO_4 \cdot 5H_2O$ (5 water molecules)

On heating, these salts lose their water of crystallisation; e.g. blue $CuSO_4 \cdot 5H_2O$ turns white.

Preparation: Baking soda (sodium hydrogencarbonate, $NaHCO_3$) is made by passing carbon dioxide through a cold, concentrated solution of sodium chloride saturated with ammonia:

$NaCl + H_2O + CO_2 + NH_3 \rightarrow NH_4Cl + NaHCO_3$

Two uses:

1. As an antacid — being mildly basic, it neutralises excess acid in the stomach and relieves acidity.
2. In baking — on heating it releases $CO_2$ which makes cakes and bread soft and spongy: $2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2$. (It is also used in soda-acid fire extinguishers.)

(a) Tooth decay: Bacteria in the mouth produce acids from sugar. When the pH in the mouth falls below 5.5, the tooth enamel (calcium phosphate) begins to dissolve. Using an alkaline (basic) toothpaste neutralises this acid and prevents decay.

(b) Aquatic life: Living organisms survive only in a narrow pH range close to 7. Acid rain lowers the pH of river and lake water, which can be harmful and even fatal to fish and other aquatic organisms.

(c) Self-defence: A bee sting injects an acid causing pain; rubbing a mild base like baking soda gives relief. A nettle plant injects methanoic acid; rubbing the leaf of a dock plant (a base) relieves the sting.

(a) The pH will decrease (become more acidic). During curdling, bacteria convert the milk sugar (lactose) into lactic acid, and the increase in $H^+$ ions lowers the pH below 6.

(b) Baking soda ($NaHCO_3$) is mildly basic. Adding a little of it makes the milk slightly alkaline (pH shifts above 6), which keeps the milk fresh for a longer time and prevents it from turning sour quickly.

(c) Because the milk has been made alkaline, the acid produced by bacteria must first neutralise this extra base before the pH can fall enough for the milk to set. Hence the curd takes a longer time to form.

(a) The chlor-alkali process is the electrolysis of a concentrated aqueous solution of sodium chloride (brine). It is named 'chlor-alkali' because two of its products are chlorine gas and the alkali sodium hydroxide.

(b) Balanced equation:

$2NaCl(aq) + 2H_2O(l) \rightarrow 2NaOH(aq) + Cl_2(g) + H_2(g)$

• At the anode (positive): chlorine gas, $Cl_2$
• At the cathode (negative): hydrogen gas, $H_2$
• In the solution: sodium hydroxide, $NaOH$

(c) Uses (one each):

• $NaOH$: making soaps and detergents (or de-greasing metals).
• $Cl_2$: water treatment / disinfecting drinking water (or making bleaching powder).
• $H_2$: as a fuel / making ammonia for fertilisers (or hydrogenation of oils to make vanaspati).

(a) With zinc metal (metal + acid → salt + hydrogen):
$Zn + 2HCl \rightarrow ZnCl_2 + H_2$

(b) With sodium carbonate (carbonate + acid → salt + water + $CO_2$):
$Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$

(c) With sodium hydrogencarbonate:
$NaHCO_3 + HCl \rightarrow NaCl + H_2O + CO_2$

(d) With sodium hydroxide (neutralisation, base + acid → salt + water):
$NaOH + HCl \rightarrow NaCl + H_2O$

(e) With copper oxide (metal oxide + acid → salt + water; blue-green solution formed):
$CuO + 2HCl \rightarrow CuCl_2 + H_2O$

Common to (b) and (c): both release carbon dioxide gas, which turns lime water milky, confirming a carbonate/hydrogencarbonate.

(a) The powder is Plaster of Paris (POP); its formula is $CaSO_4 \cdot \frac{1}{2}H_2O$ (calcium sulphate hemihydrate).

(b) $CaSO_4 \cdot 2H_2O \xrightarrow{373K} CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O$

(c) On adding water, Plaster of Paris takes up water and is converted back into gypsum ($CaSO_4 \cdot 2H_2O$), a hard solid mass. This setting reaction fixes the shape.

(d) In a damp place it would slowly absorb moisture from the air, set into a hard mass of gypsum in the container itself, and become useless. Hence it is kept in a moisture-proof (airtight) container.