(A) (a) Give the chemical name and formula of Plaster of Paris.

(b) Write the chemical equation of its preparation.

(c) Give any two uses of it.

OR

(B) (a) Name the acid present in ant's sting.

(b) Give reason:

(i) While diluting an acid, it is recommended that the acid should be added to water.

(ii) Baking soda is used as an antacid.

This is an internal-choice question; a candidate answers either option (A) or option (B) fully.

Option (A) is grounded in Section 2.4.4 'Are the Crystals of Salts really Dry?': Gypsum, $CaSO_4.2H_2O$, on heating at 373 K loses water molecules and becomes calcium sulphate hemihydrate, $CaSO_4.\frac{1}{2}H_2O$, which is Plaster of Paris — 'the substance which doctors use as plaster for supporting fractured bones in the right position.' The chapter also states 'Plaster of Paris is used for making toys, materials for decoration and for making surfaces smooth.' The half-water-molecule notation arises because 'two formula units of $CaSO_4$ share one molecule of water,' explaining the $\frac{1}{2}H_2O$ in the formula and the coefficient $1\frac{1}{2}$ on the product side of the equation shown (the book gives the reverse, setting reaction: $CaSO_4.\frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4.2H_2O$).

Option (B) draws on two separate parts of the chapter: Table 2.3 ('Some naturally occurring acids') lists methanoic acid as the acid responsible for ant stings and nettle stings, consistent with the 'Self defence by animals and plants through chemical warfare' box, which notes 'Bee-sting leaves an acid which causes pain and irritation' and 'Stinging hair of nettle leaves inject methanoic acid causing burning pain.' Part (b)(i) is grounded in Activity 2.10 and its explanation: dissolving an acid in water is highly exothermic, and adding water to a concentrated acid (rather than the reverse) causes rapid, localised heating that can make the mixture splash out and cause burns, or crack the container. Part (b)(ii) is grounded in the 'pH in our digestive system' box and the 'Uses of Baking soda' list, which explicitly states that being alkaline, baking soda ($NaHCO_3$) neutralises excess stomach acid and provides relief from indigestion, hence its use as an antacid.