(a) A few crystals of ferrous sulphate were taken in a dry boiling tube and heated. Tiny water droplets were observed in the tube after some time. (i) From where did these water droplets appear? Explain. (ii) What colour change will be observed during heating? (iii) How many molecules of water are attached per molecule of $\mathrm{FeSO_4}$ crystal? Write the molecular formula of crystalline forms of (I) Copper sulphate, and (II) Sodium carbonate. (iv) State how is Plaster of Paris obtained from gypsum. Write two uses of Plaster of Paris. OR (b) An acid 'X' present in tamarind when mixed with 'Y', produces a mixture 'Z'. 'Z' on addition to a dough when heated makes cakes soft and spongy. 'Y' is prepared from common salt and helps in faster cooking. (i) Write the common names of 'X', 'Y' and 'Z', and the chemical formula of 'Y'. (ii) How is 'Y' prepared and how does it help in making cakes soft and spongy? Illustrate the reaction with suitable chemical equation. (iii) Write the name and chemical formula of a mild base other than 'Y' used as an antacid.

PART (a)

(i) Origin of Water Droplets:

The water droplets appeared due to the loss of water of crystallization from the hydrated ferrous sulphate crystals (green vitriol, $\mathrm{FeSO_4 \cdot 7H_2O}$). On heating, these water molecules escape as water vapour and condense on the cooler upper inner walls of the dry boiling tube.

(ii) Colour Change:

• The initial color of the ferrous sulphate crystals is green.
• Upon mild heating, it loses water of crystallization and turns white (anhydrous $\mathrm{FeSO_4}$).
• On further strong heating, the anhydrous salt decomposes into a reddish-brown solid (ferric oxide, $\mathrm{Fe_2O_3}$) with the evolution of suffocating gases ($\mathrm{SO_2}$ and $\mathrm{SO_3}$).

(iii) Water Molecules and Chemical Formulae:

• There are $7$ molecules of water attached per molecule of $\mathrm{FeSO_4}$ crystal.
• (I) Crystalline Copper sulphate: $\mathrm{CuSO_4 \cdot 5H_2O}$
• (II) Crystalline Sodium carbonate: $\mathrm{Na_2CO_3 \cdot 10H_2O}$

(iv) Plaster of Paris from Gypsum:

Plaster of Paris (calcium sulphate hemihydrate) is obtained by heating gypsum (calcium sulphate dihydrate) carefully at a controlled temperature of $373\text{ K}$ ($100^\circ\text{C}$):

$$\mathrm{CaSO_4 \cdot 2H_2O \xrightarrow{373\text{ K}} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O}$$

• Two uses of Plaster of Paris:

1. Used by doctors as a plaster support for fractured bones to keep them in the correct position.
2. Used for making decorative items, toys, casts, and for making building surfaces smooth.

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OR

PART (b)

(i) Identification of X, Y, and Z:

• Acid 'X': Tartaric acid (present in tamarind).
• Compound 'Y': Baking soda (Sodium hydrogencarbonate). Chemical formula: $\mathrm{NaHCO_3}$.
• Mixture 'Z': Baking powder (a mixture of baking soda and a mild edible acid like tartaric acid).

(ii) Preparation of 'Y' and its Role in Baking:

• Preparation of Baking Soda ('Y'): It is prepared on an industrial scale using sodium chloride, ammonia, water, and carbon dioxide (Solvay process):

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

• How it makes cakes soft and spongy:

When baking powder ('Z') is mixed with water in a cake dough and heated, sodium hydrogencarbonate ($\mathrm{NaHCO_3}$) reacts with hydrogen ions ($\mathrm{H^+}$) from the tartaric acid:

$$\mathrm{NaHCO_3(aq) + H^+(aq) \rightarrow CO_2(g) + H_2O(l) + \text{Sodium salt of acid}}$$

The carbon dioxide gas ($\mathrm{CO_2}$) generated during this reaction forms bubbles in the wet dough, causing it to expand and rise, making the cake soft and spongy.

(iii) Alternative Antacid:

• Name: Magnesium hydroxide (Milk of Magnesia)
• Chemical Formula: $\mathrm{Mg(OH)_2}$