(a) On heating $X'$ at $373\mathrm{K}$ , it loses water molecules and becomes $Y'$ . $Y'$ is a substance which doctors use for supporting fractured bones in the right position. (i) Identify $X'$ and $Y'$ . (ii) How can $X'$ be reobtained from $Y'$ ? OR (b) Two solutions M and N give Red and Blue colour respectively with a universal indicator. (i) In which solution will the hydrogen ion concentration be more? Justify your answer. (ii) If both M and N solutions are mixed and the resultant mixture is tested with a universal indicator, it turns green. What is the nature of the salt formed? Justify your answer.

Q: (a) On heating $X'$ at $373\mathrm{K}$, it loses water molecules and becomes $Y'$. $Y'$ is a substance which doctors use for supporting fractured bones in the right position. (i) Identify $X'$ and $Y'$. (ii) How can $X'$ be reobtained from $Y'$? OR (b) Two solutions M and N give Red and Blue colour respectively with a universal indicator. (i) In which solution will the hydrogen ion concentration be more? Justify your answer. (ii) If both M and N solutions are mixed and the resultant mixture is tested with a universal indicator, it turns green. What is the nature of the salt formed? Justify your answer.

### Option (a) **(i) Identification of $X'$ and $Y'$:** * **$X'$** is **Gypsum** (Calcium sulphate dihydrate): $\mathrm{CaSO_4 \cdot 2H_2O}$ * **$Y'$** is **Plaster of Paris** (Calcium sulphate hemihydrate): $\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}$ **(ii) Reobtaining $X'$ from $Y'$:** Gypsum ($X'$) can be reobtained by mixing Plaster of Paris ($Y'$) with an adequate quantity of water. It sets into a hard solid mass of Gypsum within a short time according to the following chemical equation: $$\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O} + 1\frac{1}{2}\mathrm{H_2O} \rightarrow \mathrm{CaSO_4 \cdot 2H_2O}$$ --- ### OR ### Option (b) **(i) Hydrogen ion concentration in M and N:** * **Solution M** will have a **higher hydrogen ion concentration** ($[\mathrm{H^+}]$). * **Justification:** A red colour with a universal indicator indicates a highly acidic solution (low pH value). Since pH is inversely related to hydrogen ion concentration (lower pH means higher $[\mathrm{H^+}]$), the strongly acidic solution M has a significantly higher concentration of $\mathrm{H^+}$ ions than the basic solution N, which turns blue (indicating a high pH and low $[\mathrm{H^+}]$). **(ii) Nature of the salt formed:** * The salt formed is a **neutral salt**. * **Justification:** When solutions M (acidic) and N (basic) are mixed, they undergo a neutralisation reaction. The green colour with the universal indicator indicates that the resulting mixture has a pH of approximately $7$, which is neutral. This shows that the acid and base have completely nullified each other's effects, producing a neutral salt and water: $$\mathrm{H^+(aq)} + \mathrm{OH^-(aq)} \rightarrow \mathrm{H_2O(l)}$$

Acids, Bases and Salts

More about Salts and pH Scale

Official Answer

Option (a)

(i) Identification of $X'$ and $Y'$ :

$X'$ is Gypsum (Calcium sulphate dihydrate): $\mathrm{CaSO_4 \cdot 2H_2O}$
$Y'$ is Plaster of Paris (Calcium sulphate hemihydrate): $\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}$

(ii) Reobtaining $X'$ from $Y'$ :

Gypsum ( $X'$ ) can be reobtained by mixing Plaster of Paris ( $Y'$ ) with an adequate quantity of water. It sets into a hard solid mass of Gypsum within a short time according to the following chemical equation:

$\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O} + 1\frac{1}{2}\mathrm{H_2O} \rightarrow \mathrm{CaSO_4 \cdot 2H_2O}$

OR

Option (b)

(i) Hydrogen ion concentration in M and N:

Solution M will have a higher hydrogen ion concentration ( $[\mathrm{H^+}]$ ).
Justification: A red colour with a universal indicator indicates a highly acidic solution (low pH value). Since pH is inversely related to hydrogen ion concentration (lower pH means higher $[\mathrm{H^+}]$ ), the strongly acidic solution M has a significantly higher concentration of $\mathrm{H^+}$ ions than the basic solution N, which turns blue (indicating a high pH and low $[\mathrm{H^+}]$ ).

(ii) Nature of the salt formed:

The salt formed is a neutral salt.
Justification: When solutions M (acidic) and N (basic) are mixed, they undergo a neutralisation reaction. The green colour with the universal indicator indicates that the resulting mixture has a pH of approximately $7$ , which is neutral. This shows that the acid and base have completely nullified each other's effects, producing a neutral salt and water:

$\mathrm{H^+(aq)} + \mathrm{OH^-(aq)} \rightarrow \mathrm{H_2O(l)}$

GypsumPlaster of ParisCalcium sulphate hemihydrateWater of crystallisationUniversal indicatorHydrogen ion concentrationNeutralisationNeutral salt

Marking Scheme

1Option (a) (i): Correctly identifying X' as Gypsum and Y' as Plaster of Paris (with chemical formulas). (1 mark)
2Option (a) (ii): Explaining how Gypsum is reobtained by adding water and writing the balanced chemical equation. (1 mark)
3Option (b) (i): Identifying Solution M has higher H+ concentration with proper justification based on pH and colour. (1 mark)
4Option (b) (ii): Identifying the salt as neutral and justifying it using the green colour (pH = 7) of the universal indicator. (1 mark)

Hint

Recall that Plaster of Paris is a hemihydrate of calcium sulphate used for plastering fractured bones, and a green colour on the pH paper represents a neutral solution.

Quick Oral Answer

Plaster of Paris is calcium sulphate hemihydrate, $\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}$ . When mixed with water, it hydrates back into gypsum, forming a hard solid mass that supports fractured bones.

Analysis & Explanation

This question tests the practical application of chemical compounds of calcium (Gypsum and Plaster of Paris) and the interpretation of the pH scale using universal indicators.

Part (a): Gypsum ( $\mathrm{CaSO_4 \cdot 2H_2O}$ ) when heated carefully at $373\mathrm{K}$ ( $100^\circ\mathrm{C}$ ) loses $1\frac{1}{2}$ molecules of water of crystallisation to form Plaster of Paris ( $\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}$ ). POP is widely used by orthopaedic doctors to immobilise fractured bones. Adding water reverses this reaction, forming a hard interlocking crystalline network of Gypsum.
Part (b): Universal indicator is a mixture of several indicators showing a spectrum of colours at different pH values. Red indicates strong acidity (pH $1-3$ ), green indicates neutrality (pH $7$ ), and blue/violet indicates alkalinity (pH $9-14$ ). Mixing an acid and a base in stoichiometric amounts leads to neutralisation, yielding a neutral salt with a pH of $7$ (green).

Common Mistakes

1Writing the formula of Plaster of Paris with 2 water molecules instead of half.
2Forgetting to write the balanced chemical equation for the hydration of Plaster of Paris.
3Confusing blue colour with high hydrogen ion concentration (blue indicates high OH- concentration, not H+).

Interesting Facts

Plaster of Paris is so named because it was historically prepared by heating abundant deposits of gypsum found near Paris, France.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2023?

This question carries 2 marks in the CBSE Class 10 Science 2023 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Acids, Bases and Salts" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "More about Salts and pH Scale" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2023 paper?

This is a Very Short Answer question from Section B in the CBSE Class 10 Science 2023 paper.