Solution for CBSE Class 12 Chemistry

(a) Explanation based on Henry's Law:

Step 1 (Governing Law): [0.5 Mark]

Henry's Law states that the partial pressure of a gas in vapour phase ($p$) is proportional to the mole fraction of the gas ($x$) in the solution: $$p = K_H \cdot x$$ where $K_H$ is the Henry's law constant.

Step 2 (Application to Bends and Anoxia): [1.0 Mark]

• (i) Bends: Scuba divers breathing compressed air at high underwater pressure experience increased solubility of atmospheric gases (especially $N_2$) in their blood. When they surface rapidly, the external pressure decreases, causing the dissolved gases to suddenly bubble out of the blood. These bubbles block capillaries and create painful, dangerous medical conditions known as "bends."
• (ii) Anoxia: At high altitudes, the partial pressure of oxygen is significantly lower than at sea level. According to Henry's Law, this lower partial pressure leads to a lower concentration of oxygen in the blood and tissues of people living at or climbing high altitudes, causing weakness and an inability to think clearly, a condition called anoxia.

Step 3 (Final Concluded Answer): [0.5 Mark]

Both phenomena are direct consequences of the solubility of gases being directly proportional to the partial pressure of the gas above the liquid surface, as defined by $p \propto x$.

OR

(b) Sprinkling of salt on snow-covered roads:

Step 1 (Governing Principle): [0.5 Mark]

The process involves the Depression in Freezing Point, which is a colligative property. The addition of a non-volatile solute (salt) to a solvent (water) lowers the freezing point of the solvent.

Step 2 (Mathematical Working): [1.0 Mark]

The depression in freezing point ($\Delta T_f$) is given by: $$\Delta T_f = K_f \cdot m$$ where $K_f$ is the cryoscopic constant and $m$ is the molality of the solution. By adding salt, the freezing point of water drops below $0^\circ\text{C}$ (e.g., to $-10^\circ\text{C}$ or lower), causing the ice to melt even if the ambient temperature is slightly below the normal freezing point of water.

Step 3 (Final Concluded Answer): [0.5 Mark]

The colligative property involved is Depression in Freezing Point. This lowers the melting point of ice, turning it into liquid water and clearing the roads.

💡 EL10 Concept Insight:

Think of salt particles as "gatecrashers" that get in the way of water molecules trying to freeze into a solid ice crystal; because they block the formation of the crystal structure, the water stays liquid at much colder temperatures!