Question

Ethylene glycol ($\text{ethane-1,2-diol}$) is widely added to car radiators as an antifreeze agent. How does its addition protect the engine in extreme climates?

• A. It raises the freezing point of water and lowers its boiling point.
• B. It lowers the freezing point of water and raises its boiling point.
• C. It lowers both the freezing point and the boiling point of water equally.
• D. It reacts chemically with the metal radiator walls to reduce thermal expansion.

Hint:

Solute addition expands the liquid temperature range of a solvent by pushing $T_f$ down and pulling $T_b$ up.

Answer 1

The Correct Option is B

Concept: When a non-volatile solute is dissolved into a volatile liquid solvent, it changes the physical behavior of the solution. The solute particles occupy space at the liquid surface, decreasing the rate of evaporation and lowering the overall vapor pressure of the solvent. This shift in vapor pressure leads directly to changes in two key colligative properties: \[ \Delta T_f = K_f \cdot m \quad (\text{Depression of Freezing Point}) \] \[ \Delta T_b = K_b \cdot m \quad (\text{Elevation of Boiling Point}) \]

Step 1: Evaluating the physical mechanism behind freezing point depression.
In pure water, molecules readily assemble into a rigid, structured crystalline lattice at $0^\circ\text{C}$. When ethylene glycol ($\text{HO-CH}_2\text{-CH}_2\text{-OH}$) is added, its molecules form hydrogen bonds with water, disrupting the regular intermolecular organization of the solvent. To overcome this disruption and force crystallization, thermal energy must be removed from the system. This drops the freezing point significantly below $0^\circ\text{C}$ ($\Delta T_f$), preventing the coolant from freezing and cracking the radiator block in cold climates.

Step 2: Evaluating the physical mechanism behind boiling point elevation.
Simultaneously, because the non-volatile ethylene glycol solute lowers the solution's vapor pressure, a higher temperature is required for the vapor pressure to equal the external atmospheric pressure. This elevates the boiling point of the radiator fluid significantly above $100^\circ\text{C}$ ($\Delta T_b$). Together, these two property shifts widen the liquid temperature range of the coolant, protecting the engine from both freezing in winter and boiling over in summer.