Question 1

Why does a non-volatile solute raise the boiling point of a solution?

Accepted Answer

At a given temperature, dissolved non-volatile solute particles lower the solvent's vapour pressure. The solution must therefore be heated to a higher temperature before its vapour pressure equals the external pressure and boiling begins.

Question 2

Why does adding a solute lower the freezing point?

Accepted Answer

A dissolved solute changes the solid-liquid equilibrium of the solvent, so solid solvent and solution are no longer in equilibrium at the solvent's normal freezing point. The solution reaches this equilibrium at a lower temperature, producing freezing-point depression.

Question 3

What are the colligative constants of water?

Accepted Answer

For water, Kb is 0.512 K kg mol⁻¹ and Kf is 1.86 K kg mol⁻¹. These values are used in the boiling-point elevation and freezing-point depression equations, respectively.

Question 4

What shifts are produced by 0.2 m aqueous glucose?

Accepted Answer

Glucose is a non-electrolyte, so i = 1. The boiling-point elevation is 0.2 × 0.512 = 0.1024 K, while the freezing-point depression is 0.2 × 1.86 = 0.372 K.

Question 5

Are boiling-point elevation and freezing-point depression chemical changes?

Accepted Answer

No. For dilute solutions, they are physical colligative effects governed mainly by the number of dissolved solute particles. The solute is assumed non-volatile, and i is included when dissociation or association changes the effective particle count.

Boiling-Freezing Shift 3D Lab

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