colligative properties pt 1

What makes a property 'colligative'?

It depends only on the number of dissolved particles, not their identity (count over kind).

Core idea of vapor pressure lowering (nonvolatile solute)

Adding solute reduces solvent molecules at the surface and increases solution entropy, lowering the tendency to escape; the new equilibrium has fewer vapor molecules and a lower vapor pressure.

Why does a concentrated solution 'grab' solvent vapor more strongly than pure solvent?

Stronger entropy gain from mixing and fewer free solvent molecules make vapor condense preferentially into the solution, keeping its liquid level higher in a closed system.

Raoult's law in words

Vapor pressure of the solvent in solution equals the pure-solvent vapor pressure times the solvent mole fraction (more dilution → proportionally lower vapor pressure).

Why do we use mole fraction in Raoult's law?

It directly represents how diluted the solvent is by solute in the liquid phase, which scales the escaping tendency of solvent molecules.

How do volatile solutes change the picture?

Both solvent and solute contribute to the vapor; total pressure is the sum of their partial pressures (Dalton), each following its own Raoult relation in ideal liquid-liquid solutions.

What makes an 'ideal solution' for Raoult + Dalton to work cleanly?

Solute-solvent interactions are similar in strength to solute-solute and solvent-solvent interactions (no special attractions or repulsions).

What causes positive or negative deviations from Raoult's law?

If solute-solvent attractions are weaker than like-like, molecules escape more easily (positive deviation, higher P). If stronger, fewer escape (negative deviation, lower P).

Where does the van't Hoff factor i enter for vapor pressure lowering?

Electrolytes dissociate into multiple particles, so effective solute particle count (and dilution) is higher than for one-to-one nonelectrolytes; larger i → larger vapor pressure lowering.

Quick reasoning: which solution has the lower vapor pressure?

The one with the larger effective solute molality or mole fraction (accounting for i): more particles → lower solvent mole fraction → lower P.

Why solids/large nonevolatile solutes lower vapor pressure but do not appear in the vapor?

They don't volatilize; they only dilute the solvent in the liquid, changing the liquid's composition and thus the solvent's escaping tendency.

Big connection to boiling point and freezing point (preview)

Lower vapor pressure at all temperatures means you need a higher temperature to reach 1 atm (boiling ↑), and the liquid resists ordering into a solid (freezing ↓).