Vapor pressure and colligative properties

In a closed container, some molecules at a liquid's surface escape into the gas above it while others return, until the two rates balance; the pressure of that gas is the liquid's vapor pressure. Vapor pressure rises with temperature, and a liquid boils when its vapor pressure equals the pressure of the air above it. That is why water boils below 100 degrees C at high altitude, where air pressure is lower, and why a pressure cooker, which raises the pressure, cooks food faster at a higher temperature. Dissolving a solute changes some properties of a solvent in ways that, in dilute solutions, depend mainly on how many solute particles are present, not on what they are; these are called colligative properties. Dissolved particles lower the solvent's vapor pressure, raise its boiling point, and lower its freezing point. Salt spread on icy roads lowers the freezing point of water so ice melts below 0 degrees C, and antifreeze in a car's radiator keeps the coolant from freezing in winter and from boiling over in summer. Because each unit of table salt splits into two ions in water, salt has roughly twice the effect of the same number of dissolved sugar molecules, which stay whole.

This explanation is AI-generated and can occasionally be wrong. 🚩 Report