Vapour Pressure Explorer

Two real liquids are mixed. Each one sends molecules into the vapour, so each adds a partial pressure. Together they set the total pressure above the mixture. Slide the composition and watch the plot build. Pick a liquid pair that mixes ideally, or one of the two pairs that do not.

Choose a liquid pair

Composition

Mole fraction of benzene x = 0.50

What you are seeing

Raoult's law

For an ideal solution, each liquid's partial pressure is its pure vapour pressure times its mole fraction: p = p° x. Add the two partials to get the total. Both partials are straight lines, so the total is a straight line joining the two pure pressures. Real mixtures bow that total above the ideal line (positive deviation) or below it (negative deviation).