If the forces were any different, the tendency to escape would change.Įxactly the same thing is true of the forces between two blue molecules and the forces between a blue and a red. If the red molecules still have the same tendency to escape as before, that must mean that the intermolecular forces between two red molecules must be exactly the same as the intermolecular forces between a red and a blue molecule. If the proportion of each escaping stays the same, obviously only half as many will escape in any given time. That means that there are only half as many of each sort of molecule on the surface as in the pure liquids. The diagram is for a 50/50 mixture of the two liquids. You might think that the diagram shows only half as many of each molecule escaping - but the proportion of each escaping is still the same. In an ideal mixture of these two liquids, the tendency of the two different sorts of molecules to escape is unchanged. At any particular temperature a certain proportion of the molecules will have enough energy to leave the surface. If you have a second liquid, the same thing is true. The smaller the intermolecular forces, the more molecules will be able to escape at any particular temperature. In a pure liquid, some of the more energetic molecules have enough energy to overcome the intermolecular attractions and escape from the surface to form a vapour.
These are mixtures of two very closely similar substances. There is actually no such thing as an ideal mixture! However, some liquid mixtures get fairly close to being ideal. The page will flow better if I do it this way around. Use the BACK button on your browser to return to this page when you are ready.Īn ideal mixture is one which obeys Raoult's Law, but I want to look at the characteristics of an ideal mixture before actually stating Raoult's Law. Important: If you haven't already read the page about saturated vapour pressure, you should follow this link before you go on. The page explains what is meant by an ideal mixture and looks at how the phase diagram for such a mixture is built up and used. It covers cases where the two liquids are entirely miscible in all proportions to give a single liquid - NOT those where one liquid floats on top of the other (immiscible liquids). This page deals with Raoult's Law and how it applies to mixtures of two volatile liquids. RAOULT'S LAW AND IDEAL MIXTURES OF LIQUIDS Raoult's Law and ideal mixtures of liquids
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