raoult's law demonstration

[hermanntrude]

I've been thinking of a demo I want to do to show the distillation fo two volatile substances. I'd like to find a pair of volatile substances which have different boiling points, no azeotropes (or at least none which will cause any errors), and here's the tricky part: they have to be easy to spot the relative concentrations.

 

So the idea is I'll have a mixture, it'll be obvious (perhaps by smell, colour or pH) that it's got both substances in it. I'll distil it for a while and it'll then be obvious that the new mixture is more concentrated in the more volatile substance. Ideally, the concentration of one substance would be fairly easy to quantify, at least on a rough scale.

 

I wondered if perhaps an acid in water might work, despite probably not being very close to ideal... or perhaps there's a coloured volatile liquid I could use? perhaps density would be an indicator? suggestions anyone?

[Mr Skeptic]

Well, ethanol and water would be a fun option. The concentration can be tested by smell or taste

[hermanntrude]

true. That was one idea I had and as long as i steer clear of the 95% area there won't be any azeotropes. However, it'd be best if I could quantify it so I could calculate the concentration as a prediction beforehand.

 

i'm thinking pH, but i don't know whether the more volatile acids form azeotropes

[ercdndrs]

You could probably do it with food colouring in water.

[hermanntrude]

You could probably do it with food colouring in water.

 

You can certainly distil the coloring from water. However, the coloring is a nonvolatile solid dissolved in water. So when distilled it'd immediately be totally separated. I want to show that with two volatile substances, the mixture you get from the condenser is richer in the more volatile substance but not pure.

 

The best I have so far is methanol and water, and setting fire to the more methanol-ly mixture. trouble with that is that it probably won't obey raoult's law because i doubt it's an ideal solution.