[Les the Scientist (OP)]

Right a 50:50 weight mixture of MeOH and EtOH.

MeOH has an s.g. of 0.791 g/ml
EtOH has an s.g. of 0.789 g/ml

Now I thought you could simply do a quick calculation to work out your new specific gravity.  In this case 0.790 g/ml.

A lad here at work says it doesn't work in practice.  Is he right ?

Like spirit levels, not too keen on chives though.

[gsmollin]

Conservation of mass tells us that the mass of the mixture or solution is the same as the mass of the constituents. It does not tell us anything about volume changes, however. For instance, water has a density of 1 g/ml. Dissolve some salt in it, and the density increases, but it is not the weighted average of the constituents. If you mix salt with sand, the density of the mixture is the weighted average of the density of the constituents. The difference is that in a solution, there is a phase change occuring in the constituents, which can affect their volumes. In a mixture, there is no phase change, so the densities remain the same.

OBTW, s.g. is a dimensionless ratio of density divided by density of water. You are using straight densities.

[Les the Scientist (OP)]

Right ok then so obviously it doesn't work for dissolving solids in liquids but in the case of mixing oraganic liquids it does work yeah ?

50g of MeOH - 63.21 mls
50g of EtOH - 63.37 mls

Total weight 100g, Total Volume 126.58 mls

density of mixture = 100/126.58 = 0.790 g/ml

[gsmollin]

Is that a question or a statement. I can't say it is true for organic liquids, in general. I can't say it is always false either. I'm surprised some chemist hasn't weighed in (little pun, snork-snork}.