Re: why does pure isopropyl alcohol evaporates faster then 70% pure isopropyl

How readily something evaporates is measured by its vapour pressure -- the amount of vapour in 
equilibrium with the liquid.

Isopropanol has a higher vapour pressure than water. Isopropanol boils at 82 deg C, water at 100 
deg C. That is the temperature at which the vapour pressure becomes 1 atmosphere. But the 
vapour pressure of water falls off more rapidly with temperature decreasing below 100 deg C than 
the temperature rate of fall-off in the vapour pressure of isopropanol. This is because water has a 
higher heat of vaporization.

So isopropanol has a much higher room temperature vapour pressure than water.

If there is an "ideal solution" of two liquids, then the vapour pressure varies linearly with mole 
fraction. That is, for a mixture with equal numbers of water and isopropanol molecules, the vapour 
pressure will be halfway between that of pure water and that of pure isopropanol, if the 
solution is "ideal". This mixture of equal numbers of molecules works out to be 77% 
isopropanol, because molecules of isopropanol are much bigger and heavier than water 
molecules. In fact, the solution is far from ideal, and an 88% isopropanol in water solution actually 
boils at a lower temperature than pure isopropanol. But by the time you get down to 70% 
isopropanol, the lower room temperature vapour pressure of water is haing a major effect, and the 
vapour pressure of 70% isopropanol is significantly lower than that of pure isopropanol.

The rate of evaporation is much more complicated: apart from the vapour pressure, which is a 
major factor, it depends on air currents at the surface of the liquid, local cooling in the liquid 
where the evaporation is taking place, and currents within the liquid. You probably know that you 
can speed up evaporation a little bit by stirring a liquid, rather more by making waves at the liquid 
surface, and a lot more by blowing a gentle air current over the surface of a liquid. Isopropanol 
vapour is denser than water vapour, and will be more likely to form an immobile layer at the liquid 
surface, reducing evaporation. Evaporation of water will cause more local chilling than evaporation 
of isopropanol, slightly reducing evaporation. But as long as you take steps to see that other 
conditions are matched when you are comparing the evaporation of two liquids, the vapour 
pressure factor will probably dominate.