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Name: Lesley R.
Status: other
Age: 10
Location: N/A
Country: N/A
Date: 1/10/2004


Question:
I am working on a science fair project with my son regarding a correlation between viscosity of a liquid and its boiling point. I know that viscosity increases with decreasing temperatures and vice versa, however, we would like to know if there is an effect on the boiling point of a liquid based on its viscosity. (i.e., do thicker liquids take longer to boil than water?). If this is the case, please explain to us why that happens.


Replies:
Lesley,

In general, thick liquids are thick because their molecules have a good grip on each other -- either bonded to each other, or tangled up in each other. Thus, it is harder to pull them away from each other -- and that is, of course, what happens when a liquid boils. So, viscosity and boiling point are related by virtue on molecular interaction. Heating reduces viscosity because heating makes the molecules move faster. Under such conditions of increased molecular motion, their grip on each other weakens.

Regards,
ProfHoff 768


An interesting project. You are correct about the temperature dependence of viscosity. There could be some pathological examples, but in general viscosity decreases with increasing temperature. You have to be careful about what you mean by "thicker liquids take longer to boil". This could mean "normal boiling temperature" or it could mean "time for a given amount of solution to evaporate". I strongly recommend that you confine your experiments to "normal boiling temperature" because this is easier to measure, but more importantly, if you boil off a significant amount of a solution -- say glycerin and water -- the concentration of glycerin will increase as the boiling process occurs and this concentration will result in an increase in viscosity as well as a decrease in the partial pressure of water. If you confine yourself to the "normal boiling temperature" you can measure that before a significant amount of evaporation occurs, so the concentration of the solution is constant.

There is a difference too in whether you are going to measure the boiling point of pure liquids or the boiling point of solutions. In the case of pure liquids, the forces between molecules that influence the viscosity also influence its boiling point; however, that relation is very complicated, and there are a limited number of pure liquids that have large differences in viscosity too. And you have to be careful to measure the viscosity of the liquid at its boiling point -- not so easy. If you are measuring the boiling point of solutions, e.g. water glycerin, there is only one volatile component (water) and the temperature range of boiling you can control by adjusting the amount of glycerin in solution. In any case it will not be vastly different from 100C. You could also use a "thickener", e.g. polyvinyl alcohol, to adjust the viscosity. And of course, you may want to use some material than water to test.

The experimental concept is good, but it is not so easy as it may sound at first glance. Some precautions that you need to keep in mind are: Raoult's Law P = Po*X where P is the partial pressure of the solvent (assuming only one volatile component), Po is the vapor pressure of the pure solvent, and X is the mole fraction of solvent.

Clausius Clapyeron equation that describes how the vapor pressure changes with temperature as a result of a change in temperature (all other things like concentration being constant): ln(P2/P1) = -H/R[1/T2 - 1/T1] where P2 and P1 are the vapor pressure at T2 and T1 (in kelvins) respectively, H is the heat of vaporization, and R is the gas constant in the same units as H. The heat of vaporiztion itself changes with temperature but you can assume it is constant over the range of any experiment you will be doing.

Good Luck and Have Fun

Vince Calder



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