Viscosity and Boiling Point
Name: Lesley R.
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
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.
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
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
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Update: June 2012