Re: Why does alcohol freeze and boil at a lower temperature than water

A quick answer is that alcohol freezes and boils at a lower temperature than water because its molecules are not held together as tightly as the molecules of water.  Here is an explanation::

Let's start with boiling.  When a liquid boils, it changes from a liquid to a gas.  What is the difference between these two states of matter?  In a liquid, the molecules it is made of are pretty close together. You can think of them as stuck"" together.  But in a gas the molecules are very far apart.  So a liquid doesn't go flying all around like a gas does.  A liquid stays together.

How does the change from liquid to gas occur?  It requires energy (that's why you have to heat it), and this energy makes the molecules move faster.  As they move faster they bounce off each other more and are able able to fly away as a gas.  So, alcohol boils at a lower temperature because it takes less energy to makes the alcohol molecules "unstick" from each other.

Why is this?  Why do water molecules stick together more tightly than alcohol molecules?  You probably know that the formula for water is H₂O.  That is, 2 hydrogen atoms and 1 oxygen atom.  These atoms are put together like this:  H-O-H.  Now, it happens that the H atoms have a positive (+) charge on them, and the oxygen atom has a negative (-) charge.  You probably know that opposite charges attract.  So the H atoms of one water molecule are attracted to the O atoms of another water molecule, and all of the water molecules are stuck together, like this:
 

Here, the solid lines are the bonds between atoms within the water molecules, and the dashed lines are the attractions between one water molecule and another. These attractions add up to something very strong, so it is hard to separate water molecules from each other and it takes a high temperature to boil water.

Alcohol molecules are also held together in a similar way, but the attractions are weaker.  Here's why:  The formula for ethyl alcohol (the kind that adults drink) is C₂H₅OH.  Notice that this formula has OH in it just as in water.  So alcohol molecules can stick together, too, like this:

But notice that the C2H5 part does not have attractions for other molecules. So the alcohol molecules are not held together by as many attractions as are the water molecules.  So it takes less energy to separate or "unstick" alcohol molecules from each other, and alcohol boils at a lower temperature.

What about freezing and melting?  The same thing.  When a solid melts, its molecules go from being held together very tightly to being held together less tightly.  Again, it takes more energy to do this for water than for alcohol, so the melting point for water is higher than for alcohol.

David Reibstein, Outreach Director, Princeton Materials Institute, Princeton University