MadSci Network: Chemistry

Re: Why does a vinegar/water solution freeze before just water?

Date: Fri Feb 15 10:59:32 2002
Posted By: Christopher Wilson, Staff, R&D, Cooper Vision
Area of science: Chemistry
ID: 1013448380.Ch

Hello Patrick,

What an excellent experiment!

I will answer your question in two parts, a general answer and then some 
specific details.

Freezing process

The most fundamental way to explain how freezing points can vary is based 
on the equilibrium (or balance if you like) of a system (in the cases you 
describe, you have four systems, 1. is water + vinegar, 2. is water + 
soap, 3. is water + water and 4. is water + salt).

When freezing a solution, there are two forces acting against one 
another.  The first is what is often termed as the 'external pressure', 
this is the pressure exerted by the surrounding air and this pushes down 
on to the surface of the solution.  The second force is called the 'vapour 
pressure' and this is caused by the molecules in the solution trying to 
push upwards (against the external pressure) and to evaporate.

All solutions exist in a state of balance or equilibrium, where some of 
the molecules manage to escape from the solution and form vapour 
(evaporation), and some of the molecules of the vapour are forced back 
into solution by the air (condensation).  This cycle repeats repeatedly, 
until the amount evaporating equals the amount condensing.  This is 
referred to as being in a 'dynamic equilibrium'.

If however, the conditions of the system are changed, then so will the 
rate of evaporation and condensation.  As the temperature is lowered, so 
the external pressure increases (and the vapour pressure decreases), and 
condensation occurs more quickly than evaporation.  In addition, as the 
temperature falls, the amount of energy in the solution decreases, in turn 
the molecules in the solution slow down.  At a certain point (freezing 
point of the particular solution), the molecules cease to move freely and 
the solution turns from a liquid to a solid, such as water turning into 

Solutions that freeze before water

To begin with, vinegar itself is actually a mixture of water and a 
chemical called acetic acid (or ethanoic acid).   The molecules of acetic 
acid form special types of bonds (called hydrogen bonds) with molecules of 
water.  This means that the acetic acid molecules will have several water 
molecules clustered around them.  The acetic acid holds the water, 
preventing it from evaporating so that the water molecules remain in the 
solution and reduce the vapour pressure.  As outlined above, when the 
vapour pressure falls, the molecules in the solution are forced together 
(cease to move freely) and the solution freezes.  Something similar 
happens with the soap and water solution.  The two ends of a soap molecule 
are very different to each other, one draws molecules of water to it 
(called hydrophilic end, or water-liking) and the other end repulses water 
away (called hydrophobic end, or water-hating).  The hydrophilic end of 
the soap molecule gathers water around it much as the acetic acid does 
with molecules of water.  In the same way, the soap stops the water from 
evaporating, reducing the vapour pressure and causing the solution to 

Solutions that freeze after water

The salt that dissolves into the water actually keeps the water molecules 
apart.  In this respect, the salt operates in the opposite way that the 
vinegar and soap do.  Because the water molecules are much more separated, 
they have more energy and evaporate much more easily.  This increases the 
vapour pressure.  To make the solution freeze, the water molecules must be 
held together.  By taking energy away from the water molecules (by cooling 
the solution down), causes the vapour pressure to decrease.  As these 
water molecules have more energy than water molecules with no salt 
present, more energy must be taken from them.  Therefore, the temperature 
must fall below the normal freezing point of water to make the salt 
solution freeze.

I hope this answers your question for you.

Chris Wilson, Research Chemist, Cooper Vision,
Southampton, England.

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