The freezing point of water is also changed by adding substances to it, however all substances added
to water will act to decrease its freezing point. This is because any other substance present will act to
get in the way of the water molecules as they try to make bonds with each other to form ice, so
temperatures lower than freezing are required to freeze the water.
This explains why salt is often added to ice on the roads to encourage it to melt – it causes the ice will
melt at a lower temperature.
Non-solubility in water
Many molecules however do not interact with water. They are called “hydrophobic”, or water-hating.
These molecules are neutral, and do not interact with water because they are not attracted to the
water molecules, so they will not preferentially mix.
For example, oil does not mix with water. This can easily be seen if oil is added to water, the oil (which
is less dense) will float to the top and not mix with the water. The hydrophobic oil molecules will “stick
together” with special bonds (called hydrophobic interactions), and the hydrophilic water molecules will
“stick together” due to the hydrogen bonds.
If the mixture is vigorously shaken, the force of shaking will break the two liquids into smaller droplets,
and the tiny droplets of oil will temporarily disperse in the water. However, as soon as this force is
stopped, the oil droplets are free to move around and combine, and as the droplets become bigger
again the two layers separate out.
In order for a mixture of oil and water to stay stably dispersed, a special sort of molecule needs to be
added. These molecules are called surfactant molecules. They possess a hydrophilic head and a
hydrophobic tail, or in simpler terms they have one end of their molecule that is soluble in water, and
one end that is soluble in oil. The principal is that these molecules surround the tiny oil droplets by
making contact with their hydrophobic parts, leaving their hydrophilic parts to contact the water part
and keep the fat droplets very dispersed. Many foods contain such tensioactive molecules which are