Chemistry Reference
In-Depth Information
A gelatine gel is made with warm water. Gelatine is a
protein. Proteins are natural polymers (Chapter 15,
p. 243) and the molecules of protein are very large.
The large molecules disperse in water to form a gel.
As the gelatine-in-water mixture cools, the gelatine
molecules are attracted to each other and form a
continuous network. In this way, the jelly you eat as a
pudding is formed. The kind of gel which you put into
your hair is made from water and an oil (Figure 2.37).
A
sol
is similar to a gel; however, the mixture will
flow, for example emulsion paint, or PVA glue.
When you pour out a glass of fizzy drink, the frothy
part at the top of the drink is a gas/liquid mixture
called a
foam
. The gas, carbon dioxide, has formed
tiny bubbles in the liquid but has not dissolved in
it. If left to stand, foams like this one collapse as the
tiny bubbles join together to form bigger bubbles
which then escape. It is possible to form solid foams
where the gases are trapped in a solid structure. This
happens in foam rubber and bread (Figure 2.38).
Emulsions
are mixtures of liquids which are
immiscible. Earlier in this chapter you found out that
when two liquids are immiscible they do not mix but
form two different layers. Oil and water are like this
but if you shake the mixture it becomes cloudy.
The apparent mixing that you see is due to the
fact that one of the liquids has been broken into tiny
droplets which float suspended in the other liquid. If
the mixture of oil and water is now left to stand the
two layers will re-form. To make emulsions, such as
mayonnaise, an
emulsiier
is used to stop the droplets
joining back together again to form a separate layer.
The emulsifier used when making mayonnaise is egg
yolk. In many countries of the world, if you examine
the ingredients on the side of many packets found in
kitchen cupboards you will find that emulsifiers have
'E-numbers' in the range E322 to E494. For example,
ammonium phosphatide E442 is used as the emulsifier
in cocoa and chocolate. Other food additives such as
colourings and preservatives are also given E-numbers
but in different ranges to that of the emulsifiers.
It is worth noting that gels, foams and emulsions
are all examples of different kinds of solutions. In true
solutions the two phases completely mix together but
in these systems the two phases are separate.
Figure 2.37
Hair gel is a mixture of water and an oil plus a perfume.
Figure 2.38
Examples of solid foams.
