A mixture of oil and water usually separates quickly, but a range of substances act as emulsifiers. In this activity, students test a range of things commonly found in the kitchen to see which stabilises an oil and water emulsion. Colloids such as these are often found in foods.
This practical is very straightforward and does not take very long, although if students shake the boiling tubes too vigorously then the mixtures can take a while to separate. It is probably worth ensuring that students understand the meaning of the terms ‘emulsifier’ and ‘emulsion’ before they begin. They should be encouraged to record the results clearly, which probably means a results table.
Students should be warned against tasting anything – e.g. the sugar – in the laboratory. Eggs have a salmonella risk and should be marked with the lion symbol. Raw egg should be handled as little as possible, and a disposable pipette should be used to transfer it to the boiling tubes.
Mustard powder (Note 4)
Egg white (Note 5)
Other substances can be used if preferred
Refer to Health & Safety and Technical notes section below for additional information.
Each experiment requires:
Boiling tubes and bungs (Note 1)
Disposable teat pipettes
Spatulas or small spoons
Cooking oil (Note 2)
Detergent (Note 3)
Health & Safety and Technical notes
1 Using boiling-tubes rather than test-tubes means that more chemicals are consumed, but it is easier to see what is going on and much easier to clean up. The boiling tubes must be very clean and must not be contaminated with detergent.
2 Corn oil is good because it is dark in colour and easier to see.
3 Cheaper detergents do not usually work very well.
4 Colman’s powder is good and powder lasts far longer than ordinary mustard so can be used from year to year.
5 If you use fresh eggs it is fairly easy to separate these. Ensure no yolk contaminates the white – the other way round is less important. Due to the salmonella risk, handling raw egg should be kept to a minimum, so provide disposable pipettes with the egg for students to transfer it to the boiling tubes.
a Put about 2 cm3 of oil into a boiling tube. Add about the same amount of water. Put a bung into the top of the tube and shake it – but not too vigorously. Remove the bung and leave the mixture to stand. Observe what happens.
b Repeat the experiment but add a small quantity of one of the substances you are testing before you shake the tube.
c Test all the substances in the same way to find out which acts as emulsifiers.
This experiment can easily be done in a kitchen as ‘making a salad dressing’ using oil and vinegar rather than oil and water. You can taste the resulting mixtures as well as observing them. If you do this, do not taste the ones containing raw egg.
An emulsifier is a substance that stabilises an emulsion (a mixture of one liquid dispersed in another.) Detergent, egg yolk and mustard are emulsifiers, the others are not. Students may observe colloidal mixtures in the other tubes, but they are not oil and water emulsions and two separate layers should be clearly seen.
Health & Safety checked February 2008
Page last updated on 31 July 2012