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Egg yolk is an emulsifier.


This GCSE Chemistry quiz is all about emulsions. Emulsions are mixtures of two or more liquids that are non-soluble with one another. They are immiscible. A good example is water and vegetable oil. These are non-soluble with each other so when you mix a vegetable oil with water, you will see two distinct layers. This is because many oils are less dense than water and therefore float on top. You make the oil and water mixtures into emulsions simply by shaking the two liquids together. The oil breaks up into droplets and becomes dispersed (scattered) through the water. We call this an 'oil in water' emulsion. If it is the water that disperses through the oil then you have a 'water in oil' emulsion. Whichever type of emulsion you make, leave it to stand and you will notice that it soon separates. We say that the emulsion 'breaks'.

It is possible to make a much more stable emulsion by using an emulsifier. This keeps the droplets dispersed for a much longer time. Emulsifiers are added to many foods in order to make sure that they are more appealing. Egg yolk contains a natural emulsifier which is what holds the vinegar and vegetable oil together in mayonnaise. Another common food additive is the emulsifier lecithin which is a totally natural emulsifier that is extracted from seeds. It isn't a single substance, it's a mixture.

For higher tier candidates, you need to know that an emulsifier is a molecule that has at least one part, the head, that likes to dissolve in water (hydrophilic or water loving) and at least one other part, the tail, that likes to dissolve in oil (hydrophobic or water hating). The hydrophilic end remains in the water and the hydrophobic end dissolves in the oil droplets, surrounding them and keeping them apart.

For the GCSE, you need to know some examples of where emulsions are used, but that's not too difficult as they are things you meet in your daily life - milk, salad dressings, paint, cosmetics ... the list is long. What is a little harder is to remember whether they are oil in water (milk, ice cream, salad cream, mayonnaise) or water in oil (margarine, butter, skin moisturising cream).

Try this quiz and see if you understand emulsions and how we can make mixtures of non-soluble liquids.

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What is an emulsion?
A mixture of two or more immiscible liquids, one dispersed in the other
A mixture of a solid dispersed through a liquid
A mixture of two compounds
A special type of compound
The two liquids will not mix and so droplets of one will be suspended in the other
Which of the following is NOT an example of an emulsion?
Emulsion paint
Gloss paint
The active ingredient in gloss paint (called the binder) is dissolved in a solvent. In emulsion paint, the binder is dispersed in water
What is an emulsifier?
A substance that stabilises an emulsion
A type of paint
A substance that enables others to separate
A substance that lowers the boiling point of water
Emulsifiers are found in many foods
An emulsifier molecule has two ends. Pick the correct combination of these two ends.
hydrophobic - head - water hating
hydrophilic - tail - water loving
hydrophobic - tail - water hating
hydrophilic - head - water loving
hydrophobic - tail - water loving
hydrophilic - tail - water hating
hydrophobic - tail - water loving
hydrophilic - head - water hating
Even an emulsion containing an emulsifier will eventually separate
Pick the combination for what the two ends of the emulsifier molecules do.
hydrophilic head - attracted to water
hydrophobic tail - attracted to oil
hydrophilic head - attracted to oil
hydrophobic tail - attracted to water
hydrophobic head - attracted to water
hydrophilic tail - attracted to oil
hydrophobic head - attracted to oil
hydrophilic tail - attracted to water
An example of a food emulsifier is lecithin
If an emulsion is left to stand, what will eventually happen?
The mixture will remain mixed
A layer of water will settle on top of the oil
A layer of oil will settle on top of the water
The mixture will evaporate
Remember that oil is normally less dense so it floats on water
Which of the following is an emulsifier?
Vegetable oil
Egg white
Egg yolk
Egg yolk is used as the emulsifier in mayonnaise to mix oil and water
When oil tankers sink and lose their oil into the ocean, detergents can be used to help clean up the damage done by the oil. Why does this work?
The detergent dissolves the oil
The detergent acts as an emulsifier and separates the oil and water long enough for the oil to be removed
The detergent forces the oil to dissolve in the water
The detergent breaks down the oil and disperses it over a much larger area
Still harmful to the environment but given time, the smaller blobs of oil will break down naturally. The biggest environmental issues are created when a huge oil slick reaches a shoreline
Why is detergent rarely used in the way mentioned in question 8?
It is more effective than other methods
It doesn't work
It is relatively expensive and the spills are usually far too large to be treated in this manner
It doesn't work quickly enough
It is usually used on smaller scale clean-up operations such as on a beach, when the oil spill has already broken into smaller areas
When sea birds that have been coated in oil following an oil spill are treated with detergent, they often don't survive. Why?
The detergent strips the birds of their natural protective oils
The detergents cause diseases
The detergents harm their skin
The detergents clog up the feathers
This also happens with other creatures such as seals. They have natural waterproofing oils on their coats which are removed by detergents
Author:  Kate Gardiner

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