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Emulsions

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'.

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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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  1. What is an emulsion?
    The two liquids will not mix and so droplets of one will be suspended in the other
  2. Which of the following is NOT an example of an emulsion?
    The active ingredient in gloss paint (called the binder) is dissolved in a solvent. In emulsion paint, the binder is dispersed in water
  3. What is an emulsifier?
    Emulsifiers are found in many foods
  4. An emulsifier molecule has two ends. Pick the correct combination of these two ends.
    Even an emulsion containing an emulsifier will eventually separate
  5. Pick the combination for what the two ends of the emulsifier molecules do.
    An example of a food emulsifier is lecithin
  6. If an emulsion is left to stand, what will eventually happen?
    Remember that oil is normally less dense so it floats on water
  7. Which of the following is an emulsifier?
    Egg yolk is used as the emulsifier in mayonnaise to mix oil and water
  8. 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?
    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
  9. Why is detergent rarely used in the way mentioned in question 8?
    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
  10. When sea birds that have been coated in oil following an oil spill are treated with detergent, they often don't survive. Why?
    This also happens with other creatures such as seals. They have natural waterproofing oils on their coats which are removed by detergents

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