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Oils

In this GCSE Chemistry quiz we look at some of the oils used in cooking - particularly the vegetable oils which are extracted from seeds, fruits and nuts.

Have you ever been on a car journey and looked out at field after field of bright yellow flowers? They are the very distinctive flowers of the rapeseed plant. As well as being very colourful and pretty, these plants are extremely useful. They produce very nutritious, oil-rich seeds and are the third largest source of vegetable oil in the world behind soyabean oil and palm oil.

Some fruits, seeds and nuts are rich in oils that can be extracted. To extract the oil, the plant material is first crushed. In some cases, for example the manufacture of olive oil or walnut oil, squeezing the fruits or nuts in a press is sufficient to obtain the oil. In other cases this does not work very well, so the oil needs to be dissolved in a solvent. The solvent can then be distilled off, leaving the vegetable oil behind. Plant extracts such as lavender, other oils used for making perfumes and sunflower oil are extracted in this way. Water and other impurities are removed leaving the oil pure and clear and ready for use.

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The use of vegetable oils with which you will probably be most familiar is in cooking. Vegetable oils have higher boiling points than water which means that foods can be cooked at higher temperatures than by boiling in water. Cooking is simply a way of making chemical reactions happen in food in order to alter the flavours and to soften it. Chemical reactions happen faster at higher temperatures so cooking food in oil is faster than boiling in water. Fats and oils also add flavour to foods. The downside to this is that oils are rich in energy so there is a danger that you could become overweight if you eat too much food that is cooked in oil. There is so much energy in vegetable oils that even used cooking oil can be processed to provide a fuel that can be used in diesel engines!

You don't need to know the structure of vegetable oils for the GCSE, but it helps you to understand the next point. A molecule of vegetable oil comprises two parts - a glycerol molecule attached to one or more long chain molecules called fatty acids. The fatty acids can be saturated or unsaturated. By now, you should understand these terms but just in case you need a reminder, saturated in chemistry means that a molecule contains only single bonds between the carbon atoms but unsaturated indicates there is at least one carbon-carbon double bond. Health-wise, eating unsaturated fatty acids appears to be better for your health than eating saturated fats. Unsaturated oils are normally liquids at room temperature but saturated fats have higher melting points and tend to be solids. Vegetable oils can be converted into solids by hydrogenation using a temperature of 60°C and a nickel catalyst - the product of this chemical reaction is a solid vegetable fat, margerine is an example. Two other words that you need to associate with vegetable oils are monounsaturated and polyunsaturated. If you are not sure what these terms mean, think about the prefixes - mono and poly, what do they ususally mean?

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  1. Where are vegetable oils found?
    These must be crushed to release the oils
  2. Pick the method that is NOT used to extract oils from seeds, nuts or fruits.
    Cracking is the breaking of long chain hydrocarbons into shorter chain hydrocarbons
  3. What do unsaturated vegetable oils tend to be like?
    Olive oil and sunflower oil are both liquids at room temperature, sunflower oil is more unsaturated than olive oil
  4. How can we detect chemically if an oil is unsaturated?
    Yellowy bromine solution will decolourise in the presence of an unsaturated solution as it reacts with the C=C double bonds
  5. Vegetable oils can be hardened to be solids. What is this process called?
    The hydrogen adds to the molecule via the C=C double bonds
  6. Hardening vegetable oils involves adding an element to the oil. What element?
    That's why the process is called hydrogenation!!
  7. What conditions are required for the hardening of vegetable oils to take place?
    Without the catalyst, much higher temperatures would be needed
  8. Vegetable oils have higher boiling points than water. What effect does this have on food cooked in oil?
    Food absorbs the oil during cooking, it also becomes coated in oil. Oil has a very high energy value. Your body finds it easier to release energy from carbohydrates and so the cooking oil can end up being deposited in your body as fat. This is one reason why too much fried food can be bad for you - you could become overweight
  9. Mayonnaise, ice cream and milk are all examples of...
    Emulsions normally break apart quickly so food scientists have developed emulsifiers that stabilise them
  10. An emulsifier molecule has two 'ends'. Pick the correct descriptions for these two 'ends'.
    A good example of a natural emulsifier molecule is lecithin

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