Mix some food colouring into some water in a jar and pour a bit of vegetable oil in. Give it a good shake and leave it for a moment. At first it might look like it has mixed, and then small droplets of oil form, and join with other droplets to make larger and larger drops, until the oil settles on the top of the water.
Water molecules are polar – they have a small positive charge at one end and a small negative charge at the other end, and they stick to each other. Oil molecules are non-polar – they have no charge. Because of this, oil molecules are more attracted to each other than to water molecules, and water molecules are more attracted to each other than to oil molecules.
Oil and water can be forced to mix together by adding an emulsifier (see 'Making an Emulsion'). This creates a stable mixture of water with droplets of oil spread through it, or oil with droplets of water spread through it, that does not settle out.
http://www.scienceprojectideas.co.uk/why-oil-water-dont-mi
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