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If we add ethanol to water, together they form a single homogeneous solution. This is what we see in a glass of beer. We say that the liquids are miscible. If, however, we combine water and oil they don't mix. Instead they form two layers, with the less dense oil on top of the more dense water. This is what we see when an 'oil slick' is formed as oil escapes from a ship at sea. Liquids that do not mix together are said to be immiscible.

Try sorting the following pairs of liquids to show which are miscible and which are immiscible.

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One of the key factors that determines whether liquids are miscible or immiscible is the strength of the bonds between the molecules in each liquid. For liquids to mix, bonds between the molecules in the separate liquids must be broken and new bonds formed between molecules from the different liquids. For this to happen, the bonds between molecules in the separate liquids must be of similar strength to the bonds formed between molecules when the liquids are mixed.