Milk has a tendency to boil over just when you are not looking. So many times it must have happened to you. Whenever your mother told you to watch it and you were careless, it boiled over, causing you annoyance. Why does milk boil over? When milk is heated it comes up in the container and finally spills out. Unlike water, milk is not a simple substance. It is a colloid and contains many substances in suspended form. It contains fats, sugar, starch, protein, vitamins and minerals. When milk is heated slowly, the proteins and fats get separated. Since they are lighter than milk, they get collected on the surface in the form of a layer called cream.
During heating, some water gets converted into water vapour. As the upper layer of the milk is covered with cream, the vapour gets trapped under it. When the milk is heated further, the water vapour expands and a thick foam is produced on top. Finally, the trapped vapour pushes the layer up and in the process bursts through. As the vapour escapes, a lot of cream and milk spill out. It is very easy to prevent milk from spilling. All one has to do is to provide a passage for the vapour to go out. For this we can keep a long spoon in the milk container so that water vapour is able to escape along the handle of the spoon. This prevents the vapour from getting deposited under the layer of the cream and thus spilling out of the milk.