Student Projects/Why does milk spill out when boiled?

When milk is heated it comes up in the container and spills out.

Do you know why does it happen?

Unlike water, milk is not a simple substance. It is a colloid and contains fat, sugar, starch, protein, vitamins and minerals. When milk is heated slowly, the proteins and fat 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 vapor. As the upper layer the milk is covered with cream, the vapor gets trapped under it. As the milk is heated further the water vapor expands and a thick foam is produced on the top. Finally, the trapped vapor pushes the layer up and in the process bursts through. As the vapor 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 vapor to go out. For this we can keep a long spoon in the milk container so that water vapor is able to escape along the handle of the spoon. This prevents the vapor from getting deposited under the layer of the cream and also spilling out of the milk.

Reference: Children's knowledge bank [book]