Why Does Milk Spill over When It Boils?
Since milk is a colloid and contains many substances in suspended form. These substances are mainly protein, sugar and fat. When milk is heated slowly, the proteins and fat get separated. Since they are lighter than the milk they collect on the surface in the form of a layer called cream.
Milk also has a large quantity of water in it. During heating some water gets converted into water vapor. The water vapor being lighter rises up. As the upper layer of the milk is covered with cream, the vapor gets trapped below it. As the milk is heated further, the water vapor expands and thick foam is produced on top.
Finally, the trapped vapor lifts the layer of cream up, quite like a hot air balloon, and finally bursts through this layer. As the vapor escapes, a lot of cream and milk spill out.
How we can prevent milk from spilling? To prevent milk from spilling all one has to do is to provide a path for the vapor to escape. For this we can keep a long-handled 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 trapped under the layer of cream and thus spilling over of milk.