Why Does Milk Spill over When It Boils? Since milkis a colloid and has many substances in suspended form. These substances are mainly protein, sugarand fat.
On heating milk slowly, the protein and fat get separated. Since protein and fat are lighter than the milk they collect on the surface in the form of a layer called cream.
Milk also has a large measure 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 milkis 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. So now you know why does milk spill over when it boils.
How we can prevent milk from spilling? To prevent milk from spillingall one has to do is to give a path for the vapor to escape.
For this we can keep a long-handled spoon in the milkcontainer 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.
Content for this question contributed by Brian Brogan, resident of Cincinnati, Hamilton County, Ohio, USA