What causes milk to rise up when we boil it?

 Milk contains 87 per cent water, 4 per cent proteins and 5 per cent lactose (milk sugar). When we boil milk, the fat, sugar, proteins and minerals get separated. Since they are lighter than milk they collect on the surface in the form of cream.

During heating some amount of water gets converted into vapour and the bubbles of water vapour rise to the top but the heat is conducted away by the layer of water and by the fat droplets that have a higher boiling point than water.

The vapour gets trapped in the creamy upper layer. As the milk is heated further the water vapour expands and thick foam is produced on the top.

As the milk is boiled continuously the water, which boils at 100 degrees Centigrade, produces more water vapour and pressure builds up in the boiling milk so that the vapour pressure raises the creamy layer. So the milk pushes the creamy layer out and milk spills out. Courtesy : The Hindu