If water and milk are heated, the water does not overflow, but what causes the milk to overflow?

Although this happens because there is a difference between the density of water and milk, my experience is that if you put the pan on a large gas filled with water on the edge, the water will boil and overflow the gas.

The rest of the answers given earlier are detailed.

Thanks for the question request

There is a big difference between milk and water.

Milk is made up of an emulsion of water and fat. Milk contains a lot of fats, proteins - mainly proteins called casein. These elements i.e. fats and casein are not soluble in water but they are floating in water.

When we heat milk, for the first time some of its casein and fat are separated from water. Since these elements are lighter than water, a layer of them forms on the water. When the milk is further heated, the water under this layer starts evaporating. This vapor goes up and gets stuck in the formed layer on the milk.

When the milk is further heated, more water evaporates. Due to the pressure of this vapor, the layer on the milk is gradually lifted and this is what we call milk overflow.

There is no such thing as milk in water so there is no question of overflow. The water evaporates and goes straight up.