Question

why does milk in a saucepan overflows when heated ?

Answer 1

Milk overflows when boiled for the first time because it contains entrapped air. The air expands on heating and tends to escape lifting the surface of milk.Once all the air is expelled, the boiling would be regular and smooth. A second boiling would not lead to overflow.

Answer 2

Liquids expand on heating.
The inclusion of the water vapour
in the bubble lowers the density of the
mixture, causing it to expand and overflows.
Also, milk tends to froth when it boils.