Milk is a pale liquid produced by the mammary glands of mammals which primarily constitutes
- Water: 83-87%
- Protein: 3.5%
- Sugar: 5% and
- Fat: 4 - 7.4%.
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Milk contains tiny globules of fat and casein which are not dissolved but suspended in water. On heating milk, there occurs a separation between fat and the water, fat being lighter than water floats as a cream leaving water below the fat (creamy layer). As the milk start boiling water turns into vapor in the form of steam bubbles, but is trapped under the fatty layer (milk cream). Further heating results in the formation of more bubbles and these bubbles expand and lift the creamy layer causing it to overflow.
In the case of water, the steam bubbles break as they reach the surface and vapor freely evaporates.
This is the reason why milk overflows on boiling while water doesn't.
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