when we boil milk and after sometime it starts coming upwards but the milk 's volume is same but also after boiling it shifts downwards why?

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Solution

Milk contains fats , protiens, sugar and water .Water being the major constituent. The fats and protiens remain suspended in the milk.When it is heated the fat being the lighter components starts floating on the upper layer and when the water starts vapourizing trapped beneath this layer of cream(fat) .With increasing temperature the trapped vapors apply more pressure on the creamy layer and lifts it up .
When the heating is turned of the vapour pressure beneath the creamy layer drops and the fat (cream) comes down.

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