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[Raviyank]

When i was taking my hot tea early morning,i found a thin layer (scum) on it as tea was hot. The liquid tea was evaporating slowly in air, but when i move away that scum by blowing, the vaporization becomes fast.
So question is : Nature tries to maintain (or if not convert into) an equilibrium condition.so why not here? Why it stops the natural occurring of heat transfer (i.e.,tea temperature = surrounding temperature) by making thin layer on tea?
[ I know thin layer mechanism : due to some milk oil and mineral stuff but what about thermodynamic laws?? ]

[samayaraj]

Ravi,

 

Liquid tea is not evaporating. Its the water which is present in tea is evaporating. Though the tea's temperature is less than 100 Deg.C, vapors will generate (by taking heat from the tea itself) and reduces the temperature. Also, tea loses its heat by convection and radiation. In any hot liquid milk product, think layer is formed when it is exposed to lower temperature. This reduces the vapor coming out of tea. As you blow out the layer, vapor can come out easily.

 

The same is happening when you boil the milk in a vessel directly on a heating surface. As the surface of vessel is more than 100 Deg. C, vapor will generate and tries to come up. Due to the formation of think layer on the top of milk, vapors cant escape easily and finally the milk rises up. The same will not happen if you use milk cooker. In a milk cooker, there is an annular space where it was partially filled with water. When you heat the milk in milk cooker, water in the annular region will first boil by seeing the hot surface and this vapor heats the milk. So the milk temperature cannot exceed 100 Deg. C.

 

I dont understand what do you mean by thermodynamic laws here...

 

#Samayaraj

Edited by samayaraj, 16 December 2014 - 04:54 AM.