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In perspective of thermodynamics,a person can survive a temperature of twenty degrees Celsius in air, but a person cannot survive in water with same temp.(it's not because he can't breathe) why is that in perspective of thermodynamics??

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This is really just an expansion on cb3's answer:

The (obvious) reason why you can't survive in water at 20C is that you lose heat to the water faster than the body can generate it. The thermal conductivity of the water does play some part in this, but it's a small part because you mainly lose heat by convection rather than conduction. Your body heats the water around it and that water rises and generates a water flow around you.

The main reason you lose heat faster in water than in air is that it has a much higher specific heat. The specific heat of water is 4.2kJ/kg and the specific heat of air is about 1kJ/kg. However the important figure is really specific heat per unit volume because it's the volume rate of the flow around you that matters. This is 4.2MJ/m$^3$ for water and about 1kJ/m$^3$ i.e. a water flow extracts about 4,000 times as much heat from you.

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The thermal conductivity of water is much higher than that of air. Stagnant water at 20C will draw heat out of the body faster than stagnant air at the same temperature.

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