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In my text book, it is given :

One calorie is defined as the amount of heat required to raise the temperature of water from 14.5 °C to 15.5 °C.

I found out in wikipedia that this is actually the definition of 15 °C calorie.

I want to know that as temperature increases, will the amount of energy for unit increase in temperature decrease? I find 15 °C calorie > 20 °C calorie. Is this relation uniform till 100 °C or is there any anomaly? And does the increased kinetic energy of the molecules of water, at an elevated temperature cause the amount of heat - required for unit rise in temperature, to be greater than the amount of heat required for the same unit rise, but at a lower temperature?

For example, the heat energy required to produce unit rise in temperature of water at 14.5 °C is 4.1855 joules and the amount of heat energy required to produce unit rise in temperature of water at 19.5 °C is 4.182 joules.

So does the increased K.E of water molecules at 19.5 °C cause the required heat energy to be lower than the heat energy required at 14.4 °C?

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Water's heat capacity is quite constant. This doesn't hold as vapor: – jinawee Jan 27 '14 at 17:45

From the data presented here:

Cp of water

Note that although the scale of the $y$ axis makes the variation in $C_p$ look big it is only about 1% over the range 0 to 100°C.

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