Chemical Engineering - Chemical Engineering Thermodynamics - Discussion

How can specific heat be negative?

First you should know what saturated state means here. Water in a saturated state is in equilibrium with water vapour, that means at a specified condition of temperature and pressure water is at its maximum possible temperature so that any further heating of it will start increasing its dryness fraction (ratio of the mass of steam to the total mass of the mixture of steam and water). It can also be said that saturated water is water which is about to be converted to steam.

So, when water is in a saturated state it is at the maximum possible temperature at the specified condition of pressure. The temperature remains constant during further heating because of thermal arrest due to ongoing phase change (from liquid to vapour). This amount of heat supplied is utilized in the phase change process and is termed as latent heat of vaporization.

Thus it is quite clear that the temperature of saturated water does not increase on heating until all of it is converted to steam or more accurately saturated steam having dryness fraction 1. By definition, specific heat is defined as the amount of heat required to raise the temperature of a system by 1 degree Celsius or Kelvin at specified physical conditions. With the addition of heat the temperature of saturated water doesn't increase until it is converted to saturated vapour. Thus, the specific heat of saturated water may be assumed as zero, infinity or negative depending upon the circumstances within the vapour dome.