The soil moisture driven off by heat, is called

ANS:B - hydroscopic water

Water found on the surface of the soil which is not capable of movement either by gravity for capillary and can only be driven off by heat is called hygroscopic water. When an oven-dry (heated to 105°C for zero percent moisture content) soil sample is exposed to the atmosphere, it takes up some moisture called hygroscopic moisture. This moisture can be removed by heat only. If more water is made available, it can be retained as capillary moisture due to surface tension (i.e. intermolecular forces). Any water, in excess of maximum capillary moisture, flows down freely and is gravitational water. Water content in soil represents (gravity water+ capillary water + hygroscopic water) which can be removed by oven drying except STRUCTURAL water.