is called a phase diagram or P – T diagram. The following figure shows the phase diagram of water and CO . Such a phase diagram divides the P – T plane into a solid-region, the vapour-region and the liquid-region. The regions are separated by the curves such as sublimation curve (BO), fusion curve (AO) and vaporisation curve (CO).
The points on sublimation curve represent states in which solid and vapour phases coexist. The point on the sublimation curve BO represent states in which the solid and vapour phases co-exist. Points on the fusion curve AO represent states in which solid and liquid phase coexist. Points on the vapourisation curve CO represent states in which the liquid and vapour phases coexist.
The temperature and pressure at which the fusion curve, the vaporisation curve and the sublimation curve meet and all the three phases of a substance coexist is called the triple point of the substance. For example the triple point of water is represented by the temperature . K and pressure . × – Pa.
(a) (b) Figure : Pressure-temperature phase diagrams for (a) water and (b) CO (not to the scale). thermometer and steam outlet through the cork of the flask (Fig. . ).
As water gets heated in the flask, note first that the air, which was dissolved in the water, will come out as small bubbles. Later, bubbles of steam will form at the bottom but as they rise to the cooler water near the top, they condense and disappear. Finally, as the temperature of the entire mass of the water reaches ° C, bubbles of steam reach the surface and boiling is said to occur. The steam in the flask may not be visible but as it comes out of the flask, it condenses as tiny droplets of water, giving a foggy appearance.
Fig. . Boiling process. If now the steam outlet is closed for a few seconds to increase the pressure in the flask, you will notice that boiling stops.
More heat would be required to raise the temperature (depending on