Define triple point of water. Why is it unique?
It is the temperature at which the three phases of water, namely, ice, liquid water and water vapour are equally stable and coexistent. The triple point is suitable because it is unique, i.e., it occurs at one single temperature = 273.15 K and one single pressure of about 0.46 cm of the Hg column. This point is unique because any temperature or pressure higher or lower will change the phase of one or more states of water.