Structure and Bonding

Matter is composed of atoms and molecules. Gases, liquids and solids are different states of matter (there can be some other states also under some extreme conditions of matter). However they can get converted into one another depending upon their stability at different pressure, temperature or variation of both. For example, as illustrated in Fig. 2.1, water molecule (H2O) can be in gaseous state (vapour), liquid state (water) or solid state (ice) depending on the temperature and pressure. Water vapour can be converted into liquid water or ice by cooling. Similarly ice can be turned into water and then vapour by heating. At the triple point, solid, liquid and gas co-exist. Critical point as shown in Fig. 2.1 is a point at which some critical values of temperature and pressure are reached. Beyond this point there is no distinction between gas and liquid phase.