1. Any matter i.e., solid, liquid or gas when experiences an increase in temperature then they change their state.

Example: Solid(Ice) + heat → Liquid(Water) + heat → Gas(Steam)

Take ice cubes in a beaker or heat them slowly, the temperature increases and the ice melts to form liquid. Heat this liquid further it will become steam.

2. On lowering down the temperature of any matter, show change in their state.

Example: Gas(Steam) + cool → Liquid(Water) + cool → Solid(Ice)

Take the steam that is coming out of a boiling water and allow it to cool down, it condenses to form water and on further cooling of this water we get ice.

3. On applying pressure and reducing temperature we can liquefy gases or change them into solid.

Example: Take carbon-dioxide gas, reduce its temperature and apply lot of pressure on it so that it changes into solid carbon dioxide, called diy ice, which is used as refrigerant for cooling. If the pressure on it is decreased it directly changes into gas. In LPG cylinders, the petroleum gas is cooled and with lot of pressure changes it into liquid state.

While using this LPG, we release the pressure exerted on it and hence it comes out in the form of gas.