Carbon dioxide-rich gas is pressurized and refrigerated until it liquefies, then the pressure is reduced. When this occurs some liquid carbon dioxide vaporizes, causing a rapid lowering of temperature of the remaining liquid. As a result, the extreme cold causes the liquid to solidify(the velovity of the molecules of CO2 reduces till they are immobile and hence compressible) into a snow-like consistency. Finally, the snow-like solid carbon dioxide is compressed into small pellets or larger blocks of dry ice. This explains directly phase changes which CO2 converts from gaseous state to solid state.
Temperature is defined as the average kinetic energy of the molecules of a substance. When a substance is cooled, the average kinetic energy of its molecules is reduced tremendously. Thus, the molecules of the substance no longer move at high speeds.
Gases, according to the ideal theory of gas molecules do not associate but are infinitely free and move at very high speeds. However, when cooled, the kinetic energy of these molecules become sufficiently low so as to allow intermolecular association. Secondly, compressing the carbon dioxide gas increases the pressure and decreases its volume. This further makes the carbon dioxide molecules to associate more rapidly. Since dry ice is sublime, CO2 doesn't pass through a liquid state, rather a highly ordered solid-state called dry ice is formed.
