Ammonia is at ambient temperature and pressure a colorless gas with a very pungent odor. Ammonia gas is very soluble in water with a strong heat evolution. Moreover, it becomes highly reactive when dissolved in water and readily combines with many chemicals. Ammonia is easily liquefied by compression or by cooling to about -33°C. In returning to the gaseous state, it absorbs substantial amounts of heat from its surroundings (i.e., one gram of ammonia absorbs 327 calories of heat). Because of this property, it is frequently used as a refrigerant gas. Ammonia must be stored and transported under pressure and/or at low temperature.