A twisted angle of a wrenched knee requires immediate first aid. Temperature must be lowered -to shrink, the broken blood vessels around the sprain and minimize the internal bleeding. Athlete uses a "cold pack" for this purpose. It is a plastic bag containing solid ammonium nitrate that is separated from water. When bag is squeezed the partition between ammonium nitrate and water is broken. Now analyse the situation and' why it is used as a 'cold pack'?