Ice Cool When Needed

When we are fever, we normally apply a cold pack on our forehead. It helps to reduce heat as heat is transferred from the higher temperature to the lower temperature to achieve equilibrium. 

However, what is inside a cold pack? Just normal water or ice?

In fact, ice pack contains chemical that can react with water endothermically. As we learn in this University Chemistry 1 course, endothermic reaction absorbs heat from the surrounding. This is what causes an ice pack to be ice cold when in use.

In cold pack, there is a strong bag containing the ammonium nitrate salt and a thin bag containing water. When the cold pack is squeezed, the bag containing water breaks, causing the water to react with ammonium nitrate. This reaction is very endothermic, where the temperature can drop as much as 18 °C.

NH₄NO_{3 (s)} + water --> NH₄NO_{3 (aq)}

ΔH = + 26 kJ mol^-1

Besides reducing fever, cold packs also used by physiologist in sports to reduce swelling and pain due to muscle or joints sprain suffered by a player. This is to cause vasoconstriction in the blood vessels of the injured section, and also deadens the pain receptors.

So, when we use cold packs next time, we know its contents !