The chemistry of cold packs

Cold packs are one of the common things used in our home in case of emergencies. It can be used on both adults and children, though we have to be careful while using on babies. Using cold packs are one of the easiest ways to treat injuries and muscle pains. Cold packs are very effective in reducing swelling and numbing pain. Cold pack helps to numb the affected area and helps to reduce the pain. If we place water in the freezer, it takes certain hours to get freeze into ice. But how is it possible for cold packs to go from room temperature to near freezing within few seconds?

In this article, we are about to see The Chemistry of Cold Packs being explained by John Pollard from TED-ED You Tube Channel.

Chemistry of cold packs:

If you use an ice pack, you have to put it in the freezer hours ago. But a cold pack can be left at room temperature until the moment you need it and within few seconds, you’ll feel the chill.

Cold packs contain water and a solid compound. Usually ammonium nitrate in different compartments is being separated by a barrier. When the barrier is broken, the solid will dissolve causing Endothermic Reaction, one that absorbs heat from its surroundings.

To understand how it works, we need two driving forces behind chemical processes and they are Energetics and Entropy. These chemical processes determine whether the change occurs in a system and how energy flow if it does.

In chemistry, Energetics deals with the attractive and repulsive forces between particles at the molecular level whereas Entropy basically describes how objects and energy are distributed based on random motion.  In Energetics, the flow of heat in chemical transformation depends on the relative strength of particle interactions in each of a substance’s chemical states. When particles have a strong mutual attractive force, they move rapidly towards one another until they get so close, the repulsive forces push them away. The stronger the attraction, the faster their movement and since heat is essentially motion. When a substance changes to a state in which these interactions are stronger, the system heats up.

But cold packs do the opposite, which means that when the solid dissolves in the water, the new interactions of solid particles and water molecules with each other are weaker than the separate interactions that existed before. This makes both types of particles slow down on average cooling the whole solution. The substance would change to a state where the interactions were weaker and wouldn’t the stronger per-existing interactions keep the solid from dissolving because of Entropy.  If there are strong attractive forces between particles, the probability of some configurations can change even to the point where the odds don’t favour certain substances mixing. But in the case of the ammonium nitrate (NH4NO3) or other substance in cold pack, the attractive forces are not enough to change the odds and random motion makes the particles composing the solid separate by dissolving into the water and never returning to their solid state.

To be short, cold packs get cold because random motion creates more configurations where the solid and water mix together and all of these have even weaker particle interaction, less overall particle movement and less heat than there was inside the unused pack. And so it’s responsible for comforting cold that soothes your pain.

Read More

Related Articles


For Worksheets & PrintablesJoin Now