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  • So you just strained a muscle and the inflammation is unbearable.

  • You wish you had something ice-cold to dull the pain,

  • but to use an ice pack, you would have had to put it in the freezer hours ago.

  • Fortunately, there's another option.

  • A cold pack can be left at room temperature until the moment you need it,

  • then just snap it as instructed and within seconds you'll feel the chill.

  • But how can something go from room temperature to near freezing

  • in such a short time?

  • The answer lies in chemistry.

  • Your cold pack contains water and a solid compound,

  • usually ammonium nitrate, in different compartments separated by a barrier.

  • When the barrier is broken, the solid dissolves

  • causing what's known as an endothermic reaction,

  • one that absorbs heat from its surroundings.

  • To understand how this works,

  • we need to look at the two driving forces behind chemical processes:

  • energetics and entropy.

  • These determine whether a change occurs in a system and how energy flows if it does.

  • In chemistry, energetics deals with the attractive and repulsive forces

  • between particles at the molecular level.

  • This scale is so small that there are more water molecules in a single glass

  • than there are known stars in the universe.

  • And all of these trillions of molecules are

  • constantly moving, vibrating and rotating at different rates.

  • We can think of temperature as a measurement of the average motion,

  • or kinetic energy, of all these particles,

  • with an increase in movement meaning an increase in temperature,

  • and vice versa.

  • The flow of heat in any 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,

  • that repulsive forces push them away.

  • If the initial attraction was strong enough,

  • the particles will keep vibrating back and forth in this way.

  • 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 our 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.

  • But why would a substance change to a state where the interactions were weaker?

  • Wouldn't the stronger preexisting interactions keep the solid from dissolving?

  • This is where entropy comes in.

  • Entropy basically describes how objects and energy

  • are distributed based on random motion.

  • If you think of the air in a room, there are many different possible arrangements

  • for the trillions of particles that compose it.

  • Some of these will have all the oxygen molecules in one area,

  • and all the nitrogen molecules in another.

  • But far more will have them mixed together,

  • which is why air is always found in this state.

  • Now, if there are strong attractive forces between particles,

  • the probability of some configurations can change

  • even to the point where the odds don't favor certain substances mixing.

  • Oil and water not mixing is an example.

  • But in the case of the ammonium nitrate, or other substance in your cold pack,

  • the attractive forces are not strong 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 put it simply, your cold pack gets 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.

  • So while the disorder that can result from entropy

  • may have caused your injury in the first place,

  • its also responsible for that comforting cold that soothes your pain.

So you just strained a muscle and the inflammation is unbearable.

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B2 H-INT TED-Ed solid pack entropy motion substance

【TED-Ed】The chemistry of cold packs - John Pollard

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