Hot and Cold Therapy
Cold therapy is an established practice used in medical profession to treat injuries like sprained or strained arms or legs muscle and joint injuries. Cold therapy helps to slow down blood flow, thus reducing further swelling and pain. Meanwhile, hot therapy is used to treat injuries and to warm body parts during cold weather. For example, athlete uses hot therapy to warm up muscles by increasing blood flow. 
Treating injuries with cold therapy
Athlete treating injuries
Cold and Hot Pack
For cold therapy, usually ice is used and for hot therapy, a hot water bag is used. Instead of using ice, a pack filled with chemical agent that reacts endothermically(absorbs heat) may use for cold therapy. Meanwhile, a hot water bag can be substituted with a pack filled with chemical agent that reacts exothermically(release heat). The packs mentioned are hot and cold pack. Both instant hot and cold can usually maintain their temperature for approximately 30 minutes. Hot and cold packs can be used in numerous ways. Have you ever used a cold or hot bag before? I’m sure majority does. But do you know how they work to give the cooling or warming effect?
A bag used for cold therapy
An ice bag
Hot water bag
I love this^^
Sponge bob hot water bag
Cold Pack
The invention of instant cold pack brings a lot of benefits as it is very convenient. I’m sure many people have came across a cold pack before.Cold packs can be used for chilling food and drink as well as for alleviating body aches and pains. But do you know how it works? For your information, a cold pack is made up of a strong and flexible outer plastic layer. The inner part of the pack is separated into two compartments. One compartment is filled with liquid(water) and another is filled with a freezing chemical mixture(salt). The cooling reaction is activated when pressure is applied to the pack, allowing the liquid and the freezing chemical to mix.
Instant cold pack
A cold pack
How cooling effect is activated?
Have you thought of what reaction that makes a cold pack has the cooling effect? The answer is simple, the ENDOTHERMIC reaction. An endothermic reaction involves the process of absorption of heat. In cold packs, usually ammonium nitrate is used as the freezing chemical agent. As mentioned earlier, when pressure is applied on a cold pack, the frangible barrier between the two compartments ruptures, causing water and ammonium nitrate to mix. The reaction between water and ammonium nitrate is an endothermic reaction as it absorbs heat from the surrounding. When ammonium nitrate reacts with water, it splits into positive ammonium ions and negative nitrate ions. In this process of dissolving the crystal, water molecule tend to ‘donate’ some of its energy, thus lowering its temperature. As result, you will feel the cooling effect on the cold pack as the water cools down.
An endothermic reaction gives the cooling effect
Ammonium nitrate
Equation showing ammonium nitrate dissociates into ammonium ions and nitrate ions
Positive ammonium ion and negative nitrate ion
Other freezing chemical agents that can be used in cold pack:
- Ammonium chloride
- Potassium nitrate
- Sodium thiosulphate
Hot pack
Have you ever used a hot pack to warm your hands during cold weather or during a muscle cramp? Hot pack is definitely a girl’s best friend especially during menstrual cramp. The hot therapy instantly relieves the suffering pain.
How it works?
Like a cold pack, an instant hot pack has the same physical structure. However, the reaction that takes place to give the warming effect of a hot pack is different. The chemical reaction that takes place in a hot pack is an EXOTHERMIC reaction, which means that it releases heat during the reaction. Like a cold pack, an instant hot pack needs a chemical agent to undergo chemical reaction. For hot pack, anhydrous calcium chloride is used. Same goes to hot pack, when the outer bag is squeezed, the film between the two compartments will rupture, allowing water and anhydrous calcium chloride to mix. Exothermic reaction between water and anhydrous calcium chloride releases heat, causing temperature of the water to increase. At this time, the hot pack is ready to be used.
An exothermic reaction gives the warming effect
Hot packs
Other substances that can be used in hot packs:
- Anhydrous magnesium sulphate
- Anhydrous copper(II) sulphate
- Calcium oxide
Reusable heat pack
Instead of instant hot pack, you can try using reusable heat pack. You can reuse the heat pack instead of using once.
How it works?
A reusable heat pack uses sodium acetate crystallization and resolution system. Sodium acetate freezes at 54˚C at normal condition. However, when sodium acetate is placed in a sealed container, this solution can be cooled well below this temperature(-10˚C). This change is called super cooling.
When the metal disc in the bag is bent, it triggers a single molecule of liquid in the bag to crystallize and slowly causing the entire solution to change from liquid to solid. This phase change causes the pack to heat. When the heat pack is triggered, the solution is ‘taught’ how to crystallize, and the temperature jumps back to its ‘freezing’ point. The heat pack produces an even heat flow with the crystals it produces at higher temperatures. A reusable heat pack can produces heat for approximately two to three hours, depending on the size of the bag. Placing the bag in boiling water redissolves the sodium acetate crystals, and thus the heat pack can be reused.
Sodium acetate
Metal disk in the bag is bent
Bending of the metal disk triggers the molecule to crystallizes
Causing the other molecule to start crystallize
The entire solution turn from liquid to solid
And now....
You know how cold and hot pack work. And it's time for us to appreciate this excellent invention because they are useful. ^^
