A Chilling Recipe: Cool pack
http://www.csiro.au/helix/sciencemail/activities/chillingrecipe.html
Try this: A chilling
recipe
You will need
- Sodium
bicarbonate (baking soda)
- Citric
acid
- Teaspoon
- Zip-lock
plastic bag
- Water
What to do
- Open
the zip-lock bag and pour in three to four teaspoons of citric acid.
- Add
the same amount of sodium bicarbonate and mix the powders together.
- Touch
the outside of the bag. How does it feel? Is it warm or cold?
- Add
a small amount of water to the bag – enough to wet the powder
sufficiently. Watch what happens.
- Touch
the outside of the bag again. Now how does it feel?
atoms. Citric acid gives
its hydrogen away, which is used by sodium bicarbonate. We can keep track of
this swap-around if we use a chemical equation:
citric acid + sodium
bicarbonate –> carbon dioxide + water + sodium citrate
H3C6H5O7 +
NaHCO3 –> CO2 + H2O + NaC6H5O7
What’s happening?
Sodium bicarbonate is what we call a ‘base’. That means when it is mixed in water, it dissolves to make a solution that has a pH over 7. What does that mean? It means that the chemicals in it love to steal hydrogen ions!
Citric acid, on the other hand, is a little different. Mixed with water, it has a pH under 7, which is technically why we call it an ‘acid’. More importantly, it loves to give hydrogen ions away.
As a powdered solid, the two chemicals can’t do very much. They need to dissolve in water to be able to spread out and react with each other. When they do, they swap some of their
The bubbles you see are the gas carbon dioxide
forming. But why does it feel cold?
Chemical reactions need energy to occur. Sometimes,
they then produce more energy than they use and release it into the
environment. However in this case, the reaction takes the energy it needs from
its surroundings, sucking heat away and locking it away as chemical bonds. This
is an ‘endothermic’ reaction, which means ‘heat goes in’, so it feels cold to
the touch (it is taking the heat away from your fingers).
Applications
Have you ever had an injury on the sporting field
and needed a cold pack? It can be troublesome keeping ice cold while you play
out on a sporting field somewhere, especially if you need to carry it around in
an esky.
Chemical cold packs create their own ‘cold’ when
you need them. There are a few different types, but they all involve
endothermic reactions or processes of some sort. The bags usually contain two
different chemicals in different compartments. Breaking a seal allows them to
mix, which absorbs heat from the surroundings. Putting it on your injury means
the reaction will steal its warmth as well, which can reduce swelling and give
your body time to deal with the trauma.
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