Cool chemistry or Hot stuff?

By Rosie Coates

We often think of chemistry as being full of explosions but some very useful chemistry happens when reactions get cool.

In this post we share a classic endothermic reaction, then show how you can take it one step further to create your own ice-cream!

We then take a look at exothermic reactions and look at how these can be used to warm ourselves up.

Try it yourself

You can use a very simple experiment to help you to think a bit more about how chemistry and temperature are linked by doing something as common-place as dissolving a solute (like salt) in a solvent (like water) to make a solution.

You will need:

  • Two glasses
  • Some warm water
  • Some iced water
  • A spoon
  • 1kg salt
  • A measuring jug
  • Weighing scales

Pour 250ml of warm water into one of the glasses and 250ml iced water into the other. Weigh out two piles of 500g of salt. Gradually add one pile of salt to one glass and the other pile into the other stirring as you add.

Stop adding salt when no more will dissolve. Weigh the remaining salt to find out how much salt dissolved into each glass and you will then be able to tell which glass of water was able to dissolve the most.

Iodised_salt

Salt. Photo: Drtony999 (CC-BY-SA)

What happened?

Which glass of water dissolved more salt?

You probably found that the warmer water was able to dissolve more salt. The warmer water has more energy and dissolving costs energy.

A reaction that overall costs energy is called endothermic, it will cool down the reactants. Both the warmer glass and the ice glass will now be cooler than when we started the experiment.

Why not repeat the experiment with a thermometer in the glasses to check.

Dissolving forms a mixture, a solution, rather than a new chemical compound. Even though bonds in the molecules (intramolecular bonds) are not broken, bonds between  them (intermolecular bonds) are. Water molecules are separated from each other and the sodium and chloride ions that make up the salt are split apart, allowing the salt and water  to mix together.

All this bond-breaking costs energy. Forming bonds releases energy but the bonds formed between salt and water don’t make up for the energy needed to break the intermolecular bonds so energy has to be taken in from the environment (in this case, the glass of water). The more energy the water can supply the more salt can dissolve. This is why the warmer water can dissolve more salt.

So what?

Now we know the chemistry we can use it to make ourselves a tasty treat: ice-cream! The first experiment showed us that dissolving salt in water reduces the temperature of the water.

The energy was transferred from the water to break the intermolecular bonds in the salt and the water. The ice-water can actually go down as low as -10 degrees celsius in some cases.

To make ice-cream you can place a sealed bag of custard into your salt-ice water mix and you it will produce ice-cream in just a few minutes.

All without the need of any fancy ice-cream makers or even a freezer. Now that is cool chemistry!

Icecream_Cone

Anyone for icecream? Photo: Slashme (CC-BY-SA)

If spring hasn’t totally sprung where you are and you’re not quite ready for an ice-cream you can still make use of this chemistry.

Just as dissolving costs energy and reduces the temperature in an endothermic reaction, solutes coming out of solution gives out energy and increases the temperature in an exothermic reaction.

You can try this out for yourself by making a reusable hand-warmer with these pupil and teacher experiment resources from the Royal Society of Chemistry. The beauty of this technology is the easily reversible reaction. Once you have benefited from your hand-warmer you simply heat it up to dissolve the crystals that have formed, and it is ready to be used again!

This video shows some of the differences between disposable and reusable hand-warmers.

It just goes to show, whether you need to warm up or chill out, chemistry’s got the solution!

Curriculum Links

Key Stage 3:

Energetics

  • Exothermic and endothermic chemical reactions (qualitative)

Chemical reactions

  • Chemical reactions as the rearrangement of atoms

Key Stage 4:
This science made simple blogpost aims to enrich and give context to:

AQA GCSE Chemistry
C.2.5 Exothermic and endothermic reactions
C.3.3.1 Energy from Reactions
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Posted in Activity, Chemistry