Exothermic and Endothermic reactions

Here at science made simple we are keen to inspire not only the next generation of scientists but also the next generation of science communicators. Supporting work experience students each year is an important part of what we do.  Our work experience students get a chance to see a range of the activities that science made simple is involved with and we always set them challenge to produce some science communication themselves, whether designing a demo, delivering a short presentation or, in this case, writing a blogpost that includes all of these!

We hope you enjoy this from one of our latest work experience students, Bilaal, we loved having him in the office and think he’s got a promising career ahead of him.


This blogpost looks at two types of reactions: exothermic and endothermic. We’ll also be looking at reversible reactions– reactions in which the products can react to remake the original reactants. Plus, a great demonstration of an exothermic reaction.


Exothermic reactions

Image: Nikthestunned, CC-BY-SA

An exothermic reaction (the thermite reaction) using Iron (III) Oxide. The sparks flying outwards are drops of molten iron trailing. Image: Nikthestunned, CC-BY-SA


An exothermic reaction occurs when the energy used to break the bonds in the reactants (the starting stuff) is less than the energy released when new bonds are made in the products (the stuff you end up with). This extra energy is given off as heat and there is a temperature rise around the surroundings of the reaction.

Combustion is an example of an exothermic reaction- you can feel the heat given off if you get too close!


exo reaction

Shamsher Singh CC-BY-SA



This graph shows that energy has been released and Delta H (energy change) is negative.

The reactants have more energy than the end products. It also indicates that the enthalpy change is negative as heat is lost to the surroundings resulting in a temperature increase.


Endothermic reactions

Cool chemistry cat! CC-BY-SA www.knowyourmeme.com

 An endothermic reaction occurs when the energy used to break the bonds in the reactants is greater than the energy given out when bonds are formed in the products. This means that overall the reaction takes in energy, therefore there is a temperature decrease in the surroundings.

Electrolysis is an example of an endothermic reaction but you can create one easily in the kitchen just by dissolving salt or sugar in water.  If you use cold water and lots of salt you can even make ice-cream see our Cool Chemistry blogpost for more details.




Shamsher Singh CC-BY-SA


This graph shows that energy has been taken in and Delta H is positive.

Similarly, this endothermic chart shows that the reactants start off with less energy this time and the end products have absorbed energy. Clearly shown in the diagram, enthalpy change of the reaction is positive because energy is being taken in from the surroundings which results in a temperature decrease.


Reversible reactions

In reversible reactions, the reaction in one direction will be exothermic and the reaction in the other direction will be endothermic.

Copper sulphate

The reaction between anhydrous copper sulphate and water is reversible. Water is driven off from hydrated copper sulphate when it is heated, so the forward reaction is endothermic – energy must be transferred from the surroundings for it to happen. The backward reaction is exothermic – energy is transferred to the surroundings when it happens. This is easily observed. When water is added to anhydrous copper sulphate, enough heat is released to make the water bubble and boil.

Towel spontaneously combusts at Wirral house

Recently in the news, the owners of a property in Prenton spilt linseed oil on the floor and as they went to mop it with a towel, it dried the oil causing it to react with the air and burst in a fire. This suggests that as the linseed oil oxidised, there appears to have been an exothermic reaction and it self-combusted.

Oil-soaked rags are a dangerous combustion hazard because as the oil oxidises it can generate heat.

If the heat cannot escape, it can build up and ignite the rags.

Fire safety advice is to use a brush to apply oil or stain instead as they are easier to clean and store.

Elephants toothpaste Experiment

We will carry out an experiment that shows foam overflowing from a bottle. This reaction gives off heat so of course, it’s an exothermic reaction! The experiment is called “Elephants toothpaste” because it looks like toothpaste coming out of a tube- but there’s enough for an elephant!

You will need:

  • A 500ml plastic bottle or container – the size of a Coke or Fanta bottle
  • 125ml cup of hydrogen peroxide (6% solution)
  • Dry yeast (we use potassium iodide in the video but yeast works well too!)
  • Washing up liquid
  • Food colouring (for effect try red and blue)
  • Warm water
  • Small glass/cup
  • Safety goggles

What to do:

1. Put safety goggles on and add about 50ml hydrogen peroxide into the empty bottle.

2. Add a tablespoon of washing up liquid to the bottle and start swishing and swirling to mix it up a bit.

3. Add roughly 8 drops of food colouring into the bottle.

4. (Miss this bit out if you’re using potassium iodide) In a separate cup, add 3 tablespoons of warm water and combine it with 1 tablespoon of dry yeast. Mix them together for about 30 seconds.

5. Here comes the fun part! Pour the yeast mixture, or one spatula of potassium iodide, into the bottle (using a funnel) and watch how awesome the foam is as it escapes through the lid of the bottle!

For a similar experiment that’s easy to do at home with primary age children take a look here.


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Curriculum Links – Exothermic and Endothermic reactions


Chemistry KS3 – Energetics

  • exothermic and endothermic chemical reactions (qualitative)


Chemical changes


The Sustainable Earth

  • investigations into the patterns of behaviour of elements and compounds and their use to describe and predict their behaviour in chemical reactions


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