Cold reactions

The investigation should take approximately two hours to complete in full. It was initially created for 11–14 year-old learners but can be adapted for other age groups. Use in a sequence of timetabled lessons, science clubs or as part of an activity day. 

Learning objectives

• Explain what is meant by ‘cold’.
• Describe some practical applications of reactions that give a temperature decrease.

Guidance notes

Slide 3 of the PowerPoint asks the question ‘What is cold?’ Give learners two minutes to discuss this question in pairs before sharing their answers as a class. Then ask them to order the six different situations from the coldest to the hottest, writing their answers down on mini whiteboards or paper.

Activity 1: cold packs

Use slide 6 of the PowerPoint to introduce endothermic reactions before the first activity. Learners will work in pairs or groups of three. They will investigate which of the six different compounds (ammonium nitrate, sodium chloride, sodium hydrogen carbonate, citric acid, calcium chloride and calcium sulfate) will produce the best cooling effect and therefore be the best to use in a cold pack. Find the method in the student workbook and instruct learners to record their and answers in their student workbooks too.

Depending on time, the pairs or groups could test all six compounds or could be allocated two or three different compounds to investigate. The class could pool their data at the end of the activity. If time allows, learners could repeat their tests. Encourage the learners to work scientifically by adding the same amount of water and solid to the beaker each time. Discuss why this is done and anything else that makes this a fair test. If there is time, ask the learners how they might improve the experiment.

Activity 2: citric acid and sodium hydrogen carbonate

Tell learners that they will mix solutions of citric acid and sodium hydrogen carbonate and measure the temperature of the resultant solution over three minutes. Ask them to consider why the reaction can’t be used in cold packs. 

Your learners may already have one or both of these solutions from Activity 1. If not, they will have to follow the instructions from Activity 1 to make these solutions.

Use slide 13 to explain why this reaction is not suitable for cold packs. Tell learners that although mixing these two solutions gives a large temperature decrease, it can’t be used in cold packs as it gives off carbon dioxide gas as well as producing sodium citrate and water.

Activity 3: making ice cream

For use in the lab, do not use sugar or flavouring. This emphasises that this ‘ice cream’ can’t be eaten due to the lab setting.

Although the method instructs the learners to measure out 150 cm³ of milk, this does not have to be exact as the method works with volumes up to 200 cm³. As the learners must shake the bag for 10 minutes, encourage them to work together on this step, passing the bag to their partner when they get too tired. While shaking the bags, ask them to think about the reasons why they added the salt to the ice. Give them a few minutes to discuss their ideas in pairs/groups before sharing their thoughts as a class.

At the end of the time get the learners to remove the bags with the ice cream inside. They can touch it and press it but must not eat it. You could set Activity 3 as homework, where the learners can try the method again at home using sugar and flavouring.

Demonstration: glow sticks

This is a great demonstration of the concept of rate of reaction. Link the rate of a chemical reaction to the temperature of the surroundings. The effect of heating or cooling the glow sticks can be assessed by the brightness of the light given out. The brighter the light emitted, the higher the rate of reaction.

Ask learners why this kind of knowledge might be useful. In a chemistry context, controlling temperature is incredibly important. This is especially true when dealing with very reactive chemicals. If the temperature gets too high, you can end up with a runaway thermal reaction which could explode.

Find the answers to all the activities in the teacher notes and slides.

Challenge

Use slide 21 to discuss how in the last 10 years there have been huge leaps in cooling the body down to preserve it and make survival from surgery more likely. Introduce cryonics (also known as cryogenic freezing), where someone can be frozen so they can potentially be revived in the future. Science fact or science fiction? Let the learners discuss this, as there really isn’t a clear answer. Task your learners to research the technique to find out more.

Health and safety

Read our health and safety guidance and carry out a risk assessment before running any live practical.

The safety equipment suggested is in line with CLEAPSS requirements. For non-hazardous substances, wearing lab coats can help to protect clothes. The safety rules might be different where you live so it is worth checking local and school guidance.

Careers

The Chemistry for All project found that recognising the value and importance of chemistry, and appreciating how chemistry can lead to interesting and well-paid jobs strongly related to learners’ future aspirations. Discover how chemistry can prepare learners to pursue apprenticeships, university courses and vocational routes, or browse job profiles and try our career options game from A Future in Chemistry.

After the starter, introduce Robert, a consumer products technician. Watch his video job profile (also available on slide 7 of the PowerPoint) and learn how he studies the behaviours of different materials to develop and improve the properties of products, such as cold packs used to treat sports injuries. Link the context of Activity 3 to careers by watching Claire’s video job profile on slide 15. She is a flavourist and innovation director and applies her chemistry knowledge and understanding to create new flavours, find raw materials and develop technologies. 

Find more activities like this, plus longer projects suitable for off-timetable days, science lessons or clubs on our Outreach resources hub.