Reactions That Get Hot or Cold
- Chemistry
- Ages 11–14 · Grades 6–8
- Hands-on experiment
What you’ll find out
- Exothermic reactions release energy and warm their surroundings; endothermic reactions take energy in and cool them.
- A hand warmer is rusting made fast; a cold pack uses a process that absorbs energy.
- Designing with reactions means choosing the energy change and controlling its rate.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Energy in, energy out
Every chemical reaction breaks some bonds and makes new ones, and the two do not cost the same. Breaking bonds takes energy; making them releases it. When making the new bonds releases more than breaking the old ones took, the extra comes out as heat and the surroundings warm up. That is an exothermic reaction: burning fuel, rusting iron and setting concrete are examples. When breaking the old bonds takes more, the reaction draws energy from its surroundings and they cool down. That is an endothermic reaction. Mixing baking soda with citric acid in water is one you can feel: the solution turns cold. A thermometer tells you which kind it is: temperature up means energy out, down means energy in.
A hand warmer
Rusting is exothermic, but it is usually far too slow to notice: a nail rusting over weeks gives off its heat so gradually that it never feels warm. A disposable hand warmer is rusting made fast. Inside is fine iron powder, which has a huge surface area, mixed with salt, water held in a powder, and charcoal. Tear open the wrapper and air reaches the iron. The salt water speeds things up, and within minutes the packet reaches about 50 °C and stays warm for hours, until the iron is used up. Every design choice controls the rate. Powder makes it fast enough to feel. The wrapper keeps air out until you want heat. The inner pouch limits how fast air gets in, so it warms rather than burns.
A cold pack, and designing one
Instant cold packs work the other way. A typical one holds a solid and an inner pouch of water. Squeeze the pack, the pouch bursts, the solid dissolves, and the process takes in energy so quickly that the pack becomes cold enough to soothe a sprain. Engineers designing one face real choices. How cold must it get, and for how long? More solid gives a colder, longer-lasting pack but costs and weighs more. Is the chemical safe if the pack leaks? The same questions apply to self-heating cans of soup, which keep an exothermic reaction in a separate compartment under the food. Designing with reactions means choosing one for its energy change, then controlling how fast it runs and how much there is.
Curriculum links: NGSS MS-PS1-6