This experiment uses cold packs to demonstrate endothermic reactions. The excitement is when the chemicals inside the cold pack are at room temperature when separate, but absorb heat when combined to feel cold. The experiment involves opening the cold pack and pouring the materials in test tubes to demonstrate the chemical reaction.
The purpose of this experiment is to demonstrate an endothermic reaction. Many people know that cold packs feel cold when activated, but they may not know the science behind it. The cold pack contains chemicals that when combined create an endothermic reaction. Endothermic is the term that means to absorb heat. The chemicals inside the cold pack are urea and water. Inside the cold pack, when the water bag inside is broken and mixes with the urea, the reaction starts and gives instant coldness. In other words, when the urea salt is dissolved in water, the ionic bonds of the salt separate. The process requires energy which it acquires from the surroundings. The chemical reaction is described by the following equation:
CO(NH2)2 + H2O + Heat = 2NH3 + CO2
For this experiment in class, we will be mixing the ingredients in test tubes and using a thermometer to accurately measure the reaction.
2 test tubes and 1 tube rack
1 pencil and a sheet of paper or a notebook
Stopwatch (or app on cellphone)
- Gather all of the required materials.
- Find the safety goggles and put them on.
- Place the empty test tubes in the rack on the scale and zero out the weight.
- Using the scissors, cut off the top of the cold pack very carefully. Inside the cold pack, you will find a liquid package and white crystals of urea.
- Remove the liquid package and set it aside.
- Use the spoon to scoop some of the crystals into a test tube until it reads 50 grams on the scale. Remove test tube rack from the scale and set on the counter.
- Carefully open the liquid package using the scissors and pour the contents into the other test tube.
- Place the thermometer in the test tube with the liquid and record the temperature with the pencil and paper. This is the starting temperature.
- Using great caution, slowly pour the contents of the test tube with the urea crystals into the test tube with the liquid and the thermometer, and then start the stopwatch.
- Record the temperature of the reaction every thirty seconds for three minutes using a digital stopwatch (or the app on your phone).
- Once the three minutes are up, clean up the demonstration by diluting the mixture with water as you pour the contents of the test tube down the drain.
- After all steps have been completed in order, talk to the students about what they observed.