This experiment shows that exothermic reactions start not always spontaneous when two (or more) substances are mixed together. Students realize that no reaction occurs when the reactants (in solid state) are mixed together. The solids usually begin to react with each other after addition of (thermal) energy (= activation energy). Mixtures of such reactants are so called "metastable systems". By supplying activation energy to a mixture in a metastable state the reaction mixture reaches an "unstable" (activated) state, which starts the reaction.
What you can learn about
- Exothermic reactions do not always start spontaneous
- Exothermic reactions need activation energy
- Metastable system