Chemists often find it useful to make a distinction between physical and chemical changes. Chemical changes result in the formation of a new substance whereas physical changes do not. In fact, there are basic similarities between both types of change because bonds are broken and made. To explore chemical and physical changes in more detail subscribe to the Analytical chemistry course (Separating and purification).

Changes of physical state are familiar to students and are a good way of introducing the idea that some changes can be easily reversed. Simple experiments like freezing water and melting ice and boiling water and condensing steam on a cold surface serve to illustrate this idea. Changes of state are developed further in the Developing and using models course. The idea of reversible changes can then be broadened to include the changes between hydrated and anhydrous salts and developed further to cover decomposition of carbonates. 

How might you use Lego bricks to illustrate the difference between a physical and a chemical change?

To illustrate a physical change move a collection of Lego models from being clustered together to being further apart. To illustrate a chemical change rearrange Lego models into a different arrangement of bricks. The use of models is discussed in detail in the Developing and using models course.

Can all chemical reactions be reversed?

In theory, many reactions can be reversed at a different temperature. For a reaction to be feasible, the free energy, ΔG, must be negative. ΔG depends on the enthalpy change of reaction, ΔH, the entropy change of the reaction, ΔS, and the temperature. When ΔH is positive and ΔS is negative, the reaction will never be feasible. You can find more on these ideas by subscribing to the Energy and change course.