In this short CAP, we wish to extend the Investigation 1 discussion of the different phases of water (solid, liquid, and gas) to speak of reversible and irreversible changes. The water cycle features entirely reversible changes in state. Liquid water evaporates and forms water vapor gas. The gas vapor can cool and condense and return to the liquid phase or if the temperature is low enough to form solid water, snow, and ice. These too can reversibly change to liquid and gas again depending on temperature.

This slide is to simply remind students about the water cycle. The various phases of water in the water cycle are shown in the illustration at the top. As suggested at the bottom of the slide, not all changes are reversible. We will consider examples of reversible changes in the next two slides and then irreversible changes on the following two slides.

This slide shows the simple conversion between two states of water that are dependent on temperature. The gas phase is omitted but of course, would occur if the liquid water was not refrozen – it would evaporate into a gas over time.

This slide is similar to the last in that it shows a reversible state change between solid and liquid according to temperature. In this case, we see the extremely useful property of metals to be solid at room temperature upon cooling from a liquid state. This property of metals allows manufacturers to recycle obsolescent scrap metal into new products. Recycling metal not only provides for cheaper steel in this case, but is also a wise use of our natural resources. Plastic waste can also be melted and reformed into brand new products when it cools.

In this slide, we see a change caused by a temperature that is not reversible. Unlike ice that melts and returns to the solid state again by lowering the temperature, the chemical reactions that occur when an egg is fried do not revert to the original egg properties when removed to a serving plate and permitted to cool. Frying an egg causes irreversible changes.

This is one of the most fundamental irreversible changes know to man, the burning of wood. It is a combustion reaction and cannot be reversed. Cooling the fire will not reconstitute the wood.

This final slide asks students to suggest other changes that are either reversible or irreversible. Examples of reversible changes: 1) Dissolve solid salt in water. Following evaporation of the water, solid salt is once again formed, 2) when you stretch a rubber band you change its shape, relax the rubber band and it returns to its original shape, 3)  blowing up a ballon increases the pressure of the air inside but letting the air escape changes the gas pressure of the air back to its original pressure. Examples of irreversible changes: 1) The rotting of wood cannot be reversed, 2) when a firecracker explodes, the changes cannot be reversed, 3) burning a piece of toast is irreversible.

As a general rule, physical changes are typically reversible while chemical changes are often (but not always) irreversible.