Why do some things get colder (or hotter) when they react? [v1.0]
This high school physical sciences unit on chemical energy starts out with students exploring a phenomenon where mixing two room temperature substances together results in a dramatic drop in temperature. This leads to disagreements as to what is happening that causes thermal energy to "disappear"—what is that energy being used to do? This question motivates a series of investigations examining salt dissolution and using magnets as a model for both inter-molecular and intra-molecular bonds.
What students figure out:
By the end of the unit, students develop ideas about the structure and properties of matter, chemical reactions, and the relationship between energy and forces. These ideas include:
- A stable molecule has less energy than the same set of atoms separated; one must provide at least this energy in order to take the molecule apart.
- When two objects interacting through a field change relative position, the energy stored in the field is changed.
- Whether or not energy is stored or released in a chemical process, can be understood in terms of the collisions of molecules and the rearrangements of atoms into new molecules, with consequent changes in the sum of all bond energies in the set of molecules that are matched by changes in kinetic energy.