Thermochemistry

Every reaction either gives out heat or takes it in. Measure it with a calorimeter, predict it by adding up the bonds that break and form, and solve Hess's law puzzles by flipping and scaling known reactions until they add up to one you can't measure directly. Then meet entropy and Gibbs free energy, the quantity that decides whether a reaction will run at all, and use it to find the temperature at which ice melts, limestone turns to lime and carbon can pull a metal out of its ore.