Short answers, pulled from the story.
A chemical reaction is a process that transforms one set of chemical substances into another. Atoms rearrange, bonds break and form between atoms, and an energy change accompanies the creation of new products. Classically the nuclei stay unchanged, so the elements present do not change.
The four basic types are synthesis, decomposition, single displacement, and double displacement. In synthesis, simple substances combine into a complex one, while decomposition breaks a complex substance into simpler parts. Single displacement has one element replace another in a compound, and double displacement swaps the anions and cations of two compounds.
Exothermic reactions release energy, often as heat, and include combustion, precipitation, and crystallization. Endothermic reactions consume heat from the environment, frequently by increasing entropy through the formation of gaseous or dissolved products. Many endothermic reactions favor high temperatures, while exothermic ones like crystallization favor lower temperatures.
A catalyst speeds a reaction by forming weak bonds with reactants or intermediates and lowering the activation energy needed for the reaction to take place. It changes the reaction's pathway but is returned to its original state and not consumed, so it can be used again. Substances that slow a reaction are called inhibitors.
Antoine Lavoisier disproved the phlogiston theory in 1785. The phlogiston theory, proposed by Johann Joachim Becher in 1667, claimed a fire-like element was released during combustion. Lavoisier found the correct explanation, that combustion is a reaction with oxygen from the air.
Slow reactions can be analyzed in real time by measuring pH and the optical absorption and emission spectra of the ingredients. A radioactive isotope can be introduced and tracked over time, a method often used to study redistribution of substances in the human body. Faster reactions are studied with ultrafast laser spectroscopy, where femtosecond lasers monitor short-lived transition states.