Le Chatelier's interest in thermodynamics and building materials such as cement and plaster led to the Le Chatelier Principle in 1884. Activity 13 in a series of 36 extensively explores chemical equilibrium. Learners read about reversible and irreversible reactions, equilibrium in different states of matter, and use the Law of Equilibrium in numerous calculations. They finish by reading, discussing, and applying Le Chatelier's Principle.