Question

What chemical principle is in a calorimeter enthalpy lab?

Answer 1

First Law of Thermodynamics: The internal energy of an isolated system is constant.
We can see a calorimeter as an isolated system.

Answer 2

There are actually a number of principles in a calorimetery lab. The First Law of Thermodynamics says that if heat is lost by the system then it is gained by the water. If heat is lost by the system then it is gained by the water. The First Law is the main principle involved here.