There isn't one single, universally accepted synonym for "big idea" in chemistry. However, depending on the context, some alternatives could be:

General alternatives:

* Fundamental concept

* Core principle

* Key idea

* Central theme

* Major theory

Specific examples:

* Law of conservation of mass: This is a fundamental concept in chemistry that states matter cannot be created or destroyed in chemical reactions.

* Periodic law: This is a key idea that explains the recurring properties of elements based on their atomic number.

* Kinetic molecular theory: This is a major theory that describes the behavior of gases and explains their properties.

Ultimately, the best alternative depends on the specific big idea you are referring to and the context of your discussion.