Because physical science is a branch of the natural sciences that studies non-living systems, a good physical science study guide answer key should include answers to questions about basic concepts from chemistry, astronomy, physics and the earth sciences. Physical science is a science opposite to the life sciences.
A very basic idea in physical science is the scientific method. The steps to the scientific method are therefore essential to any good answer key for a physical science course. The definition of a hypothesis and a theory is a good place to start. It would also help to include an explanation of how a scientific theory differs from an opinion or a belief about the world. This would solidify what it means to have reasoned knowledge about the world as opposed to simply have opinions or arbitrary beliefs.
Other good concepts to include on the answer key are definitions of physics, chemistry, astronomy and earth science. A diagram or mapping of the differences between the sciences would help separate them but also show how they are interrelated as separate projects within scientific inquiry as an enterprise. The concepts of energy, work, matter, atmosphere and light are central to physical science studies and are therefore excellent choices for a good physical science answer key.