Changing States of Matter

Elements and compounds can move from one physical state to another and not change their basic atomic

parts. Oxygen (O₂) as a gas still has the same properties as liquid oxygen. The liquid state is colder and denser, but the molecules (the basic parts) are still the same. Water (H₂O) is another example. A water molecule is made up of two hydrogen (H) atoms and one oxygen (O) atom. It has the same molecular structure whether it is a gas, liquid, or solid. Although its physical state may change, its chemical state remains the same.

So you're asking, "What is a chemical change?" Let's start with a glass of pure water. If the formula of water were to change, that would be a chemical change. If you could just add a second oxygen atom, you would have hydrogen peroxide (H₂O₂). The molecules in your glass would not be water anymore. A chemical change happens when the atoms in a molecule are moved around or when atoms are added or taken away. Chemical changes happen when bonds between atoms are created or destroyed. Changing physical states of matter is about changing densities, pressures, temperatures, and other physical properties. The basic chemical structure does not change when there is a physical change.