"Iron reacts with oxygen in the presence of water to form rust."

Here's why this statement describes a chemical property:

* It describes a change in the composition of the substance. Iron (Fe) and oxygen (O) combine to form a new substance, iron oxide (Fe₂O₃), which is rust.

* It involves the formation of new chemical bonds. The chemical bonds within the iron and oxygen atoms are broken, and new bonds are formed to create rust.

Here are some other examples of chemical properties:

* Flammability (how easily a substance catches fire)

* Reactivity (how readily a substance reacts with other substances)

* Acidity (how acidic a substance is)

* Toxicity (how poisonous a substance is)

In contrast, physical properties can be observed without changing the substance's chemical composition. Examples of physical properties include:

* Color

* Density

* Melting point

* Boiling point