Here's why:

* Chemical properties describe how a substance behaves in chemical reactions. They tell us how a substance might change its composition to form a new substance.

* Physical properties, on the other hand, describe the appearance and behavior of a substance without changing its chemical makeup.

Examples:

* Chemical Property: Flammability (how easily a substance catches fire and burns). This describes how the substance might react with oxygen to form new products.

* Physical Property: Boiling point (the temperature at which a liquid turns into a gas). This property doesn't change the substance's chemical composition, just its state of matter.

In Summary:

When we talk about how a pure substance might form new substances, we are discussing its chemical properties because these properties describe its ability to undergo chemical reactions and change its composition.