* Pure Substances: A pure substance has a fixed composition and uniform properties throughout. Examples include elements (like gold) and compounds (like water).

* Mixtures: Mixtures are combinations of two or more substances that are not chemically bonded. They can be separated by physical means and their composition can vary.

Alloys are mixtures because:

* Variable Composition: Alloys are created by mixing different metals (and sometimes non-metals) in varying proportions. This means their composition is not fixed.

* No Chemical Bonding: The metals in an alloy are not chemically bonded to each other. They are physically combined.

* Separable Components: The components of an alloy can be separated using physical methods, such as melting and electrolysis.

Examples:

* Brass: A mixture of copper and zinc.

* Bronze: A mixture of copper and tin.

* Steel: A mixture of iron and carbon (and other elements).

Let me know if you have any other questions!