Here's why:

* Pure Substance Definition: A pure substance is a material that consists of only one type of particle, either atoms or molecules.

* Examples of Pure Substances:

* Elements: Gold (Au), Oxygen (O2), Helium (He), etc.

* Compounds: Water (H2O), Sodium Chloride (NaCl), Carbon Dioxide (CO2), etc.

What makes water special:

* Water is a very common and essential pure substance for life on Earth.

* It has unique properties like being a good solvent and having a high heat capacity.

Let me know if you'd like to learn more about pure substances!