* Pure substances have a fixed composition and cannot be separated by physical means. Examples include water (H₂O) and table salt (NaCl).

* Mixtures are combinations of two or more substances that can be separated by physical means.

Seawater contains:

* Water (H₂O): The primary component.

* Dissolved salts: Including sodium chloride (NaCl), magnesium chloride (MgCl₂), and many others.

* Dissolved gases: Oxygen, nitrogen, carbon dioxide.

* Other substances: Trace amounts of minerals, organic compounds, and even microscopic organisms.

Since seawater contains multiple substances that can be separated (like by evaporation to obtain salt), it is a mixture.