* Pure substances have a fixed composition and cannot be broken down into simpler substances by physical means. Examples include elements like gold and compounds like water (H₂O).

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

Seawater contains a variety of dissolved substances, including:

* Salts (primarily sodium chloride)

* Minerals (like magnesium, calcium, and potassium)

* Dissolved gases (like oxygen and carbon dioxide)

* Organic matter

Because these substances are not chemically bonded, they can be separated by methods like evaporation (to obtain salt) or distillation (to obtain fresh water). This indicates that seawater is a mixture, not a pure substance.