* Elements are pure substances that cannot be broken down into simpler substances by chemical means. Examples include gold (Au), oxygen (O), and hydrogen (H).

* Compounds are formed when two or more elements combine chemically in a fixed ratio. Examples include water (H₂O) and salt (NaCl).

* Mixtures are combinations of two or more substances that are not chemically bonded. The substances in a mixture retain their individual properties.

Seawater contains a variety of substances, including:

* Water (H₂O): The main component of seawater.

* Salts (NaCl, MgCl₂, etc.): These dissolve in the water and give seawater its salty taste.

* Dissolved gases (oxygen, nitrogen, carbon dioxide): These are essential for marine life.

* Organic matter: Includes living organisms, dead organisms, and their waste products.

* Minerals: Various minerals are dissolved in seawater.

Because seawater contains multiple substances that are not chemically bonded, it is classified as a mixture.