* Solution: A solution is a homogeneous mixture where one substance (the solute) is dissolved evenly into another substance (the solvent).

* Seawater: Seawater contains dissolved salts (mainly sodium chloride), minerals, and gases like oxygen and carbon dioxide. These dissolved substances are uniformly spread throughout the water.

Why it's not a mechanical mixture or a pure substance:

* Mechanical Mixture: A mechanical mixture has components that are easily identifiable and can be separated by physical means. Seawater's components are not easily separated by simple methods like filtration.

* Pure Substance: A pure substance has a fixed composition and cannot be broken down into simpler substances by physical means. Seawater has a varying composition depending on location and can be separated into its components.

In summary: Seawater is a solution because its components are dissolved and uniformly distributed, making it a homogeneous mixture.