1. Different forms of water in the world: liquid, solid and gas.
2. The hydrologic cycle.
3. Occurrence of salt water and fresh water: oceans, rivers, lakes, ground water and atmosphere.
Why are the Oceans Salty?
As water flows in rivers, it picks up small amounts of mineral salts from the rocks and soil of the river beds. This very-slightly salty water flows into the oceans and seas. The water in the oceans only leaves by evaporating (and the freezing of polar ice), but the salt remains dissolved in the ocean – it does not evaporate. So the remaining water gets saltier and saltier as time passes.