Water is made up of two elements, hydrogen and oxygen. Each molecule of water is made up of two hydrogen atoms bonded to a single oxygen atom. The existence of water is essential for life on Earth. Water has three different states, liquid, solid and gas. The word water usually refers to water in its liquid state. The solid state of water is known as ice while the gas state of water is known as steam or water vapor. Water covers around 70% of the Earth’s surface. The three largest oceans on Earth are the Pacific Ocean (largest), the Atlantic Ocean (second largest) and the Indian Ocean (third largest).
Found in the Pacific Ocean, the Mariana Trench is the deepest known point in the world’s oceans. Ocean tides are caused by the rotation of the Earth and the gravitational pull of the Moon and Sun acting on ocean water. Water from a sea or ocean is known as seawater. On average, every kilogram (2.2lb) of seawater contains around 35 grams (1.2 oz) of dissolved salt. The freezing point of water lowers as the amount of salt dissolved in at increases. With average levels of salt, seawater freezes at -2 °C (28.4 °F). The longest river in the world is the Nile River, it reaches 6650 kilometers in length (4132 miles). The second longest river in the world is the Amazon River, it reaches 6400 kilometres (4000 miles) in length. Water makes a good solvent with many sugar, salts and acids easily dissolving in it. On the other hand oils and fats don’t mix well with water. The water cycle involves water evaporating (turning into a gas), rising to the sky, cooling and condensing into tiny drops of water or ice crystals that we see as clouds, falling back to Earth as rain, snow or hail before evaporating again and continuing the cycle. Learn more about the water cycle. Water in the form of ice is found at the polar ice caps of the planet Mars, some scientists have also suggested the possibility of liquid water on the red planet. Pure water has no smell and no taste, it also has a pH level around 7. While most people know that water boils at 100 °C (212 °F), this is at the normal conditions of sea level. The boiling point of water actually changes relative to the barometric pressure. For example, water boils at just 68 °C (154 °F) on the top of Mount Everest while water deep in the ocean near geothermal vents can remain in liquid form at temperatures much higher than 100 °C (212 °F).
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