Earth is covered in water. Today, the Earth contains the same amount of water as it did when dinosaurs were the most advanced life forms on the planet. Water is constantly being cycled and it is never created nor destroyed, it merely changes states between solid, liquid and gas. Approximately 97% of the water found on Earth is ocean leaving around 3% of Earth’s water as freshwater, covering a much smaller area and which is often divided. There is an obvious difference between ocean water and freshwater in that the ocean has a high salt content (over 3.5% salt) and fresh waters have less than 1% of salts. To understand how both the oceans and fresh waters are linked, read on and learn about water cycle.

Properties of water

Water has the chemical formula of H₂0 and is made up from a single oxygen atom and two hydrogen atoms bonded together as can be seen in the diagram below.

Water is a unique substance as it can be found in three states at normal temperatures on Earth. Water freezes and becomes a solid at 0°C, boils and becomes a gas at 100°C and between these temperatures, water is a liquid. This happens because of the changes in the arrangement of the water molecules. When frozen, the molecules are tightly packed together and ordered. When water is a liquid, the molecules become less ordered and fluid. When water becomes a gas, the molecules are arranged randomly and are more spaced out.

 

 

 

 

 

 

 

 

Figure 1. A water molecule showing the oxygen and hydrogen atoms

The Water Cycle, also known as the Hydrological Cycle describes the continuous movement of water below, along and above the surface of Earth. There are four main process that occur in the cycle, these are evaporation, condensation, precipitation and flow.

 

• Evaporation: (liquid to gas) Heat from the sun warms the surface of the water.  Some particles of the water turn to vapour which rise into the air
• Condensation: (gas to liquid) Water vapour rises up through the air until the air surrounding it becomes cool. The water vapour then cools and changes back into a liquid state and ‘stick’ to each other forming water droplets.
• Precipitation: (rain or snow) This is when the water droplets get so large and heavy, they fall to earth as rain. Snow is formed when the air is so cold, the droplets change from a liquid to a solid state.
• Channel flow: When rain falls, it either soaks into the ground into bedrock or runs off the land surface. This water is then channeled down hill either through man-made drainage systems in urban areas, underground-water flows or through river tributaries. This water then ends up in a freshwater body such as a stream or a lake which discharges its load into the ocean.

To read more about the Water Cycle click on the link to take you to our FreshwaterLife pages.