Water is a molecule that consists of two elements: oxygen and hydrogen.

Water is a good solvent. This means that different substances, especially sugars and salts, dissolve into water easily. This is especially beneficial for plants, because they can receive important nutrients and minerals alongside the water that they collect with their roots. 

Water is also used by plants to trasnport the sugar they produce in their leaves into other parts of their structure. The sugar dissolves with water, and moves through the plant's vascular system. Similarly, the human body uses water to transport salts and sugars between various cells and organs. This is why both plants and humans are mostly made of water. 

Some gases also dissolve into water. For example, oxygen that dissolves into water is used by fish to breathe through their gills. However, water contains significantly less oxygen than the air around us. 

Although the air is full of oxygen, fishes will quickly die on dry land. This is because their gills are not suited to breathe without moisture. On dry land, the gills dry out and stick together, making the fish suffocate. 


Enough oxygen is dissolved into water for fish to breathe through their gills.

A solution is a mixture of substances, where one substance is dissolved into another. For example, salt molecules quickly dissolve in water so that they cannot be distinguished with the naked eye. In the images below, the test tube on the left contains water that has been mixed with quartz sand. The test tube on the right contains water that has been mixed with salt. The picture on the left shows the starting point, and the picture on the right shows the situation some minutes after the two substances have been mixed together. The picture on the right clearly shows how the salt solution on the right is more transparent than the mixture of quartz sand and water on the left. 

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Salt (test tube on the left) dissolves into water easier than quartz sand (test tube on the right).

If you want to separate the salt from water once again, the water must be evaporated. This method is used to separate sea salt from sea water. The heat of the Sun helps to evaporate water from shallow beaches, and people can gather the crystallized sand into large piles.


Evaporation is used to separate sea salt from sea water.