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  • Looking down at the Earth from space, you can get an idea of just how much water there is. In fact, around 71% of the Earth’s surface is covered in water. Oceans make up about 97% of the world’s water, but because this is salt water, we are unable to drink it without first removing the salts. That leaves us with just 3% of fresh water, meaning we rely on our rivers, lakes, ice and rain for this essential life liquid.

    What is water?

    Water is a molecule made up of two Hydrogen (H) atoms and one Oxygen (O) atom. Liquid water consists of polar molecules with hydrogen bonds between the molecules.

    When we say the word water, we usually imagine it as a liquid. But water is able to pass through all states of matter. This means it can be a gas (steam), a liquid (water) and a solid (ice).

    We can use words like gas, liquid or solid to describe water’s physical properties. We can also say water is clear and colourless. Clear means we can see through it, colourless means it has no colour of its own. These two terms are often confused but are, in fact different, from each other. For example, if you made a cup of tea (without milk) and looked at it, you would see that the tea was clear (because you could see through it) but brown.

    Why does water freeze and become ice?

    Molecules are constantly moving because they have energy. In a liquid form, water molecules have more energy than in a solid – they move around quickly, essentially bouncing off of one another. As the liquid cools down, the amount of potential energy is reduced and the molecules start to move slower. When the water temperature reaches around 0°C, the molecules stick together and form a solid – ice. Even in this solid stage, the molecules are still moving – we just can’t see it.

    There is one more thing needed for water to turn into ice – a crystal-seed. This is a small impurity from which an ice crystal will grow. The impurity may be another particle in the water, or it may be caused by the container that holds the water. This is why not all water freezes at 0°C. Under some conditions, water can get as cold as -40°C – a process called super cooling. This occurs in completely pure water and usually in a very smooth container.

    So water turning into ice is not as straight forward as just getting cold. Other things also affect water freezing, including dissolved solids, water density, pressure and movement (such as currents).

    Related content

    The water cycle involves water moving from one form to another, through the different Earth systems so that it is constantly being recycled.

    Activity ideas

    Investigating sea water – in this activity, students investigate some of the properties of seawater.

    Water temperature – in this activity, students look at what happens when hot and cold water meet.

      Published 19 July 2007 Referencing Hub articles
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