8.2: States of Water

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What are the three states of water in this photo?

Water is present as liquid, solid, and gas in this photo. We take water for granted here on Earth, because there is a lot of it!

The Water Molecule

A molecule of water (Figure below) is made of three atoms bonded together. Two of these atoms are hydrogen, and one atom is oxygen. The hydrogen atoms have a partial positive electrical charge. The oxygen atom has a partial negative electrical charge. The hydrogen atoms are located on one side and the oxygen atom on the other. This means that the side with the hydrogen ions has a slightly positive charge. The other side, the side without the hydrogen ions, has a slightly negative charge. The water molecule is a polar molecule.

Diagram of a water molecule

A water molecule. The side with the hydrogen atoms has a slightly positive charge. The side with the oxygen atom has a slightly negative charge.

Water has remarkable properties. Water expands when it freezes, even though all other substances contract. This means that in the winter ice on a pond or lake will float. Water has high surface tension, because polar molecules tend to stick together. That's why drops stick together or some bugs can walk on the surface of a pond. Without water, life might not be able to exist on Earth. If it did exist, it would not be as diverse or as complex.

Three States of Matter

Water is the only substance on Earth that is present in all three states of matter. This means that water is present as a solid, liquid, and gas. Earth is the only planet with a significant amount of water present in all three states. All three phases may be present in a single location or in a region. The three phases are solid (ice or snow), liquid (water), and gas (water vapor). See ice, water, and clouds pictured below (Figure below).

Ice, liquid, and water vapor are the three phases of water

(A) Ice floating in the sea. Can you find all three phases of water in this image? (B) Liquid water. (C) Water vapor is invisible, but clouds that form when water vapor condenses are not.

Summary

Water is a polar molecule. On one side there is a positive electrical charge. On the other side there is a negative charge.
Water is the only substance on Earth that is stable in all three states.
Earth is the only planet in the solar system that has water in all three states.