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States of Water

Water is a polar molecule and the only substance that can be any of the three states of matter on Earth.

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States of Water

H - two - O. Why is something so simple so important?

Water is the most important substance on Earth. Think about all the things you use water for? If your water access were restricted what would you miss about it?

The Water Molecule

Water is simply two atoms of hydrogen and one atom of oxygen bonded together (Figure below). The hydrogen ions are on one side of the oxygen ion, making water a polar molecule. This means that one side, the side with the hydrogen ions, has a slightly positive electrical charge. The other side, the side without the hydrogen ions, has a slightly negative charge.

Diagram of a water molecule

A water molecule. The hydrogen atoms have a slightly positive charge, and the oxygen atom has a slightly negative charge.

Despite its simplicity, water has remarkable properties. Water expands when it freezes, has high surface tension (because of the polar nature of the molecules, they tend to stick together), and others. Without water, life might not be able to exist on Earth and it certainly would not have the tremendous complexity and diversity that we see.

Three States of Matter

Water is the only substance on Earth that is present in all three states of matter – as a solid, liquid or gas. (And Earth is the only planet where water is abundantly present in all three states.) Because of the ranges in temperature in specific locations around the planet, 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 (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.


  • Water is a polar molecule with a more positive charge on one side and a more negative charge on the other side.
  • 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.


  1. What is a polar molecule?
  2. What makes water a polar molecule?
  3. What are the three states that a substance can have?
  4. Where in the solar system is water found in all three states?

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Use this resource (watch up to 5:50) to answer the questions that follow.

  1. What is the only substance that occurs naturally on Earth in all three states of matter?
  2. Why do scientists look for water in other places in the solar system?
  3. Describe the bond that keeps the water molecule together.
  4. What does it mean to say that the water molecule is polar?
  5. What are hydrogen bonds?
  6. Why does water form a droplet if it is placed on waxed paper or teflon?
  7. Why does water experience adhesion rather than cohesion with glass?
  8. What causes water in a straw to rise higher than the surface level of the water in the beaker the tube is in?
  9. Why is water such a great solvent?
  10. Why doesn't oil mix with water?

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water vapor Water in the form of a gas. Water vapor is invisible to humans; when we see clouds, we actually are seeing liquid water in the clouds.

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