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. 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.
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.
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 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).
(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.
Use this resource to answer the questions that follow.
1. What is water made of?
2. Explain how water becomes a solid.
3. What happens when a solid melts?
4. How do molecules move in a liquid?
5. What are the major differences between a solid, a liquid, and a gas?
1. What is a polar molecule?
2. What are the three states that a substance can have?
3. Where in the solar system is water found in all three states?