Water
Objectives- describe the unique properties of water
One molecule of water is made up of 2 hydrogen atoms and one oxygen atom. The formula for
water is H2O. There are at least 1 billion billion (not a typo) molecules of water in a drop of water. In
order to understand some of water’s properties, you need to understand its shape. In the center of the
molecule is the oxygen atom. The oxygen atom has a slight
negative charge, while the hydrogen atoms have a slight
positive charge. The opposite charges are attracted to each
other holding the atoms together. The force that holds
atoms together is called a bond. The bond type present in
water is strong. As you can see in the diagram, the atoms
form a boomerang shape. When the molecules come
together, they come together in geometric shape. This
explains the shape of a snowflake.
Water’s density is unique. Under most temperatures, it does what is expected- becomes less
dense as it is heated and more dense as it is cooled. More dense objects sink and less dense objects
float. However, between a very narrow temperature range of 4-0°C (39-32°C) it does the opposite.
Instead of becoming more dense, it becomes less dense. When water freezes, it expands (ever freeze
water or a can of pop?) which makes it’s density go down. That explains why ice floats. Since ice floats
when it freezes, the water below is insulated from the cold temperatures and does not freeze.
Water dissolves more substances than any other. This is due to charges on the surface of the
water molecule being attracted to the charges of other atoms.
Have you noticed how water appears to form a bubble at the top of a full glass of water and yet
it does not spill? That is because water has a high surface tension. Surface tension allows insects like
water striders to float on the surface. Surface tension is created by the attraction of water molecules to
each other. The molecules at the surface are attracted more to each other than to the molecules in the
air, creating like a skin on the surface.
Questions:
1. What would happen to the water in a lake if it did not become less dense as it froze? What would
happen to living organisms if water did not become less dense as it froze and water froze from the
bottom of the lake upwards?
2. Suggestion a reason why rubbing alcohol evaporates more quickly than water.
3. Why does a dry sponge absorb very little water, but a damp sponge absorbs a lot?