Liquid
OK here's the story, it's England, 1827. There's a scientist, Dr. Robert Brown, who's a botanist. He made a microscope slide of pollen grains in water, and took a look. He was amazed at what he saw. The pollen grains were moving around in random directions inside the water. This movement, called Brownian Movement, is caused by the molecules of water hitting the pollen grains. This tells us that the molecules of a liquid are constantly moving in unpredictable paths.
Cohesion occurs when molecules that are alike stick together. Adhesion is the opposite. It's the attraction of unlike molecules to one another. If you were to push a spoon into some honey it would take force to pull it out. This is due to the honey molecules sticking to each other. If you were to lick the honey off the spoon it would take force as well. That's because honey likes spoon molecules, too. This shows cohesion in a liquid, and adhesion between a liquid and a solid. The strengths of these forces tell us how the liquid behaves.
If a clean glass rod were dipped into a beaker of water, then removed, there would be droplets of water on the rod. We say that the water has wet the rod. That means that some water molecules were pulled off by the rod due to high adhesion between glass and the water. If the rod were dipped into mercury instead, when removed nothing would be on the rod. This is because of high cohesion in the mercury. Cohesive and adhesive forces vary in different liquids.
Have you ever seen that trick where someone will make a razor blade float on water? Ever wonder how they do that? Here's how: surface tension. Even though the blade is several times more dense than the water, it will stay on top. The water has a thin, flexible film on it. This film creates an upward force that balances the weight of the blade. That's surface tension
All liquids have surface tension. Mercury has very high tension, but not all liquids do. Part of the way soaps clean is by breaking down the surface tension of the water, helping it to get into those hard to reach spots.
Here's an experiment to amaze your friends with. All you'll need are a few things:
The change of phase from a solid into a liquid is called melting. When a solid melts, heat energy is applied to it. This energy lets the molecules move faster and farther away from each other. If enough heat is applied, eventually some of the bonds holding the molecules together will break, and the solid will become a liquid. The change of phase from a liquid to a solid is called freezing. With freezing, just the opposite happens. Heat is taken away from the liquid, molecules slow down, bonds form, and the liquid becomes a solid.
Normally when a substance is heated it expands, and when cooled, it contracts. Water is the most important exception to this rule. When you put an ice cube tray of water in the freezer, the level is straight across. When the ice cubes form they look distorted. This is because ice takes up 1.1 times the space that the same amount of water does. The particles that form ice are more widely spaced out than when in liquid form.
What have we learned about liquids?
- The molecules of a liquid are constantly moving in random directions
- Like molecule are attracted to each other. -Cohesion
- Unlike molecules are attracted to each other. -Adhesion
- Liquids have a thin, flexible skin on which you can float some things.
- Most substances expand when heated and contract when cooled, unlike water.
Well that was quite a bit to have learned in such a short period of time. I hope you will remember some of it. Keep expanding your knowledge by visiting another one of our pages. See you soon!
