Main->Readings->4th Grade Readings->Matter ->Part 4: States of Matter
Vocabulary |
Remember that matter is made out of atoms. Most of the time, atoms are not found all by themselves. They are joined together with other atoms in tiny things called molecules (MOLL-uh-kyoolz). Molecules are still too small to see. Yet we know they are there. Just as with atoms, molecules are always moving. They vibrate, which means they wiggle back and forth. Sometimes they are spaced close together, and other times they are farther apart. The vibration and the spacing are so small you cannot see them, but they are there.
There are many ways scientists know about this. For instance, if you put food coloring into a glass of water, even if the water seems to be very still, eventually the food coloring spreads until all the water is colored. This is because the molecules of water are moving. Otherwise the food coloring would just stay in a blob.
Another example is odor. You can smell things in the air, whether it is popcorn or perfume, because tiny molecules of the popcorn or perfume are brought to your nose by the air.
Another reason scientists know about molecules is the phases of matter, and in the next section you will learn why this is so.
There are four main phases of matter: solid, liquid, gas, and plasma. We call them phases because they are temporary situations. Any kind of matter can be a solid, a liquid, a gas, or a plasma if conditions are right. Air can be a solid. Steel can be a liquid. It is just that most of the time, the conditions are not right for air to be a solid or steel to be a liquid. Yet every day in factories, steel is melted so it can be formed into tools and other useful things. When it is melted, it is a liquid, but it is still steel.
When something is solid, like copper, steel, rock, or ice, it is stiff. It does not change shape easily, and it does not change size. In fact, that is the definition of a solid: matter which has a definite volume and a definite shape.
Why do solids behave this way? The molecules in a solid, though they vibrate, do not move very far. When they wiggle, they wiggle in place. They fit closely together and keep their position.
Matter when it is solid usually does not take up as much space as matter in other phases. This makes sense. The molecules are crowded together and do not move away from each other. They can jam into a smaller space.
A liquid like honey, cooking oil, or water can flow and move. It changes shape easily, and takes the shape of its container, though its size stays the same. The definition of a liquid is matter which has a definite volume and no definite shape.
Liquids behave this way because in a liquid, the molecules are moving around more than the molecules in a solid. They slide over each other.
In a liquid, matter can take up a bit more space than in a solid. Because the molecules are free to move around each other, they can spread out more, but they don't go far from each other.
A gas like oxygen, carbon dixide, helium, or steam can expand to fill any space it is in. If you open a helium balloon, the escaping helium goes everywhere in the room. Gas is matter which has no definite volume (it can be any size) and no definite shape.
The molecules in a gas are moving around like crazy, bouncing everywhere. They do not have to stay close to one another.
A gas takes up a great deal more space than a liquid or a solid. It takes up as much space as there is! However, it can also be squeezed, or compressed, unlike a liquid or a solid. That is why a deep sea diver can carry enough oxygen for long dives in a tank on his back, and why a small helium tank can inflate dozens of balloons.
The phase of matter, as you have seen, depends on how its molecules are moving. The molecules in a gas are moving a great deal. The molecules in a solid are moving much less. A gas has a great deal of energy, a liquid has less, and a solid has the least.
Can there be a phase where molecules are not moving at all? Scientists have gotten matter very close to that state. Matter at this phase behaves in very strange ways. Many of its properties change. When molecules are not moving at all, scientists call that state absolute zero. It is a very cold temperature: about 273 degrees below 0° Celsius.
Is there a phase with more energy than a gas? A plasma is the state where the atoms have so much energy and are moving so much, their outsides come off! When you turn on a fluorescent light, the gas inside becomes a plasma.
You can see that the phase of matter depends on how much energy its molecules have. The energy they have is heat energy. Heat is the energy of moving molecules. The hotter something is, the more energy its molecules have and the more likely it is to be a gas or a plasma. In the next part, we will talk about the way heat makes matter change phase.
Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:
Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.
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Go on to Part 5: Changes In Matter
This page last modified August 15, 2002
Copyright ©2000 Delia Marshall Turner. All rights reserved.
Questions? Send me a note at dturner@haverford.org