November 24, 2012 2:08 pm

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You cannot drink a steak or nail chunks of air together. You cannot chew milk or breathe wood.
Steaks, air, milk, and wood seem very different from one another. Yet all these things have one thing in common. They are types of matter. Matter is anything that takes up space. Look around. Every object you see is made of matter.
Matter comes in different forms. Matter can be solid like steaks and wood. It can be a gas like air. It can be a liquid like milk. Scientists call solids, liquids, and gases the ordinary “states of matter.”
You know a solid when you see one. You can hold a solid rock in your hand. You can push on a solid brick wall. Scientists say a solid is something that has shape. A solid resists any change in its shape.
You can change the shape of solids if you work at it. If you hit a piece of glass with a hammer, it will break into pieces. You can saw a stick of wood in two. The pieces of glass and wood are still solids.
You know a liquid when you see one. You turn on a faucet and water pours out. The water flows down into the sink until you put it in a glass. The water takes the shape of the glass. You pour milk from a carton into a bowl for your cat. The milk takes the shape of the bowl.
A liquid has no shape of its own. A liquid does have a volume of its own. Volume is the amount of space that something takes up. You pour one cup of milk into the cat’s bowl. One cup is the volume of the milk. The volume of milk in the bowl will not change until your cat drinks it up.
Do you know a gas when you see one? You can’t hold a gas in your hand like a solid. You can’t pour a gas into a bowl like a liquid. Many gases are invisible. So how can you know what a gas is?
Scientists say a gas is something that expands to fill any container it is in. A gas does not have its own shape. A gas does not have its own volume. A gas does have its own weight.
You can weigh air and other gases. Suppose you weighed an empty rubber balloon and then filled it with air. The air-filled balloon would weigh more than the empty one. The added weight is the weight of the gas.
Matter is made of tiny bits called molecules. How these molecules line up and move around is what makes solids, liquids, and gases.
Molecules in solids are packed tightly together and do not move around much. They give a solid its shape.
Molecules in liquids are not so tightly packed. They move around more. The moving molecules are far enough apart to let liquids flow, but they are close enough to give liquids volume.
Molecules in gases zoom around every which way. They expand, or keep going outward, until they run into something. They fill any size container they are in.
Matter can go from solid to liquid to gas. You can change a material’s state of matter by changing its temperature or pressure. You can make something hotter or colder. You can squeeze the molecules more tightly together.
It is easy to understand how temperature can change states of matter by looking at ordinary water. Water is normally a liquid. But water can be a solid or a gas.
You change water to a solid when you make ice cubes. You fill an ice cube tray with water and put it in your freezer to chill. Lowering the temperature makes the water molecules slow down. When the water gets cold enough, the molecules slow down enough to freeze into solid ice. This change happens at 32° Fahrenheit (0° Celsius). This temperature is called the freezing point of water. Above 32° Fahrenheit (0° Celsius), ice starts to melt and turn back into a liquid. So this temperature is also called the melting point of ice.
You change water into a gas when you boil water. You set a pot on the stove and turn on the burner. The water gets hotter and hotter. The water molecules move faster and faster. When the molecules are zooming every which way, the water starts to bubble as steam rises up into the air. Water changes into steam, a gas, when its temperature reaches 212° Fahrenheit (100° Celsius).
Different materials have different melting and boiling points. Some of these points are very hot or very cold.
At 2795° Fahrenheit (1535° Celsius), iron melts. It becomes a red-hot liquid that can be poured into molds to make engines and other products. The metal tungsten doesn’t melt—change from a solid to a liquid—until about 6190° Fahrenheit (3420° Celsius)! It has the highest melting point of any metal. Helium changes from a gas to a liquid at -452° Fahrenheit (-268.9° Celsius). It is the most difficult gas of all to turn into a liquid.
You can change a gas into a liquid without lowering its temperature. Instead, you increase its pressure. Pressure is the force of gas molecules banging against a container wall. Stuffing gas into a smaller container increases the pressure. It pushes the molecules closer together. When the molecules are squeezed close enough, the gas changes into a liquid.
Your gas barbecue uses a liquid gas called propane. Propane gas stuffed into small bottles or tanks becomes a liquid. When you turn on your grill some propane shoots out of the tank. Pressure on the propane lessens, and it turns back into a gas. The gas starts to burn when it hits the flame.
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