3.1  Solids, Liquids, and Gases (continued)

Describing the States of Matter

If you were asked to classify some materials as solids, liquids, or gases, you would probably find the task fairly easy. But could you describe what method you used to classify the materials? You might notice that some materials have a definite shape and volume and some materials do not.   Materials can be classified as solids, liquids, or gases based on whether their shapes and volumes are definite or variable. Shape and volume are clues to how the particles within a material are arranged.

Solids Think about these familiar objects: a pencil, a quarter, a book, and a cafeteria tray. What do these four objects have in common? They all have a recognizable shape and they all take up a certain amount of space. The materials in these objects are all in the solid state. Solid is the state of matter in which materials have a definite shape and a definite volume.

The term definite means that the shape and volume of a pencil won't change as you move the pencil from a desk drawer to a pencil case to a backpack. Changing the container doesn't change the shape or volume of a solid. However, the term definite doesn't mean that the shape or volume can never change. After all, you can change the shape of a pencil by sharpening it. You can change the shape of a copper wire by bending the wire.

Figure 2 shows the arrangement of atoms in a copper wire. The copper atoms are packed close together and are arranged in a regular pattern. Almost all solids have some type of orderly arrangement of particles at the atomic level.

Figure 2  Samples of solid copper have definite volume. Copper atoms are packed close together in an orderly arrangement.

Liquids How good are you at estimating whether the juice remaining in an almost-empty bottle will fit in a glass? If your estimate is not accurate, you will run out of space in the glass before you run out of juice in the bottle.

Appearances can be deceiving. Imagine a narrow glass and a wide bottle side by side. Each contains exactly 350 milliliters of juice (about three quarters of a pint). There will seem to be more juice in the glass because the juice rises almost to the rim of the glass. There will seem to be less juice in the bottle because the juice forms a shallow layer. What can you learn about liquids from this comparison?

A liquid always has the same shape as its container and can be poured from one container to another. Liquid is the state of matter in which a material has a definite volume but not a definite shape.

Mercury exists as a liquid at room temperature. The drawing in Figure 3 shows the arrangement of atoms in liquid mercury. Compare this arrangement to the arrangement of copper atoms in Figure 2. The mercury atoms are close together but their arrangement is more random than the arrangement of atoms in copper.

Figure 3  At room temperature, mercury is a liquid. Drops of mercury on a flat, clean surface have a round shape. Mercury in a container has the same shape as its container.

Gases If you were asked to name a gas, what would you say? Air, which is a mixture of gases, is probably the most obvious example. You might also mention natural gas, which is used as a fuel for heating homes. Gas is the state of matter in which a material has neither a definite shape nor a definite volume. (The adjective form of the word gas is gaseous (gas e us), as in gaseous state.) A gas takes the shape and volume of its container.

The balloons in Figure 4 are filled with helium, a colorless gas that is less dense than air. Two of the balloons are teardrop-shaped and two are disk-shaped. The “shape” of the helium in a balloon is the same as the shape of the balloon itself. The volume of the helium in a balloon is equal to the volume of the balloon.

Figure 4  Helium gas takes the volume and shape of its container.

The helium atoms in a balloon are not arranged in a regular pattern, as shown in the drawing in Figure 4. They are at random locations throughout the balloon. There is more space between two helium atoms in a balloon than between two neighboring atoms in solid copper or liquid mercury.

Because of the space among helium atoms, a large amount of helium can be compressed into a metal cylinder. When helium flows from the cylinder into a balloon, the helium atoms spread out. If 200 balloons are filled from a single cylinder, the total volume of the balloons will be much larger than the volume of the cylinder.

Other States of Matter On Earth, almost all matter exists in a solid, liquid, or gaseous state. But ninety-nine percent of all the matter that can be observed in the universe exists in a state that is not as common on Earth. At extremely high temperatures, such as those found on the sun or other stars, matter exists as plasma. You will read more about the properties of plasmas in Chapter 10.

In the 1920s Satyendra Bose, a physicist from India, wrote a paper about the behavior of light. After Albert Einstein read the paper, he realized that the behavior described could apply to matter under certain conditions. Einstein made a bold prediction. He predicted that a fifth state of matter would exist at extremely low temperatures. At temperatures near −273°C, groups of atoms would behave as though they were a single particle. In 1995, scientists produced this fifth state of matter, which is called a Bose-Einstein condensate (or BEC). It behaved as Einstein had predicted decades before.