Describing Materials

As we deal with observing things and describing them, it is important to realize that there is an important difference between a material and an object. If you will picture in your mind a cast iron skillet, that should help clarify the difference. "Cast iron" is the material. "Skillet" is the object. Materials are the "stuff" aspect of things. Some of the qualities or properties of materials are color, phase, clarity, degree of homogeneity, density, hardness, and sometimes the texture and crystalline shape. Also important are the interactions with heat and cold and with other materials.

Phase

In dealing with materials, let's start with the simple appearance. One aspect of appearance that is extremely important to chemists is the state of matter or phase. State refers to whether the material is a solid, liquid, or gas. I'd like to point out a few things about these phases that you might not be completely aware of. Solids are rigid: they maintain their own shape; their shape is not influenced by the shape of their containers; they have a fixed volume and a fixed shape. Powders are solids too. Even though the powdered material as a whole doesn't retain its shape, the individual particles do.

Liquids, on the other hand, are not rigid. They are fluid. Their shape adapts to fit the bottom of whatever container they are in. The top surface of a liquid is approximately flat and level. Their volume is fixed but their shape is not. There are some minor exceptions to these statements about liquids so keep your eyes open.

Gases are also fluid in that they are not rigid. Their shape is not fixed. Also, their volume is not fixed. The volume occupied by a gas is not determined simply by the amount of gas that is present. Any gas, by itself or mixed with others, will spread out to completely fill whatever container it is in. Perhaps I should say that a gas will fill whatever closed container it is in because if the gas is put into a container having a hole, the gas will spread beyond the walls of the container.

The three states, by the way, are dependent on temperature. If you were to heat a solid, it might melt or vaporize. A liquid could boil. If a gas is cooled enough, it will liquify; and if a liquid is cooled enough, it will freeze into a solid. We will look at these phase changes later on in this lesson.

Color

Color is another very important property of material-one which on the surface wouldn't seem to need any explanation or detail. If something is red, it's red; and if it's blue, it's blue. But it turns out there is quite a bit more to color than just its appearance. The color that we see is actually a combination of many different colors. I would like you to be aware of this and in a moment I'll have you do an exercise to demonstrate a few aspects of color.

As you probably know, Newton did an important experiment with light some time ago. He ran white light through a prism and broke it up into a full spectrum with all the colors in the rainbow. Rainbows themselves are white light that has been broken up into a variety of different colors, the component colors of white light. Things we see that are white are able to reflect all colors that hit them. Whereas something that is red can either be red because it absorbs green and reflects everything else or it might absorb everything but red and reflect only the red. So, there are a variety of different aspects to the colors.

Generally, when describing something, the overall color (blue, pink, green, whatever) is the important feature rather than the individual parts of the spectrum that are present. Occasionally, the component colors in the spectrum are the important feature.

Clarity

Clarity is another property having to do with appearance. Something is clear or transparent if light can shine through it. If you can see through it, it is clear. A material can be clear whether or not it has color. Quite often clear means colorless to many people, but in this context that is not really true. For example you can have a clear blue sky. The sky does have color, so don't confuse the term "clear" with the term "colorless". They have different meanings.

If only a limited amount of light will pass through something, like waxed paper or frosted glass, it is called translucent rather than transparent. The light can come through but the image will not. Unless you have the material real close to what you are looking at, the light will diffuse and you will lose the sharp image that you would get with a transparent material. If no light will pass through, it is called opaque.

Homogeneity

Another important aspect of the appearance of a material is whether the material is homogeneous ("ho-mo-gee-nee-us") or heterogeneous ("het-er-o-gee-nee-us"). That is, does the material appear to be the same throughout or does it contain things which can be visibly distinguished from one another? Materials which have the same appearance all the way through the sample are said to be homogeneous. Homogenized milk, for example, is the same all the way through--it does not have separate layers of milk and cream. On the other hand, a material which has visibly distinguishable components is said to be heterogeneous. Dirt or concrete or seawater would be fairly good examples of heterogeneous materials. On reasonably close inspection, each of these materials contains within it a variety of distinguishable components.

Describing Objects

Materials vs. Objects

It is sometimes necessary to distinguish between materials and objects. I said before that materials are the "stuff" aspect of things. Samples and objects are the "item" aspect of things. A few examples should help to show what I mean. A pencil is an object--graphite, wood, paint, metal, and rubber are materials from which it is made. Another example is a cast iron skillet. The material is the cast iron and the object is the skillet. Saying that it is round, ten inches across, and has a handle describes the skillet but not the cast iron. The size and dimensions are sometimes called extensive properties. On the other hand, to say that it is solid, dense, hard, dark gray, opaque, and attracted to a magnet describes the cast iron, the material out of which the skillet is made. These are often called intensive properties. Quite often it is very easy to make such distinctions, but sometimes it gets a bit cloudy. For example the word "cork" can be used to identify an object and also to identify a material. So be aware of that.

Objects

Some of the extensive qualities that objects have are size, weight, and shape. Some of these qualities describe the amount of material that you have. Some of these qualities describe the condition that a material is in.