Main->Readings->4th Grade Readings->Matter- >Part 3: Describing Matter
Vocabulary |
Scientists who study matter are called chemists (KEM-ists), and chemists are a lot like detectives. They can take a few clues about a piece of matter, even if it is very tiny, and they can tell you exactly what kind of matter it is.
They can do this because each kind of matter has its own properties. A property is something that describes matter. For example, if you say that an apple is red or a pear is yellow, you are talking about the property of color. Color is also a property chemists can use to identify matter.
However, sometimes a pear can be red, and an apple can be yellow. You have to think of something else to tell them apart. For instance, shape is a property. Pears are thinner at the top than at the bottom, and apples are more like spheres.
An apple is not always red, and a pear is not always yellow.
What if a pear and an apple could be the same shape? You would probably use the property of taste to tell them apart. You can see that you might use several properties to tell the difference. Just as with identifying fruits, chemists often use more than one property to identify matter.
You use shape to tell pears and apples apart, but shape is something that can change. If you cut pears and apples into cubes, they would have the same shape, but they would still be different. There are other properties that can change without changing the kind of matter, such as volume and mass. For instance, a ton of bricks is large and heavy, while a chunk of brick is small and light, but both of them are still brick.
Chemists try to find properties of matter that do not change, such as color, hardness, shininess, and density. In the next section, you will find out what density is and how to measure it.
You may have noticed that some things seem to weigh more for their size. If you have ever carried a bucket full of water, you know it is heavy, but if you try to fill the same bucket with rocks, you would find it much harder to carry. Rocks weigh more for their size than water. If you filled the same bucket with wood chips, it would be easier to carry than either the water or the rocks.
Another way to see the difference between rocks, water, and wood is to drop a rock and a piece of wood into the water. The rock sinks. The wood floats.
We say rocks have more density than water, and we say that wood has less density than water. Density is the amount of mass packed into a particular volume.
How can you measure density? Well, since the definition of density talks about mass and volume, you have to measure those two things and divide.
Suppose you wanted to measure the density of water. In the metric system, one cubic centimeter of water weighs about one gram. So we say its density is 1 gram divided by 1 cubic centimeter. 1 divided by 1 is 1. The density of water is about 1 gram per cubic centimeter.
The funny thing is that density does not change when you change the amount of water. 2 cubic centimeters of water weighs 2 grams. 2 divided by 2 is still 1. And it keeps going. A bucket of water, a bathtub of water, and a lake of water, all have a density of 1 gram per cubic centimeter.
Other kinds of matter act the same way. Iron, for instance, is very dense. If you drop an iron bolt in the swimming pool you are going to have to dive for it, because it will not float. The same thing is true for an iron frying pan, or a wrought iron chair. They will all sink. The size does not matter. Every part of it still has the same density.
On the other hand, styrofoam is not dense. If you have a styrofoam cup, it will float in the water. So will a styrofoam cooler. If you threw a hundred styrofoam cups in the pool, they would still float, all of them. Density is a great property for chemists, because it stays the same.
As you may have noticed, it is easy to compare the density of materials with the density of water, because things which have a high density sink, while things with a low density float. It can be fun to test the density of household objects in your bathtub. Just don't test anything you don't want to get wet.
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 4: States of 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