Carolyn N. Kinder
On the earth, matter can exist in three states. The states of matter are solid, liquid, and gas. A solid has a definite shape. A liquid takes on the shape of its container.
A gas does not have a definite volume or a definite shape. A gas fills all the space in a container, regardless of the shape or the size.
Most materials can take any of the three forms, with no change in their chemical composition. Steam, water, and ice are common names for the three forms taken by a single material.
The best way to picture the difference in the states of matter is to think about water which can be changed into a solid by freezing it to produce ice, melting it to produce a liquid, and heating it to produce a vapor or gas.
In order to understand the states of matter, you must know something about molecules. The different substances that exist are made up of small particles. A molecule is the smallest part of any substance that still has all of the properties of that substance. The behavior of solids, liquids, and gases can be explained in terms of the arrangement and movement of molecules.
In the solid, atoms are close together. They vibrate but cannot move past one another. In the liquid the atoms are almost as closely packed as in the solid, but they can move past one another. In the gas the atoms are widely separated, and can move almost independently.
Figure 1.1 The Three States of Master
(figure available in print form)
Molecules Are Small: How Small?
The kinetic theory gives us a clear idea of how molecules in a gas are affected by changes in temperature, volume and pressure and how they change from one state of matter to another. There are other things that we know about molecules; we can calculate their speed, their number, their size and their relative weights with a considerable degree of accuracy.
The speed of a molecule will depend upon the temperature; it will also be affected by collisions with other molecules. The molecules in a gas will move at a wide range of speeds. The average is high; it is something like 1,000 miles an hour for oxygen molecule at 0°C. Heavy molecules move more slowly than light ones at the same temperature.
It is easy for us to grasp a figure like 1,000 miles an hour since airplanes have already reached speeds several times greater than that rate. It is harder to have a clear understanding of the very large number of molecules and their extremely small size. For example, in every cubic inch of air about us there are some 500,000,000,000,000,000,000 molecules. Suppose these were apportioned, share and share alike, among all the people of the world. Suppose also that the United Nations offered each person five cents per million for his share, provided he counted his molecules accurately. Would it be worth your while to turn in your share to the UN? The answer would be yes and no. The amount to which you would be entitled would be a respectable $8,500. But if a machine counted your molecules at the rate of three per second, day and night, you would have to wait something like 1,750 years, before you received your money according to
The Book of Popular Science
, Other Facts About Molecules, volume 1, p. 172.
The great Italian Chemist Count Amadeo Avogadro (1776-1856) calculated that 18 grams of water, which is a little more than half an ounce have 602,000,000,000,000,000,000,000 molecules. This huge figure is known as Avog adro’s number; it has proved to be valuable in many calculations. It may be abbreviated to read 6 x 10
23
would be 6 with 23 zeros after it or 602 with 21 zeros after it.. 6 x 10
23
is 602 with 21 zeros after it.
We calculate the volume of a single water molecule by using Avogadro’s number. We assume that the molecules are so closely packed in liquid water that the amount of empty space is negligible in comparison to the volume of the molecules themselves. Hence we may divide the total volume of water in 18 grams by the total number of molecules (6 x 10
23
) in order to find the volume of one molecule which is about 0.000,000,000,000,000,000,000,03 cubic centimeters, or 3 x 10
23
cubic centimeters. This calculation is in terms of the molecules of a liquid. The individual water molecule remains unchanged; it will have the same volume in the gaseous and solid states that it had in the liquid state.
Avogadro’s hypothesis is that “equal” volumes of gases at the same temperature and pressure contain equal number of molecules. This means that at a given temperature and pressure, if 6 x 10
23
molecules of water vapor occupy 50 liters, then 50 liters of any other gas, such as oxygen, for example will contain 6 x 10
23
molecules.
There is still more to be learned about molecules. You may want to investigate later how they are transformed in nature, and how man deliberately changes their patterns to produce a large number of useful chemicals.
Inside the Atom
Atoms are made up of even smaller particles called subatomic particles. There are three particles that make up atoms. They are electrons, protons, and neutrons.
Protons and neutrons are tiny specks that have almost the same mass. Protons are particles that carry a positive electric charge. Neutrons are particles that have no charge, which means that they are electrically neutral. Together these two kinds of particles make up the nucleus, the center of the atom. All around the outside of the atom are electrons moving around the nucleus like insects swarming around a street light. In an ordinary uncharged atom there are exactly as many electrons as protons, and there is just as much positive as negative charge.
As small as the atom is, it possesses mass. We cannot weigh an atom as we would weigh flour, or potatoes. The reading of the most delicate scale at our disposal would not be affected in the slightest if we added a thousand atoms to one of its trays. We can weigh an individual atom by indirect methods. We know, for example, that an oxygen atom weighs .000,000,000,000,000,000,000,026 grams. We call the absolute weight of the oxygen atom, because we consider it by itself and not in relation to the weight of the other atoms.
We use the relative weight or atomic weight to compare atoms. For example, we take an isotope of carbon as the basic unit of the system of atomic weights and give it the value of 12,000. When we weigh equal volumes of carbon 12 and hydrogen, we find that the carbon is about 11.905 times as heavy as hydrogen. In the atomic scale, hydrogen has the value of 12,000 divided by 11.905, or 1.008 (1.00797, to be more exact.)
Using the appropriate methods, we can compute the ratio between the weights of all atoms, which are listed in the Periodic Table. There are ninety-two naturally occurring atoms. For example the sulfur atom is 2.672 times heavier than the carbon atom; its atomic weight is 2.672 x 12 or 32.064. The weight of each atom is the sum total of the weights of each of the subatomic particles of which the atom consists. They are held within the atom by electrical forces. When we say that they are held together, we do not mean that they are closely packed. We mean that they occupy very little of the space within the atom since matter is made up of atoms. In spite of the fact that most of the mass of the atom is in the nucleus, the proton and neutron are extremely tiny. In a pound of any substance you think of iron, gold, cork or air as something like having 270,000,000,000,000,000,000,000,000 protons and neutrons.
Atoms are very small in size and in general we have no exact picture of their structure. However, models of atoms have been constructed to help people understand the basic structure and behavior of atoms. See figure 2
(figure available in print form)
(Figure 2. Drawing of a planetary model of an aluminum atom. According to this model, the electrons travel in fixed paths.
Electrons are found in a region called the electron cloud. This represents the space in which electrons are likely to be found. Within the electron clouds, electrons are arranged in energy levels closest to the nucleus. Electrons with higher energy are found in energy levels farther from the nucleus. See figure 3
(figure available in print form)
Figure 3. Drawing of an electron cloud model of an aluminum atom.