�Before You Begin:
This section does not require any specific background knowledge.
Classification of Matter
In order to make sense of the tremendous amount of information that we can learn about matter, chemists have developed categories of kinds of matter in which to organize the information. If we examine a thing, and decide which category it belongs in, we are classifying it. Keep in mind that there are several different systems of categories which are not mutually exclusive. When we classify organisms in biology, for instance, we can decide that a particular animal is male and that it is a reptile.
Pure versus mixture
A sample of matter is pure if it has only one identity throughout (there is one and only one ‘thing’ in it). A mixture has two or more different substances physically blended together such that each retains its own identity (two or more ‘things’ in the same chunk). Pure things are rather rare. Your computer is a mixture, a single thing, yet it each contains lots of different parts that you can see. Air is a mixture, too. It contains oxygen, nitrogen, carbon dioxide, and lots of other things, even though we can’t see them. Virtually everything in the room with you is a mixture, but you probably have a few pure things in your kitchen. Pepper is a mixture, but salt is pure (unless it is iodized salt, which is a mixture). “Good grief, how can anyone ever tell the difference?” you may be tempted to ask. Nearly everything that occurs naturally is a mixture. In order to have pure things, we have to spend time, energy and money to get the stuff we don’t want removed from the stuff we do want. When we manage to accomplish this, we have purified something.
Element versus compound
An element is a sample of matter that cannot be simplified (note that is 'simplified' instead of 'purified.' We’ll distinguish the two, soon). A compound is a sample of matter that can be changed into two or more elements. Since it contains more than one of the simple types of matter, it is more complicated than a substance with only one. Gold is an element. We cannot change gold into anything simpler than gold. Water is a compound. We can change water into hydrogen and oxygen, both of which are simpler than water.
If you are truly a beginner to the study of chemistry, you may be amazed and discouraged when the students in your class can volunteer other examples, such as aluminum is an element and rust is a compound. How can you tell the difference between an element and a compound? You can’t, unless you have access to fairly sophisticated chemical equipment. How can the other students tell the difference? They can’t. They just memorized some examples before they got to class. The good news is that there are only about a hundred elements to learn. The bad news is that there are an almost infinite number of compounds.
So what is the difference between a mixture and a compound? They are both things that have more than one kind of stuff in them. The difference lies in how the different kinds of stuff are combined. In a mixture, two or more things are blended but each retains its own identity. The properties of the mixture are similar to the properties of the things in the mixture. So if I add salt to water and stir, I get a mixture that looks like water but tastes salty. A compound has two or more elements that are combined so that they make a thing with a completely new identity. This new stuff has properties that are different from those of the elements that were used to make it. Iron is an element; it is shiny, strong, metallic, and magnetic. Oxygen is an element; it is a colorless, odorless gas. When iron rusts, it combines with oxygen in the air to form the compound iron oxide. Iron oxide not only has a different name, it has different properties from those of iron and/or oxygen. It is reddish brown, a solid, crumbly rather than strong, and non-magnetic.
4Concept Check: Here is an analogy that highlights the difference between a mixture and a compound: a mixture is to an element as an eye-chart is to a poem. Can you explain this analogy?
Answer: An eye-chart and a poem are both collections of letters printed on a page. However, when we read an eye-chart, we read each letter separately--they do not make sense as a set. When we read a poem, however, groups of the letters make up words that express different (and hopefully meaningful) ideas.
Explain this analogy on your own: an element is to a compound as bricks are to a house.
Try to write your own analogies for chemistry concepts. They can be a powerful tool to help you develop a deeper understanding.
States of mater
There are three major states of matter: solid, liquid, and gas. A solid is something that has a definite shape and volume. A liquid has a definite volume but takes the shape of its container. A gas takes the shape of its container and it expands to fill the entire container.
© Copyright 2005, Kelley Whitley, ChemProfessor. All rights reserved.
This site was last updated 05/12/05