The Language of Chemistry - types of substances

Substances | Symbols

Everything is made up of atoms. Atoms are the basic units of elements, which cannot not be broken down by any means (water can be separated into hydrogen and oxygen gas by applying electricity, so it is not an element. However, for helium this is impossible no matter what you do, so it is an element*). They are like the fundamental building blocks in chemistry, since from the elements we can make anything in this world.

The atoms are arranged in some way to make molecules, which are the basic units of compounds. Not everything in this world is a compound, though. To be a compound, the substance has to be of a "definite composition". Think about water. You probably know that it's "H₂O", meaning that there is one oxygen atom for every two hydrogen atoms. This ratio must be maintained or else it won't be water. In pure water, if there are 70 oxygen atoms, there are 140 hydrogen atoms. If there are only 139 hydrogen atoms, it is not pure water. Elements are similar: in pure gold, 100% of the atoms must be gold atoms. FYI: elements and compounds are called "pure substances" because of the need for constant composition.

But ice cream doesn't have to be in a "definite composition". If you add just 1% less sugar, you still end up making ice cream. There is no compound called "ice cream". This is because ice cream is not a pure substance; rather it is a "mixture": composed of several compounds whose ratios can vary.

There is a very broad difference between ice cream and stirred salt-water, though, that illustrates two types of mixtures. They are both made up of several compounds, but the salt in the water is evenly mixed throughout, while the ice cream may not be as uniform (some more sugar here, a different flavor there, etc.). Salt-water is called a "homogeneous mixture" because of its uniformity (homo- = same, gen- = kind), and ice cream is an example of a "heterogeneous mixture" (hetero- = different). Big words, but important ideas.

You may have also heard of the term "solution". This is a special type of mixture: one of the compounds must be in a liquid. In salt-water, the water is a liquid, so the salt-water is a solution. Soda is also a solution because the carbon dioxide, sugar, and flavorings are all mixed with water.

Another common word is "alloy". This is a mixture of metals. Brass is not an element or a compound. Rather, it is a mixture of the elements zinc and copper.

*When you add sodium to water, an explosion (chemical reaction) occurs. Yet sodium is an element. Why? The sodium does not "break down" in this case; it actually adds oxygen to create a compound. So, "breaking down" is not synonymous with "reaction". There are only 110 or so elements, so you won't have to try decomposing substances to see whether they are elements or not - just check the "periodic table", which lists all them (in a special way)!