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Phases of Matter X - Elements, Compounds, and Mixtures

Written by Translator on July 3, 2008 – 8:00 pm -

Normal matter (not exotic matter like black holes or neutronium) is either an element, compound of two or more elements, or a mixture of elements, compounds, or both. This sounds pretty basic, but is important and will be needed for future installments.The classic definition of an element goes something like, “a material that cannot be decomposed into simpler materials”. That is pretty good, but a more fundamental one is that an element is a material composed of atoms that all have the same atomic number. (The atomic number is the number of protons in the nucleus). This is a better definition because we now know that we can fission uranium, for example, into simpler elements.

Elements come in several varieties, called isotopes. While atoms in an element all contain the same number of protons, they can differ in the number of neutrons (uncharged particles, protons have a single positive charge). Only one stable element has no neutron in its nucleus, and that is the common isotope of hydrogen, the nucleus of which consists of a single proton. Another isotope of hydrogen, called deuterium, contains a proton and a neutron, while a third isotope, tritium, contains a proton and two neutrons. Tritium is radioactive and must be produced artificially. One way that can be done is to bombard lithium with neutrons. This is how thermonuclear bombs work.

It is often said that isotopes are chemically identical, but that is not really true. While any given isotope forms exactly the same compounds as another isotope of the same element, the rate of reaction is different for different isotopes. This has some important ramifications. Plants and animals given deuterium oxide (”heavy water”) fail to thrive and eventually die because deuterium has very different rates of reaction in the body than protium (the single proton one), but the products are the same. This also allows identification of sources of, for example, sugar in beverages. A friend of mine did the mass spectral work that showed that the Beech-Nut Company was selling flavored sugar water as pure apple juice. It turns out that sugar from apples has a different carbon-12/carbon-13 ratio than sugar from cane or beets, and cane and beet sugar are different from each other, too. He has a photocopy of the $1 million check that the company paid in fines.

A compound is a material composed of one or more elements that have undergone rearrangement of their electrons so that the atoms are bonded to different atoms in a definite ratio. Thus, water always has two hydrogen atoms bonded to one oxygen atom. Any different and it is not water. One of each gives the hydroxide ion, which makes caustic soda caustic, and two of each gives hydrogen peroxide. We talked quite a bit about compounds earlier in the series, so this is all that I will say about them.

Mixtures are much more varied than compounds. While there are millions of compounds, there are potentially trillions of mixtures. Two or more elements can form a mixture, such as zinc and copper forming brass, a very useful alloy. An element and a compound can form a mixture, such as oxygen dissolved in water. Finally, two of more compounds can for a mixture, such as sugar dissolved in water.

The fundamental difference between compounds and mixtures is that permanent chemical bonds do not form in mixtures. Thus, there is no requirement for definite ratios. Thus, a mixture of water and alcohol can be made from near zero alcohol content to near zero water content. They have different properties, but these differences in properties change gradually, not suddenly like in compounds. For example, using water, hydroxide ion, and hydrogen peroxide as an analogy from above, two-to-one alcohol to water is not a whole lot different than one-to-one alcohol to water. Both will burn (two-to-one faster), and both will make you wasted. Two-to-two is the same as one-to-one, unlike hydrogen peroxide and hydroxide ion.

Most materials that we encounter are mixtures, with some exceptions. Pure elements are rarely encountered with the exception of copper, which has to be very pure to conduct electricity well, so is pretty much a pure element. In am trying to think of other examples, and diamond is the only other one that I can think of off the top of my head. And aluminum foil, if you discount the invisible layer of aluminum oxide that protects it from corrosion, unlike iron oxide.

Pure compounds are more common, but not as much as you would think. Salt and sugar come to mind, but most salt contains a bit of an anticaking substance, and often iodine, making it technically a mixture. Distilled water is pretty much a pure compound except for dissolved air. Sorry, pure grain alcohol is a mixture of 5% water and 95% alcohol. The reason for this is the fact that this mixture has a lower boiling point than pure alcohol, so this is what you get from distillation. 100% alcohol can be made, but not by distillation. I can think of few other pure compounds commonly encountered.

The mixture that we take most for granted is air. Composed of, by volume, roughly 78% nitrogen (an element), 21% oxygen (an element), 1% argon (an element), 0.038% carbon dioxide (a compound, essential for plant life but a contributor to climate change), and traces of a few other gases. Without air, none of us would be communicating at present. Water is the next most essential mixture, and we could not live very long without it. It is extremely difficult to make ultrapure water, because is dissolves traces of almost everything. Likewise, water contaminates almost everything. When I did laboratory work that required exclusion of water it took many steps, lots of them severe, to remove the last traces. So it could be said that just about everything that you encounter is a mixture with water being at least a minor component.

There are two distinct types of mixtures, and next time we will discuss them. I will hang around for questions, comments, and whatnot for a while. Warmest regards, Doc.

Tags: Compound, Element, Mixture, Phases of matter, Teaching
Posted in Diaries |