When I first created the last post, I had no idea that I would be breaking it down into a series of separate posts. However, I now would like to note that there is much more to say about the classification of matter.
One of the first things that comes to mind when I think of chemistry is the periodic table of elements, which contains 118 elements, of which 91 of these are naturally occurring. What, then, is an element? You do not need to know all elements of the periodic table by heart because you will often be provided a periodic table of elements on different examinations, from chemistry class to the MCAT, but it is certainly important that you know what an element is. An element is defined as being a kind of matter with the distinct property of being unbreakable into more than one pure substance, or in other words, a substance that cannot be diminished into simpler substances using any chemical reaction (Masterton and Hurley 2015; Ebbing and Gammon 2009).
Let's think of some scenarios in which we might recognize elements. When we use our outdoor grills, such as for grilling chicken, we often use charcoal in order to facilitate that process. Did you know that charcoal is composed mainly of carbon, one of the 118 known elements? We also often use household elements that are made of aluminum. And another element we know of is much less used these days due to its toxicity: lead (Masterton and Hurley 2015).
Another important concept to understand is that of compounds. Many things that we know of are compounds, and compounds are described as substances which have constituents consisting of atoms of two or more elements joined together (Petrucci et al. 2010). Perhaps the most familiar compound to us all is water, which is composed of the elements of hydrogen and water. You should be aware, as well, that compounds always have fixed compositions. In more detail, what this means is that for any given compound, the elements will always be the same, and they will always exist in the same percentages by mass of that compound. Again, let's use the example of water. Water is composed of hydrogen and oxygen, we know. However, the respective percentages by mass are 11.19% hydrogen and 88.81% oxygen. This is the same for any sample of pure water that exists. This is not the same for mixtures (Masterton and Hurley 2015).
Mixtures, on the other hand, are very different. In an earlier post on matter, I mentioned mixtures and you should know easily that mixtures are not pure substances and have greatly different properties to pure substances. Mixtures are able to be separated into two or more substances, using some physical means, meaning that composition can vary with mixtures (Ebbing and Gammon 2009). I also mentioned the two types of mixtures in my earlier post. There are homogenous mixtures, which are known as solutions, as well as heterogenous mixtures. Homogenous mixtures have properties throughout a given sample that are always uniform. On the other hand, heterogenous mixtures have physically distinct parts, and these different parts also have different properties associated with them (Ebbing and Gammon 2009).
A lesser used term but one that is also good to know is what a "phase" is. A phase is described as any homogenous substance that exists in a portion or sample of matter that is being studied. For instance, sugar and salt when mixed together form a sample of matter consisting of two separate and distinct phases of sugar, and of salt. The same can be said for ice cubes in water: two phases exist, the ice cubes and the water (Ebbing and Gammon 2009). Please do not confuse this with the basic phases of matter that we discussed earlier as being solid, liquid, and gas. This is a somewhat different concept.
That's all for this post folks. This may have been a bit to take in but study it well and you will be well-prepared to learn more, and to take the MCAT when you've studied enough! Thanks for reading.
A prospective medical student, looking to help others succeed.