So matter. What"s up with matter? Let"s see... it"s all around us and we are constantly interacting with it. We are it as a matter of fact. We chemistry folks like to break matter into specific groupings. It helps us understand it much better. That is a "science-y" thing to do, and were are being science-y in this class. So let"s start.
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First, matter is everything, solid, liquid, gas - those are the 3 common states of matter. You nerdy types will say "what about plasmas?" and I"ll say, let"s not go there. When was the last time you interacted with a plasma? Let"s just acknowledge it and then move on - another class can deal with plasmas.
Pure Substances A pure substances is matter that is the same thoughout all the way down to a molecular level. The "down to the molecular level" is very important here. Every single piece of a pure substance has the exact same make up or chemical formula. Pure water is always H2O and never ever anything else. Pure oxygen gas is O2 - you"re breathing it right now. And pure quartz is SiO2 (repeat) - I say repeat because that formula is repeated over and over in a solid crystalline lattice. Quartz is the primary component of sand. We chemists/scientists like to work with pure substances because we know exactly what we have and even have an exact formula to describe it.
Mixtures Anytime you combine two or more pure substances, you have a mixture. Most of the matter that we encounter here on earth is some form of mixture. Many mixtures are well defined and have names to go with them. Air is a mixture of gases that make up our atmosphere here on earth. Some mixtures sound like they"d be a pure substance but they are actually mixtures. For example, the metal called bronze is a mixture of primarily copper and a bit of tin, and possibly some other metals too...which illustrates my point to be made: Mixtures can have variable compositions. A little more of this, a little less of that... a dash of the other. Go nuts, mixtures are generally quite variable in their compositions. But remember, we chemists and science guy types can define that composition and make it exact. Mixtures are variable but also definable - remember that.
Homogeneous Mixtures Now we are going to split up the two divisions of matter, mixtures and pure substances. Let"s split up mixtures first. If and only if the pure substances you combine mix all the way down to a molecular level, you will have a homogeneous mixture (aka a solution). Any macroscopic sampling (big enough to see) of a homogeneous mixture you will get the same exact composition of substances. The mixture is the "same throughout" - that is homogeneity. This is true all the way down to where you can"t see anymore. Your vision peters out around 50 microns (µm). Read my blurb on molecular true homogeneity vs our day to day perceived homogeneity.
Heterogeneous Mixtures Any (perceived) heterogeneous mixture, you can actually see the differences in the mixture. It is obvious that there are different substances in the mixture. An overly obvious heterogeneous mixture is sand in water. The sand is easily seen in a glass of water... heterogeneous. A chocolate chip cookie is heterogeneous. There are the chocolate chips and there is the rest of cookie. A glass of ice tea is heterogeneous because the ice is a different substance than the tea. If all the ice were to melt though, you"d have a homogeneous mixture.
That was fun splitting up mixtures, now lets split up pure substances.
Elements If you take any form of matter and just keep breaking it down to smaller and smaller bits, you will eventually reach the molecular level and then you reach single atoms of stuff. Those atoms are elements that make up all the matter of the universe. The elements are the simplest form of matter in that there is only one kind of atom in an element. If you only have iron atoms, you"ve got the element iron (Fe). There is a real easy check for elements, look at a periodic table. If it is on the periodic table, it"s an element. If it is not on the periodic table, it is not an element.
Compounds If you combine, via a chemical bond, two or more different elements - you"ve got a compound. Compounds are very defined in there elemental ratios as well. Sodium chloride (table salt) is NaCl and will always exist as one atom of sodium to one atom of chlorine in its makeup. It will never vary. Water is a molecule that is always two atoms of hydrogen bonded to one atom of oxygen, H2O.
Blurb: True Homogeneity vs Perceived HomogeneityThere is a difference in the physical properties of solutions when they are truly homogeneous down to the molecular level versus just being homogeneous to a perceived level. I"m a big believer in "watch you see is watch you get". The classic gotcha! question in chemistry textbooks and I suppose on exams is to ask what category is milk?
If you look really close - like zoom way in, maybe get a microscope and look. You"ll see that there are differences in the tiny bits. Milk is really a bunch of fat suspended in an aqueous solution. The fat doesn"t really dissolve fully at all - but instead of just being a big giant wad of fat (easy to see there), it is broken up into teeny tiny globules of fat and those get emulsified with some proteins to make all the parts completely mix and be homogeneous on a human perceived scale. In order to know that it is really heterogeneous, you have to have equipment to look further - or you"ve now been told and can carry on the tradition of doing the whole gotcha! thing.
I (Dr. katifund.org) believe you call "em as you see "em. We generally make our first assessments based on simple human observation of a sample. If it looks homogeneous, then I"d check that box on the information form. I don"t believe in tricking you to think its homogeneous and then turn around and point out that at a 10 µm level, there are little bits that are different and therefore heterogeneous.
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LATER... after much experimentation and examination... I might conclude the mixture is heterogeneous. But my first look says it"s homogeneous for milk, blood, and even paint. There IS a sort of in between state called colloids. They LOOK homogeneous, but they are actually a suspension of tiny-ass particles.... just like the milk and blood. Study colloids on your own. I"m not going to go there.