Chemists are concerned about MATTER. Very concerned. It occupies all their waking hours. What is “matter?” It’s anything that has mass and volume. So if you put a piece of matter on a scale,you can measure its mass in grams, or tenths of grams, or millionths of grams. You can measure its volume – how much spaceit takes up – in terms of cubic meters, or cubic millimeters, or something even smaller.
In chemistry, we’re usually talking aboutvery SMALL bits of matter – things like atoms and molecules. Here’s how we know we’re in CHEMISTRYworld and not the real world – in chemistry class, we’re always talking about PURE samplesof matter. What does potassium do. How does magnesium behave when you put itin water. What’s the pH of hydrochloric acid. In the real world, we generally don’t stumbleupon perfectly pure samples of anything. We have to actually do a lot of work to purifymaterials before we can study their chemical.
Behavior in the lab. The more pure a sample of matter is, the betterwe can predict how it will behave, based on chemical principles. This is one reason why chemists CLASSIFY matter,based on how pure of a sample it is. Here’s the basic flowchart chemists referto when they’re classifying matter: {show flowchart} All of matter can be classified into eitherpure substances, or mixtures. A pure substance has a definite compositionthat doesn’t change. That’s why Chemists want PURE SUBSTANCESto work with.
Imagine trying to run an experiment on thechemical behavior of something, and all your samples had different compositions. You wouldn’t be able to conclude anythingfrom your experiments! When we say “pure substances,” those mightbe either elements, or compounds. An ELEMENT is composed of just ONE kind ofatom. Like a gold bar – it’s pure gold, all theway through, made up only of gold atoms. Elements CANNOT be separated into simplersubstances. A COMPOUND, on the other hand, is made upof more than one element, but it always has the same chemical formula.
H2O, for instance, water, is a compound madeof two kinds of atoms, hydrogen and oxygen. This compound always, always has two atomsof hydrogen bound to one atom of oxygen. If you somehow force those atoms apart fromeach other, for instance, by electrolysis – running electricity through it – it’sno longer that compound. This notion that compounds always have theexact same formula is known as the Law of Constant Composition, or the Law of DefiniteProportions. It was proposed around 1800 by French chemistJoseph Louis Proust. There are a few fundamental ideas like thisin chemistry that we almost take for granted – of course water is always 2 hydrogen atomsfor every 1 oxygen atom!.
But it still has to be stated, and occasionallyeven defended. For instance, you may have come across peoplewho think a vitamin from a vegetable is different from a vitamin synthesized in the lab. Nope. A pure sample of a compound is chemicallyidentical to any other pure sample of a compound, no matter where you got it from. There are DIFFERENT compounds you can makeof the same two kinds of atoms – for instance, hydrogen peroxide, H2O2 is made of the samekinds of atoms as water. Hydrogen peroxide always has this exact composition- two atoms of hydrogen bound to two atoms.
Of oxygen. Water and hydrogen peroxide are two DISTINCTcompounds, even though they are made of the same kinds of atoms – the hydrogen and oxygenatoms are combined together differently in the two compounds, and form different 3-dimensionalshapes. You wouldn’t confuse these two compounds,because their chemical composition gives them very different chemical behavior. Even if they look similar standing next toeach other, clear liquids in a beaker, you could tell the difference between them ifyou test their physical and chemical properties. We have a video about physical and chemicalproperties if you want to learn more about.
This idea. Now let’s turn to MIXTURES, which is morelike real life. We run into mixtures in nature all the time. A mixture is a PHYSICAL blend of two or moresubstances. But unlike a compound, where the substancesare there in fixed ratios, always identical – a mixture can vary in its composition. For instance, we call the blend of gases thatwe breathe “AIR.” Air doesn’t have an identical compositioneverywhere at all times. In general, it has a lot of nitrogen, someoxygen, some carbon dioxide, and a few other.
Gases. But air in a polluted city is going to havemore methane, more carbon monoxide…Air is a mixture, not a compound – there’s no Lawof Definite Proportions here. One important thing to keep in mind aboutmixtures is that all the component parts keep their original chemical behavior. So all those gases mixed together in air – theydon’t have a different chemical behavior because they are physically mixed together. Nitrogen still behaves like nitrogen. Carbon dioxide still behaves like carbon dioxide.
It’s not until atoms react with each otherin a CHEMICAL reaction that they exhibit different chemical behavior as part of a new compound. The compound HCN, hydrogen cyanide, is a completelydifferent chemical species than a mix of those three kinds of atoms. Another real life example of a mixture wherethe composition varies is LEMONADE. There isn’t a set formula for making lemonade,although everyone thinks their recipe is the best. One classic recipe follows the ratio 1: 2:4 – 1 cup sugar, 2 cups lemon juice, 4 cups water.
That’s WAY too sweet for me! My recipe is 2 Tablespoons of sugar, 2 cupsof lemon juice, 4 cups of water. My lemonade is tart! But we still call it lemonade. It’s a mixture. Chemists also distinguish between a Homogeneousmixture and a Heterogeneous mixture. Can you figure out the difference betweenthese based on a little etymology? “Homos” is a Greek prefix meaning thesame, and “heteros” is a Greek prefix meaning “other,” or “different.”.
A Homogeneous mixture has the same compositionthroughout – it’s a uniform mixture all the way through. Think of when you mix paint, and if you mixit well enough, you get the same colour all the way through. That’s a homogeneous mixture. A heterogeneous mixture, on the other hand,does NOT have a uniform composition all the way through. You can identify different “phases” inthe mixture. Think of a bowl of cereal with milk.
Each time you stick in your spoon, you’llpull out a different, unpredictable ratio of components. Sometimes you’ll get a marshmallow. Tang vs fresh squeezed orange juice. One is a homogeneous mixture, and the otheris a heterogeneous mixture. Tang was one of those “better living throughchemistry” products from the 1950s. It’s a powdered orange drink you mix intowater. It’s not very good, but the first astronautsdrank it, so I liked to mix it up as a kid and pretend I was going into space.
You add a tablespoon to a glass of water,mix it well, and you have a glass of uniform, bright orange Tang. No difference between the first sip and thelast. Tang is a homogeneous mixture. Freshly squeezed orange juice, on the otherhand, has distinct phases in it. There are pulpy bits floating around, andeach time you take a sip you’ll sometimes get these little orange bits, and sometimesyou won’t. It’s a mixture of various things includingwater, and fructose, and little pieces of orange – but it’s not uniform all the waythrough.
Fresh squeezed orange juice is a heterogeneousmixture. Socratica Friends, if you're preparing to take a test on this subject, my best advice is to study what you DON'T KNOW. And you can figure out what you don't know by taking a PRACTICE TEST. We've made a practice test complete with an answer key Available for purchase for a reasonable price on our website, Socratica.com Your purchase helps us make these videos free for the world. Time for a Pop Quiz – how would you classifythe following? Helium – Pure substance – an element, He.
Milk – Mixture – a homogeneous mixture. Here’s a caveat – I’m talking about milkfrom the grocery store – It even says “homogenized” on the label. That’s a mechanical process where they shakethe milk really hard or, sometimes they spray the milk through tiny holes, as a way to breakup the fat droplets so they’re very small, and they’ll stay suspended evenly throughoutthe milk. If you don’t homogenize milk, like whenyou get it straight from a cow, that’s actually a heterogeneous mixture. If you let it sit for a bit, you’ll seecream rise to the top.
That makes it very clear there are phasesin that mixture. Moving on. Glucose – a pure substance, a compound. It has a specific formula C6H12O6. Gravity – that’s not matter, that’s aforce. Carbon Dioxide – that’s another pure substance,a compound, with the formula CO2. A sunbeam – that’s not matter. Photons don’t have any mass. Trailmix – that’s a mixture (it’s in thename, trail MIX).
Heterogeneous. Sometimes you just get a bunch of raisins,ugh. Salt, or table salt – that’s a pure substance, a compound. NaCl. Beach sand – mixture, heterogeneous. It’s made up of silica compounds, salt (NaCl), andlittle bits of crushed shells and who knows what else. And beach sand varies, depending on whichbeach you’re visiting. Let’s do one more exercise, to get a feelfor how these concepts can be used by chemists. What if you got a bucket of that beach sand- heterogeneous mixture – and you want to.
Separate out the component parts. Let’s say, specifically, you wanted to purifyout the salt. How would you do it? You’d use your knowledge of the propertiesof salt and sand, to separate out the components of this mixture. We know that salt dissolves in water, butsand doesn’t, so let’s add water to the mixture. Now you’ve dissolved the salt, and you cansee sand settling out along the bottom of your bucket.
But to catch all the sand, you might wantto pour the water into a strainer or filter it through some cloth or a coffee filter,something where the holes are smaller than the grains of sand and little pieces of shells. Just the salty water can run through the sieve. How do we get the salt away from the water? You could boil the water (or just leave itout in the sun for a long time) to evaporate out the water, leaving the salt behind. Sea salt! People pay a lot for that in the shops!.
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