The easiest way to explain mixtures in chemistry is to talk about food.
Think about mixtures in terms of cooking, and there isn’t much I need to tell you. A salad, like the one pictured above, is a mixture. So is chicken noodle soup. So is…hmm, so is lemonade.
I told you we’d be talking about lemonade soon.
Even air is a mixture—of different gases. Believe it or not, we don’t just breathe in oxygen. In fact, if we breathed in pure oxygen, it would be poisonous. We inhale a little bit of nitrogen, too.
A mixture is—by textbook definition—a physical blend of two or more substances.
Note that I said physical. This is not a chemical combination of compounds. A mixture still just deals with physical properties. All of the substances within a mixture stay exactly what they are—they don’t get changed into something new.
One key aspect of mixtures is that their compositions may vary. In English, that means that even the same mixture isn’t always going to be made of the same stuff.
For example, when you make a salad, do you get the recipe exactly the same every time, right down to the smallest difference in cucumber and tomato slice sizes?
And I can bet you that salad’s not always going to have exactly the same number of slices of anything, either.
What’s more—even air varies. In cities, it’s a lot more smoggy, and in more open, country areas, it’s a lot clearer. Better for stargazing, might I add. I, personally, am not a fan of city air.
While we’re talking about natural mixtures, even blood is a mixture that varies from individual to individual. It’s made up of water, various chemicals, and cells.
There are two basic kinds of mixtures: heterogeneous and homogeneous. To explain what these mean, let’s examine the words themselves. Who cares about what -geneous means? The prefix hetero- means “different,” and homo- means “same.”
So…a heterogeneous mixture is one like salad, where if you separate different portions of it, there’s a high chance you’ll get more greens in one serving than in another.
A homogeneous mixture, though, will behave like a substance if you do the same thing. Lemonade is a good example of a homogeneous mixture.
Yay! I finally get to talk about lemonade!
I’ve had a lot of experience with lemonade, recently. At my college’s dining hall, lemonade is my choice drink. Mostly because I can. Back home, lemonade is something served mainly at restaurants.
Here’s the thing with lemonade. Ignoring any ice or floating lemon slices, the lemonade itself is pretty much uniform and constant in its makeup. What I mean by that is, you can’t really see the sugar and lemon juice particles floating about in the water.
It all looks very much like a substance. And in case you don’t remember my last chemistry post, “What Matters?“, a substance is matter that has a uniform and definite composition.
Looks can definitely be deceiving. But fortunately for you, I’ve spent a lot of time around lemonade, and I know it very well.
When I get it in the dining hall, I can always tell if it’s been recently replenished—or if the lemonade mix is about to run out. If it’s been replenished recently, it tastes a lot more tart than I really enjoy. And if it’s about to run out, it tastes…well…pretty much like water.
Boring. I came for the lemonade, not the water…
Anyway. The reason lemonade tastes different depending on how recently more mix has been added is because it’s a mixture, not a substance. A substance, like H2O (water), will always have the same distribution of particles.
But lemonade can have any amount of mix mixed into the water. It could have so little of the actual lemonade part that it tastes like water. Or it could have so much that it tastes a bit too strong.
See, if you add extra hydrogens or extra oxygens to water and don’t keep the same 2:1 ratio of hydrogen to oxygen, you’re going to get a different molecule.
But with lemonade, it doesn’t matter—mix water, sugar, and lemon juice and you have lemonade. No matter how much of any of it you add in. Because it’s a mixture.
It is, however, different from a heterogeneous mixture. Lemonade is a homogeneous mixture, meaning that for any one mix, all servings are going to have pretty close to the same amounts of the ingredients.
A heterogeneous mixture doesn’t have that restriction. In a salad, you can very easily pick out all the tomatoes and ignore the greens. (That’s what I do!)
See what I mean? Talk about mixtures in relation to food, and it all makes sense.
Let’s get back to homogeneous mixtures for a sec. Another name for them that you’ll hear a lot in chemistry is solutions. Not solutions like the answers to a puzzle, but…literally…homogeneous mixtures. They’re the exact same thing.
Why do chemists give homogeneous mixtures their own special name? Because they are extremely important.
Literally anytime you do an image search for “chemistry,” this kind of thing is what pops up. Solutions are practically the face of chemistry. You see a picture like this and you think chemistry. That’s how important solutions—homogeneous mixtures like lemonade— are.
Solutions don’t even have to be composed of the same states of matter. There are gas-gas, liquid-gas, liquid-liquid, solid-liquid, and even solid-solid solutions.
It may seem surprising that you can mix solids in a homogeneous mixture, but sterling silver—composed of copper and silver—is a prime example.
Within any mixture, there are phases—parts of the mixture with the same uniform composition. Therefore, homogeneous mixtures like lemonade just have one phase. But heterogeneous mixtures have multiple phases.
Salads, for example, have so many phases I don’t even want to count them. Hmm, one for each part of a tomato, one or more for the dressing depending on the type…nah, don’t feel like counting.
You may have noticed that with certain salad dressings, you have to shake them a bit before pouring.
Salad dressings make it easy for you to separate out their phases. They do it for you, annoyingly enough. But would you believe there’s a whole part of chemistry devoted to separating the phases of mixtures?
It’s easy with salad dressing, and even easier with, say, the ingredients of a taco. (Come on. I’ve seen my brother pick food out of his tortilla so many times, I definitely know that one.) But what about, say, a mixture of sulfur and iron particles?
Not necessarily an easy task—the filings are often very thoroughly mixed together. But because iron is magnetic and sulfur isn’t, we can attract the iron filings with a magnet and leave the sulfur behind.
What really is harder, though, is separating a homogeneous mixture of water and other dissolved substances.
Chemists use a process called distillation. It’s based on the idea that the different phases in the mixture are going to have different boiling points. If you heat water up so it becomes water vapor, you’re going to leave the other substances mixed in behind.
Basically, the water vapor is transported into a different beaker, while the other substances stay behind in the first beaker. If you keep the heat on after the experiment’s over, you’ll burn the other substances a bit, but while they’re still in water they can’t burn.
Water is what we use to put out fires, remember?
Distillation works for solid-liquid solutions like salt water because the solid is definitely not going to boil up into a gas at the same time as the liquid. It doesn’t necessarily work as well for solutions involving two liquids or a liquid and a gas.
Why? Because the substances might have similar boiling points. The whole idea is to leave one substance behind.
Anyway, that’s it for this chemistry post—next up tomorrow morning is climate science, in which I’m thinking of talking a bit about the famed greenhouse effect.