>> host: On "Chemistry Matters", we're always looking for the best ways to utilize chemistry.
And we can't think of anything better than using it to make amazing frozen treats. That's what our friends
at King of Pops in Atlanta do every day. They use all-natural ingredients, along with
a lot of the tools and methods we're about to investigate, like freezing point depression, solubility, and concentration
to make, well... to make chemistry delicious!
They know what they're doing.
♪♪
Welcome back to "Chemistry Matters", Unit 7, Solutions, Acids and Bases. In this unit, we'll explore
the properties of solutions, how to describe them, how to measure their concentrations,
and we'll look at different types of solutions, such as acids and bases. But before we investigate
solutions, I think it's worthwhile to go back and revisit a couple of definitions we explored
in previous units. When we investigated matter in unit 2, we used terms such as mixtures
and pure substances. Examples of pure substances are elements like gold and silver,
and compounds like water and sodium chloride. All pure substances can only be made up
of one specific ratio of their constituent parts, like the H2O of water. Or they're made of only
one thing, like pure gold. In other words, they have a definite composition, and these parts cannot
be separated from each other using a physical process like filtration or chromatography.
A mixture, as you'll recall from unit 2, is a combination of two or more pure substances
in which each pure substance retains its individual chemical properties. These substances can be
separated from each other without breaking chemical bonds. And when classifying matter,
there are two subdivisions under mixtures, heterogeneous mixtures and homogeneous mixtures.
Heterogeneous mixtures are a combination of two or more substances in which the original substances
are separated into physically distinct regions with differing properties. In other words,
the different parts throughout the mixture are usually distinguishable from each other,
like students in a classroom. In homogeneous mixtures, the components of the mixture are not distinguishable.
So the mixture looks the same throughout, like coffee with cream in it. Homogeneous mixtures,
or solutions, are the focus of this unit. Homogeneous mixtures are a combination
of two or more substances that have uniform composition and chemical properties throughout.
The important distinction between mixtures and pure substances is that mixtures do not have
definite composition, whereas pure substances do. It's important to remember those definitions because
a big part of this unit deals with solutions, which is another word for homogeneous mixtures.
So let's go out to our classroom to further explore solutions.
>> professor: As you know from previous units, solutions are homogenous mixtures. They can be liquid,
gaseous, or solids. The type of solutions we're primarily going to focus on in this unit are
liquid solutions. In liquid solutions, one or more substances are dissolving into another.
The substance that's being dissolved is referred to as the solute.
The substance that's present in the greater amount is known as the solvent. The solvent is the substance
that's actually doing the dissolving. This is a very specific process. First the solute particles
are surrounded by the solvent particles, then the solute particles are separated and carried out
into the solution by the solvent particles. Kat and Josh, in your notebooks I'd like you to draw a model
of sugar dissolving in water.
And Mariah and Caid, you two make a model of NaCl, or salt, dissolving in water.
Then I'll put your models on the monitor and we'll look at the differences. >> student: Why would they
dissolve differently? >> student: I'm not sure... Well, sugar has covalent bonds and salt is ionic.
So that's different. >> student: Oh, I think I know what to do now. >> student: Yeah, me too.
>> professor: Okay, guys, let's look at your work. Kat and Josh, tell me about your model.
>> student: So we showed the water circling around a molecule of sugar that was on the edge
of this sugar cube. >> professor: Good. What about the salt group? >> student: Well, we drew
the water encircling the salt, but we separated the salt into sodium and chloride ions. >> student: Wait, why?
We didn't do that. >> student: Sugar is covalent and it isn't made of ions. >> student: Right.
Salt is ionic and we remembered that aqueous ionic compounds were shown as ions in net ionic equations
from when we learned about chemical reactions. >> student: Ionic compounds that dissolve, will separate
into ions. And we know that salt dissolves in water. >> professor: Nice work
all of you. Most covalent substances dissolve into single molecules, while ionic substances
dissolve into ions.
Can anyone think of another example of a solution? >> student: How about chocolate milk?
I love making the kind with the powder that dissolves into milk! >> professor: Great example.
When making chocolate milk, what would be acting as the solvent and what would be the solute?
>> student: Well, you said the thing that was present in the greater amount is the solvent,
so the milk would have to be the solvent. >> student: Yeah, and the powder would have to be the solute.
>> professor: Correct! Of course, milk is mostly made of water, so water's really the solvent.
>> student: But couldn't you get a small cup of milk and just dump the chocolate powder in it?
Would that make the powder the solvent since it would be present in the greater amount? >> professor: That's
a great question. The milk would still dissolve as much of the powder as it could, so it would
still be the solvent. There would just be a whole lot of leftover powder there. But that raises another
interesting question. Can any of you guys think of a way to get the milk to dissolve
more chocolate powder? >> student: You could stir it. >> professor: Good idea! In order for something
to dissolve, the solute and the solvent must come in contact. If we increase the contact
like Josh suggested, we'll increase the fresh solvent contact with the solute particles,
and this increases the rate which a solute dissolves in a solvent. You may also hear this called
the rate of dissolution. Any other examples for getting more milk to dissolve in the chocolate powder?
>> student: How about heating it up? I make tea sometimes, and I've noticed it's
a lot easier to make it sweet with sugar once you heat it up. Would that work with chocolate milk?
>> professor: Absolutely! We could get the milk to dissolve more chocolate powder more quickly
by increasing the temperature, because increasing the temperature increases the kinetic energy
of the solvent. This causes the particles to move around more, which means there'll be
greater number of collisions between the solvent particles and the solute. Let's go back to the tea
example that Caid brought up. How many of you guys drink tea?
If you're trying to sweeten it, what works faster, regular sugar or a sugar cube? >> student: Regular sugar.
>> professor: Right. Why? >> student: Because the pieces are smaller? >> professor: Exactly.
What you're doing by having a lot of smaller pieces is you're increasing the surface area of the solute,
and this can be done by crushing or grinding a solute to make a powder. By doing this you decrease
the particle size, but you increase the surface area of the solute that comes into contact
with the solvent, making it dissolve faster. I have another question. Let's say you guys all had
the same amount of tea, and you have plenty of sugar, but all of your tea is at different temperatures.
Which tea could dissolve the most sugar? >> student: The hottest tea. I notice when I make tea,
hot tea dissolves in sugar faster, But it can also dissolve more. Why is that?
>> professor: The amount of solute that can be dissolved at different temperatures is related to the solubility
of the solvent. And we're just about to get to that. But first, there's one more
way to increase the amount of solute that will dissolve in a solvent, but it's a little harder to picture.
It's increasing the pressure. When a solution, like this bottle of soda, is under pressure,
more gas can be dissolved in the solution. As soon as I open it, listen.
The external pressure is decreased and what you heard was the carbon dioxide gas
escaping from the solution. This is what we mean when we say soft drinks are carbonated, and why
they go flat if you leave the cap off. Now I want you to try and think of examples
of how these factors affect the formation of solutions in your everyday life. >> student: Well, a couple
of weeks ago, I had a sore throat and so I had to gargle some salt water.
So I stirred it up first. I guess I was increasing the frequency of contact between the particles?
>> professor: That's exactly right. And if you'd used warm water, that would've also helped
dissolve the salt also. Anyone else? >> student: I might have one. My uncle's a scuba diver.
And he told me if he comes up to the surface too fast, it does something bad to the nitrogen in his blood
because of the change in pressure. But I'm not sure I understood. >> professor: That's
a great example, and it's kind of like the soft drink. Deep underwater, he's breathing
highly pressurized gas from his scuba tank. If he comes to the surface too quickly,
the rapid reduction in pressure will cause nitrogen to collect and form large bubbles in his blood, and that
can be very dangerous. So your uncle probably ascends very gradually when he's deep sea diving.
Now, just because we put a powder into a liquid doesn't mean it will always dissolve to form a solution.
Let's investigate how much of each of the following salts will dissolve in 100 grams of water.
Make a data table where you can record how temperature impacts the dissolution of your salts.
And we'll show your results on the monitor when you're done. The salts we'll use today are calcium sulfate,
sodium chloride, and potassium nitrate. First, you'll try to dissolve 50 grams of each salt into
water at 5 degrees Celsius. For any salts that don't dissolve, raise the temperature
to 30 degrees Celsius and stir to see how well they dissolve at this temperature. Finally, if any of the salts
still won't dissolve, raise the temperature to 50 degrees Celsius. Feel free to stir while you
make your observations.
>> student: Whoa, that's kind of strange. The calcium sulfate isn't dissolving at 5 degrees Celsius.
>> student: Yeah, the potassium nitrate is kind of dissolving, but the sodium chloride is definitely dissolving.
>> student: Hmm... The potassium nitrate is dissolving now at a higher temperature.
>> student: Yeah, but the calcium sulfate still isn't dissolving no matter how much I stir.
>> student: Huh, this is weird. I thought we said increasing the temperature speeds up how fast
the solution forms. >> student: Yeah, the calcium sulfate doesn't seem to dissolve
at any temperature. It's certainly not dissolving at 50 degrees!
>> host: Our students are off to a pretty good start with this lab. So we'll take a break here
to let you get started on it as well. When you're done, and you've completed your data table,
I'll see you at the top of the next video on the playlist.