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- [Instructor] I used to[br]always think of solutions

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as liquids of some kind,

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but turns out that it's wrong.

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Not all liquids are solutions.

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And more importantly,[br]solutions can be solids,

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liquids, or gases.

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So, in this video,

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let's understand exactly[br]what solutions are.

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So, let's take a concrete example.

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Let's put some salt in water[br]and stir it very nicely.

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We get salt water, but[br]what exactly is salt water?

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Let's analyze.

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Do you think salt water is an element?

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Elements are made of[br]just one kind of atoms.

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For example, you have oxygen

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which is made of only oxygen atoms

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or gold made of only gold atoms.

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Well, salt water has different[br]kinds of atoms in them.

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There's water, therefore[br]there's hydrogen and oxygen,

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and we also put salt.

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So, there's sodium and[br]there's also chlorine.

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So, since salt water is made[br]of several different elements,

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it is not an element.

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What else could it be?

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Hey, what about compounds?

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Remember in compounds,

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two or more atoms are chemically[br]bonded in fixed ratios.

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For example, water is a compound

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because hydrogen and oxygen[br]are chemically bonded

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in the ratio two is to one,[br]two hydrogen for every oxygen.

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Carbon dioxide is a compound,

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because carbon and oxygen[br]are chemically bonded

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in the ratio one is to two,

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one carbon for every two oxygens.

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So, what about salt water?

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Is that a compound?

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No, it's not, and this is important,

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because salt and water[br]are not chemically bonded,

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they are physically mixed over here,

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and they don't have a fixed ratio.

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You can have different ratios[br]of salt and water here.

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Now, of course, salt is a compound,

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because sodium and chlorine[br]are chemically bonded in.

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One is to one fixed ratio,[br]and water is a compound.

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But salt water?

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That's not a compound at all.

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And more importantly,[br]compounds are new substances

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with different properties altogether.

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When hydrogen and oxygen form a compound,

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you get water, which is a new substance.

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It has its own properties,[br]it has its own taste,

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it has its own boiling point.

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Similarly, sodium and[br]chlorine coming together,

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you get salt, which is a[br]new substance altogether.

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It has its own taste,

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it has its own melting[br]point, boiling point.

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But when you dissolve salt in water,

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you don't get a new substance.

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You basically get salt water,

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and you don't have new properties.

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It's still salty, it's still watery.

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And finally, remember, the[br]atoms of that compounds

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cannot be physically separated.

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For example, when you heat[br]water to its boiling point,

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the hydrogen and oxygen[br]atoms do not separate.

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The water molecule just move[br]farther away from each other

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as it changes to gas.

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But here, salt and water[br]can be physically separated,

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just evaporate the water,[br]salt will be left behind.

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So, hopefully it's clear[br]that this is not a compound.

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But what is this?

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Well, when you mix two[br]substances together,

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we get what is called a mixture.

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And since this mixture[br]is uniformed throughout,

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you cannot, with your[br]naked eyes, see the salt

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and the water to be different over here.

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We call such mixtures homogenous mixtures.

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In contrast, mixtures in[br]which the different components

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can be easily seen, like[br]for example, water and oil,

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you can clearly see oil over here,

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you can see water here,[br]you can see the boundary.

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In such cases, we call them[br]heterogeneous mixtures.

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But anyways, in our example,

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salt water is a homogenous mixture.

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And guess what?

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These homogenous mixtures[br]are what we call solutions.

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So, let's look at solutions in[br]a little bit more detail now.

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Solutions usually contain two things.

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We have the solute and the solvent.

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Here, you can think of solute[br]as something that is dissolved

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in the solvent to form a[br]solution, but in general,

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you can think of solute as a substance

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which is in less quantity,

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and solvent is is the substance[br]that is in more quantity.

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But now, let's look at more examples.

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What about air?

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Well, air is a homogenous[br]mixture of oxygen, nitrogen,

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carbon dioxide, and many other gases.

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So, air is a solution.

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So, look, solution need[br]not be just liquids.

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Solutions can be gases.

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And then think about alloy,[br]which are made by mixing metals.

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Like, say, steel is made[br]by mixing iron and carbon,

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and bronze is made by[br]mixing copper and tin.

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These are also homogenous mixtures.

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The metals are mixed throughout uniformly.

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You cannot see them separately,

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and therefore these are also solutions.

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So you can see, solutions[br]can also be solids.

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Now, a key thing about solutions[br]is their concentration.

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If you have a lot of solute, relatively,

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compared to the solvent,

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we say it's a very concentrated solution.

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And if you have less solute[br]compared to the solvent,

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we say it is a dilute solution.

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But why should we care[br]about the concentration?

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Well, here's the thing,

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although individual components will have

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their own separate properties[br]because it's a mixture,

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it turns out that the property[br]of the solution itself

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can vary, depend upon the concentration.

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For example, usually, water freezes

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at about zero degrees[br]Celsius, but guess what?

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When you add salt to it, the[br]freezing point of this solution

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is below zero degrees Celsius.

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Similarly, if there is[br]water molecules in the air,

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we say there is humidity.

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A lot of water molecules means[br]there is a lot of humidity.

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Humidity changes the[br]properties of the air.

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For example, the speed of sound changes,

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its density changes, and so on.

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And similarly, steel is made[br]by adding carbon to iron.

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And by increasing the[br]carbon concentration,

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we can make steel harder.

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We can make it stronger and more durable.

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So, look, by changing the[br]concentrations of the solute,

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we can vary the properties[br]of the solution itself,

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and that is pretty awesome.

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So, putting it all together,

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matter can be classified into two parts.

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We have pure substances,[br]which are basically elements

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which are made of just one kind of atoms,

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or they can be compounds

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which are made of atoms that[br]are chemically bonded together

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in fixed ratios, like[br]water or carbon dioxide.

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And when you physically[br]mix substances together,

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we get mixtures.

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Now, mixtures can be heterogeneous.

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If you can see the[br]individual component apart

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with our naked eye, you can[br]see the boundary, like say,

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when you add oil to water,[br]or they can be homogenous.

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In this case,

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the components are distributed[br]uniformly throughout,

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and so you cannot tell them apart.

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We also call them solutions.

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The component which is[br]in the less quantity,

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we call them as the solute,

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and the one with more quantity,[br]we call them as a solvent.

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What's important is that the[br]individual physical properties

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of the components, like[br]their boiling point,

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melting point, that does not change.

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And so, we can use that to[br]separate them out physically.

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But a cool thing about solutions

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is that by varying the concentration,

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we can vary the properties[br]of the solutions

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to our advantage.