- [Instructor] I used to always think of solutions
as liquids of some kind,
but turns out that it's wrong.
Not all liquids are solutions.
And more importantly, solutions can be solids,
liquids, or gases.
So, in this video,
let's understand exactly what solutions are.
So, let's take a concrete example.
Let's put some salt in water and stir it very nicely.
We get salt water, but what exactly is salt water?
Let's analyze.
Do you think salt water is an element?
Elements are made of just one kind of atoms.
For example, you have oxygen
which is made of only oxygen atoms
or gold made of only gold atoms.
Well, salt water has different kinds of atoms in them.
There's water, therefore there's hydrogen and oxygen,
and we also put salt.
So, there's sodium and there's also chlorine.
So, since salt water is made of several different elements,
it is not an element.
What else could it be?
Hey, what about compounds?
Remember in compounds,
two or more atoms are chemically bonded in fixed ratios.
For example, water is a compound
because hydrogen and oxygen are chemically bonded
in the ratio two is to one, two hydrogen for every oxygen.
Carbon dioxide is a compound,
because carbon and oxygen are chemically bonded
in the ratio one is to two,
one carbon for every two oxygens.
So, what about salt water?
Is that a compound?
No, it's not, and this is important,
because salt and water are not chemically bonded,
they are physically mixed over here,
and they don't have a fixed ratio.
You can have different ratios of salt and water here.
Now, of course, salt is a compound,
because sodium and chlorine are chemically bonded in.
One is to one fixed ratio, and water is a compound.
But salt water?
That's not a compound at all.
And more importantly, compounds are new substances
with different properties altogether.
When hydrogen and oxygen form a compound,
you get water, which is a new substance.
It has its own properties, it has its own taste,
it has its own boiling point.
Similarly, sodium and chlorine coming together,
you get salt, which is a new substance altogether.
It has its own taste,
it has its own melting point, boiling point.
But when you dissolve salt in water,
you don't get a new substance.
You basically get salt water,
and you don't have new properties.
It's still salty, it's still watery.
And finally, remember, the atoms of that compounds
cannot be physically separated.
For example, when you heat water to its boiling point,
the hydrogen and oxygen atoms do not separate.
The water molecule just move farther away from each other
as it changes to gas.
But here, salt and water can be physically separated,
just evaporate the water, salt will be left behind.
So, hopefully it's clear that this is not a compound.
But what is this?
Well, when you mix two substances together,
we get what is called a mixture.
And since this mixture is uniformed throughout,
you cannot, with your naked eyes, see the salt
and the water to be different over here.
We call such mixtures homogenous mixtures.
In contrast, mixtures in which the different components
can be easily seen, like for example, water and oil,
you can clearly see oil over here,
you can see water here, you can see the boundary.
In such cases, we call them heterogeneous mixtures.
But anyways, in our example,
salt water is a homogenous mixture.
And guess what?
These homogenous mixtures are what we call solutions.
So, let's look at solutions in a little bit more detail now.
Solutions usually contain two things.
We have the solute and the solvent.
Here, you can think of solute as something that is dissolved
in the solvent to form a solution, but in general,
you can think of solute as a substance
which is in less quantity,
and solvent is is the substance that is in more quantity.
But now, let's look at more examples.
What about air?
Well, air is a homogenous mixture of oxygen, nitrogen,
carbon dioxide, and many other gases.
So, air is a solution.
So, look, solution need not be just liquids.
Solutions can be gases.
And then think about alloy, which are made by mixing metals.
Like, say, steel is made by mixing iron and carbon,
and bronze is made by mixing copper and tin.
These are also homogenous mixtures.
The metals are mixed throughout uniformly.
You cannot see them separately,
and therefore these are also solutions.
So you can see, solutions can also be solids.
Now, a key thing about solutions is their concentration.
If you have a lot of solute, relatively,
compared to the solvent,
we say it's a very concentrated solution.
And if you have less solute compared to the solvent,
we say it is a dilute solution.
But why should we care about the concentration?
Well, here's the thing,
although individual components will have
their own separate properties because it's a mixture,
it turns out that the property of the solution itself
can vary, depend upon the concentration.
For example, usually, water freezes
at about zero degrees Celsius, but guess what?
When you add salt to it, the freezing point of this solution
is below zero degrees Celsius.
Similarly, if there is water molecules in the air,
we say there is humidity.
A lot of water molecules means there is a lot of humidity.
Humidity changes the properties of the air.
For example, the speed of sound changes,
its density changes, and so on.
And similarly, steel is made by adding carbon to iron.
And by increasing the carbon concentration,
we can make steel harder.
We can make it stronger and more durable.
So, look, by changing the concentrations of the solute,
we can vary the properties of the solution itself,
and that is pretty awesome.
So, putting it all together,
matter can be classified into two parts.
We have pure substances, which are basically elements
which are made of just one kind of atoms,
or they can be compounds
which are made of atoms that are chemically bonded together
in fixed ratios, like water or carbon dioxide.
And when you physically mix substances together,
we get mixtures.
Now, mixtures can be heterogeneous.
If you can see the individual component apart
with our naked eye, you can see the boundary, like say,
when you add oil to water, or they can be homogenous.
In this case,
the components are distributed uniformly throughout,
and so you cannot tell them apart.
We also call them solutions.
The component which is in the less quantity,
we call them as the solute,
and the one with more quantity, we call them as a solvent.
What's important is that the individual physical properties
of the components, like their boiling point,
melting point, that does not change.
And so, we can use that to separate them out physically.
But a cool thing about solutions
is that by varying the concentration,
we can vary the properties of the solutions
to our advantage.