Let’s look at the effect that pressure has
on the solubility of gases that are dissolved in liquid.
We’ll use an example with soda to see what the general trend is, and then we’ll also
see how this pattern of pressure and gas solubility
is very important for Scuba diving.
A major concept here is gas solubility.
Solubility tells us the maximum amount of solute that can dissolve in a certain amount
of solvent.
Here, gas is the solute and water is the solvent, so we wanna know much gas can dissolve in
a certain amount of water.
We don’t always think about gases dissolving in water, but it happens a lot.
For example, soda is a solution of carbon dioxide gas dissolved in water.
There’s some other stuff too, like sugars and coloring, but we’re really just worried
about the water and the carbon dioxide gas.
And the solubility of that gas—the amount that can dissolve—changes, depending on
pressure.
You can see how gas solubility changes with pressure whenever you open a bottle of soda.
Let’s watch this happen.
We’ll start off with the bottle closed.
Now this bottle is closed, so we have the water here with the carbon dioxide dissolved
in it, but there’s also carbon dioxide gas in this area above the liquid.
That carbon dioxide gas presses down on the surface of the liquid, and that creates high
pressure.
This red arrow shows the pressure.
Now here’s the trend: as we increase the pressure of the gas above or on the liquid,
we increase the solubility of the gas that’s in the liquid.
So, higher pressure, higher solubility.
Relatively speaking, there’s a high amount of carbon dioxide dissolved in the liquid,
because of the high pressure above.
When the cap is on, there is a lot of gas pressure above the liquid.
But here’s what happens when you lower the pressure.
All that gas pressing down on the liquid?
When you open the cap, you release that gas.
You hear that pssst, that’s the gas escaping.
So there’s less carbon dioxide above the soda.
That’s less pressure pushing down.
OK?
Now we have relatively low pressure.
So what happens to the carbon dioxide dissolved in the soda?
Its solubility is linked to pressure: lower pressure, lower solubility.
So its solubility decreases.
Now, less carbon dioxide can dissolve.
And the extra carbon dioxide that can’t dissolve anymore,
it comes out of the solution.
You can see it bubbling out: that's what gives soda its fizz.
We see bubbles when gas can’t dissolve anymore.
We didn’t see bubbles when the bottle was
at high pressure, because the carbon dioxide was all
dissolved.
But here, a lot of carbon dioxide can’t dissolve anymore, so it bubbles out.
So, the higher the pressure of the gas above the liquid, the higher the solubility of the
gas dissolved in the liquid.
The lower the gas pressure above the liquid, the lower the solubility of the gas.
And…the gas that can go longer dissolve, it bubbles out.
This relationship between pressure and solubility of a gas is often referred to as Henry’s
Law.
It is what we call a direct relationship: the two things move in the same direction.
As the pressure of the gas above the liquid goes up, the solubility of the gas in the
liquid goes up as well.
As pressure goes down, so does the solubility.
Now here’s an example that’s super interesting.
Henry’s law really comes into play during Scuba diving, and it can be a life-or-death
situation.
Here, it's not about gas dissolved in a bottle of soda, it’s about gas dissolved in a diver’s
blood!
This is a little zoom-in magnification of the diver’s blood vessels.
So, just like many other liquids, blood has gases dissolved in it: there’s oxygen
and nitrogen, carbon dioxide, bunch of different gases.
And these gases are all under pressure.
Here, a lot of that pressure is coming from the water pushing on the diver’s body.
Here’s what happens.
As divers go deeper and deeper under the water, the pressure on their bodies increases.
That means that more gas can dissolve in the blood.
High pressure, high gas solubility.
Compared to the surface, more gas can dissolve down here.
And that extra gas stays dissolved in the blood, because of the high pressure under
the water.
But a problem happens if divers come up too quickly.
As they come up, the pressure decreases, which means the gas solubility decreases as well.
All of that extra gas that was dissolved in the blood down here can no longer stay dissolved.
And so the extra gas that used to be dissolved, it literally bubbles out of the blood.
This is happening at a small scale, but it’s essentially like a microscopic version of
a soda bottle opening up PSST in the diver’s blood vessels.
Oooh, you can imagine that hurts.
And this creates a syndrome named decompression sickness.
But it’s often referred to as “The Bends.”
It is extremely painful, and it can be deadly.
To prevent the bends, divers need to surface slowly,
so that the pressure changes slowly, and the extra gas comes out of their blood gradually,
instead of all at once.
Now, if someone is suffering from the bends, what can be done about it?
Well, they’re probably gonna need medical treatment.
The diver is placed in a high-pressure chamber.
This allows doctors to increase the pressure The high pressure increases the gas solubility
again, so that extra gas dissolves back into their blood.
Then, the pressure is decreased very slowly.
This lets the extra gas come out gradually, just like swimming very slowly to the surface.
If Henry’s Law got them into trouble, Henry’s Law is used to get them back out!
So that’s an explanation of the relationship between gas pressure and solubility.
The higher the pressure of a gas on a liquid, the higher its solubility in the liquid.
This is a direct relationship referred to as Henry’s Law.
You can see this demonstrated dramatically when the pressure drops quickly.
This causes bubbles to form in soda when the cap is removed: the pressure lowers, and so
does the solubility of the gas.
The bubbles are the gas that can’t dissolve anymore.
We also see this when Scuba divers come to the surface too quickly.
More gas dissolves in their blood at high pressure deep down, but the pressure decreases
as they rise to the surface.
That gas can dangerously bubble out of the blood as its solubility decreases as well.
So that's Henry's Law, gas pressure, and gas solubility.