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Concentration and Pressure
Written by Tim Sheppard MBBS BSc. Last updated 10/11/10

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What is concentration?

Concentration is one of the scientific basics that is incredibly important for you to get your head round. If you don't understand concentration, a lot of parts of science - and especially aspects of reactions and kinetics - will be incredibly difficult to follow. Unfortunately, it's also not an easy topic to explain if you don't know what it is!

You might have seen on a can of beer there is a reference to how much alcohol is in it - probably somewhere in the region of 5%. That means that out of all the liquid in that can, 5% (or 1/20) is actually alcohol. The other 95% is something else. This is explaining how concentrated the alcohol is. In terms of a liquid, it's saying how much of a mixture of liquids is made up of the liquid of interest - in this case, how much of the beer is made up of alcohol.

You can also talk about concentrations of a solid in a liquid. Imagine you're having a mug of coffee, and you put three teaspoons of sugar in that drink. If you wanted to make a whole flask of coffee which had the same sweetness, you couldn't just put three teaspoons of sugar in. If the flask holds 2 mugs of coffee, you'd put 6 teaspoons in. You are maintaining the concentration of sugar automatically, and you're not even thinking about it, but you are controlling the amount of solid within a fixed volume.

You could say that the concentration in the original mug of coffee was '3 teaspoons per mug'. In the flask, there were '6 teaspoons per flask', but because the flask holds 2 mugs of coffee, it was the same as '6 teaspoons per 2 mugs', or '3 teaspoons per mug'.

In science, we tend to use more specific measurements than 'teaspoons' and 'mugs', but it gives a reasonably good idea. Concentration is often measured in molarity, but that's quite a complicated topic. Basically concentration gives an idea of how much of a substance is contained within a certain volume. If it's a liquid such as alcohol, then it's how much of that solution is made up of alcohol. If it's a solid such as sugar, it's how much of that solid is dissolved in the liquid. Concentration could also be measured in weight per volume (e.g. 40 grammes per litre).

If you dissolve less of the solid into the liquid, then it's going to be more spaced out. The concentration will be lower, and this means that the particles of solid that are floating about will be further from each other. Imagine we're talking about hydrogen ions floating about, as in an acid. If there is a high concentration of hydrogen ions, that means there's lots of them in the liquid (or in the 'solution'). If you drop something into a strong acid (where there is a high concentration of H+ ions), it's much more likely to react because the hydrogen ions are more packed and are more likely to come into contact with the object you dropped into the acid. However, in a weak acid, where the H+ ions are far apart, the object is less likely to come into contact with the hydrogen ions and therefore the reaction will not happen so readily.

I suppose concentration is a measure of how close together particles are within a solution. A high concentration means they're very close together, and a low concentration means they're far apart. This can have important effects on how quickly a reaction takes place.

What is pressure?

I always see pressure like concentration but for gases. Pressure is another important feature in determining how quickly a reaction takes place, and it reflects how closely together the particles of a gas are. You can apply pressure to a liquid, but liquids are less deformable (they're not going to squash so easily) so it's more relevant in terms of gases.

A low pressure means the particles involves are spaced out. They have plenty of room to fly about freely. Realistically the pressure is a result of the force that the particles place on the boundaries of what they're contained in. If you allow particles to fly about in a huge space, they're not going to bump into the sides very much, because they've got plenty of room to roam.

However, if there is only a small space, the particles will be very close together, so there will lots of bumping into the sides, and therefore the pressure will be high. The force exerted on the boundaries of the container will be greater because so many particles will be bumping into the sides.

Pressure is more difficult to follow than concentration, but it's also very important because it effects how quickly a reaction takes place in the same way as concentration. If the particles are close together (i.e. high pressure) the reaction will happen more readily / quicker because the gas is more likely to come into contact with whatever it is reacting with. However, in low pressure, the particles of the gas are further apart and therefore they're less likely to come into contact with the reacting object, so the reaction doesn't happen so readily.

A more accurate, mathematical definition of pressure would be:

Pressure = Force / Area

In the case of squashing a gas into a smaller space, you are reducing the area that the gas particles are bouncing against, so the pressure increases. Similarly, if you put more gas particles into the same space, the area wouldn't change but the force would increase because there would be more gas particles exerting a force, and the pressure would increase.

Blobs are generally all the same size. They are made of the same stuff, and they have the same weight. Therefore if you put any blob on a platform, that platform would apply the same amount of force. If it were a small platform, then the force would be constant, but the area would be small, so the pressure would be high.

If you put a blob on a big platform, then the force would still be the same, but the area would this time be a lot bigger, which means the pressure would be low.

To that extent, pressure is a bit like concentration of force. Instead of weight per volume, though, you are considering force per area. It might be that you are applying 5 Newtons per m2, or 5 Newtons per cm2. Clearly the second one is higher pressure because the force is more concentrated. Ultimately that is the concept of pressure.

What is partial pressure?

The partial pressure of a gas in a mixture of gases is the pressure which that gas would exert, were it the only gas occupying that space. Air is a mixture of gases. Normally, at sea level, air is at a pressure of 1 atmosphere or 100 kPa (kilopascals). If we were asking the partial pressure of nitrogen in air, what we're really asking is what the pressure would be if it was the only gas in the air - what would air pressure be if only nitrogen were around.

Partial pressure of a gas in a mixture of gases is calculated using the following equation:

Partial Pressure of Gas Z = Molar Fraction of Gas Z x Total Pressure

This equation basically considers how much gas Z is responsible for the total pressure. If gas Z only makes up 5% of all the gas in the mixture, then it's only contributing 5% of the total pressure, which means that it's partial pressure will be 5% of the total pressure (i.e. the pressure that it would exert if it was the only gas there would be 5% of the total pressure).

You can also have the partial pressure of a gas in a liquid, although this is a more difficult concept to understand. It doesn't really make the same logical sense as partial pressure of a gas in a mixture of gases, but it is a definition worth knowing, because it can be quite relevant. If people talk about 'blood gases' (which is a test doctors often have to take), they are talking about the partial pressures of particular gases dissolved in blood.

If a gas (e.g. oxygen) is soluble in a liquid (e.g. water) then while it's floating about above the surface, some of it will dissolve into the liquid. Similarly, if a gas is dissolved in a liquid, some of it will diffuse from the surface of the liquid into the air.

At a particular pressure, the gas dissolved in the liquid and the gas above the liquid are in equilibrium; there is as much gas leaving the liquid as there is dissolving in it. This is the partial pressure of a gas in a liquid.

If a liquid is more soluble to a gas, then the gas will dissolve into it more easily. However, the partial pressure will still be the same - that is, the gas dissolved in the liquid and the gas floating about above the liquid will still be in equilibrium, it's just that the concentrations will be different, because more or less will be dissolved.

Partial pressure is denoted by P; e.g. the partial pressure of oxygen gas, O2, could be written as PO2.

What is a molar fraction?

A molar fraction is defined simply as the number of moles of a particular gas, divided by the total number of moles.

This effectively gives us an idea of how much of the mixture of gases is made up of a particular gas. Volume is proportional to number of moles according to Avogadro's law, which means moles is a really good indication of what proportion of a mixture of gases is made up of a particular gas. This is why molar fraction is so immensely helpful in calculating the partial pressure of a gas in a mixture of gases.

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