Critical and Pseudocritical Temperature

Image via Wikipedia Critical Temperature is the temperature above which, no matter how much pressure you apply, you cannot force a gas to become a liquid. Interestingly enough, though, if you apply sufficiently high pressures, you can form a solid. Essentially, distinct liquid and solid phases of a substance no longer exist.

If you measure the vapour pressure of a substance at the critical temperature, that pressure is called the critical pressure. Alternatively it could be defined as the pressure which is required to liquefy a vapour at its critical temperature.

A substance is a vapour when it is in equilibrium with the substance in another phase, and a gas when there is no liquid or solid present. Therefore, by definition, except at the extremely high pressures mentioned above, any substance above its critical temperature, is a gas. A liquid does not have to boil, nor a solid to sublime (change state directly from solid to vapour/gas-Ed.) to form a vapour. You can draw a serious of lines, plotted on a graph where the x-axis shows volume, and the y-axis shows pressure, which correspond to different temperatures and called isotherms, which demonstrate what will happen to a substance as you increase temperature with a given volume (or pressure). The one with most relevance of course is nitrous oxide...(see here).

Pseudo-critical temperature is the critical temperature of a mixture of gases. In anaesthesia it is commonly used to describe the temperature at which a 50:50 mixture of oxygen and nitrous oxide separates (laminates) forming liquid nitrous oxide and gaseous oxygen, which occurs at (depending on the pressure) temperatures in the range -7 to -5.5 degrees Celsius in cylinders, and lower temperatures in a pipeline (due to lower pressures) at around -20 degrees Celsius.

Random Factoid: Perlite

Image via WikipediaEver wondered what Perlite is?

It's the stuff that goes in between the two layers of steel in a Vacuum Insulated Evaporator (VIE) which is used for storing about 10 days worth of oxygen on hospital grounds.

It's a form of volcanic glass, which when heated expands from being a glass to being about 10-15 times the original volume. This means that it becomes very low density, and also acts as a good insulator.

It is also used in plaster (as in for walls) and in hydroponics because it's very good at holding water.

Usually drink, usually dance, usually bubble

EI has relations with a Street Style blog over at Stitsh.com, and recently a little vid caught our attention over there. Click here and scroll down to 28.06.08.

It reminded us of a little law that the examiners sometimes like to question, that is Henry's Law:

At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Okay, so what does that mean.

Most of the time we refer to Henry's law by the formula p=kc (that's one way of looking at it - Ed).

Another way is to say:

where:

is approximately 2.718, the base of the natural logarithm

is the partial pressure of the solute (the gas being dissolved) above the liquid in which is being dissolved.

is the concentration of the solute in the solution

is the Henry's Law constant, which has units such as L·atm/mol, atm/(mol fraction) or Pa·m³/mol (this is so that the dimensions all work out correctly - the funny thing about constants is that they usually can be expressed in many different units, depending on what units the rest of the equation is being calculated in....more on that another time).

(In other words, most of the time, we take the natural logarithms of both sides).

The pressure above a solution dictates how many collisions occur between the gas and the liquid. So if you increase the pressure above the solution, the partial pressure of the gas increases, the number of collisions increases, and more gas is dissolved. What will then happen is that an equillibrium will be achieved, where the number of molecules of gas crashing into the surface of the liquid will be the same as the number of molecules leaving the surface of the liquid.

The more observant amongst you will have realised that temperature hasn't been mentioned yet except in the definition....

So what effect does temperature have?

Well, think of a can of "fizzy pop" (you're showing your age there - Ed). When it comes out of the fridge, it's not that fizzy, is it? However, the longer you leave it standing around, the closer it's temperature comes to room temperature, and then when you go back to the can, first it will seem quite gassy, and then eventually it will go flat. This is because the gas in the drink is coming out of solution. The gas solubility relationship with temperature is very similar to the reason that vapor pressure increases with temperature. (This is Gay-Lussac's Law: The pressure of a given number of moles (given amount) of gas, is directly proportional to its temperature in Kelvin (absolute temperature scale), when the volume is kept constant. Better known as P/T=k).

Increased temperature causes an increase in kinetic energy, which in a gas causes either expansion or an increase in pressure, or in this instance, more movement of the molecules, which break free of the surface of the solution! (The surface could be the gas side of a small bubble of gas trapped within the solution, which is one reason we get bubbles!)

If you want to see another demonstration of Henry's law in action, look at a pan of water. As you warm the pan, small bubbles start to form, well before the pan reaches 100°C (373K). Those bubbles are air coming out of solution.

So why do the examiners like this concept: the Bends.

Decompression Sickness occurs when gas (specifically nitrogen) is breathed at higher than atmospheric pressure, and the diver then returns to atmospheric pressure without allowing the gas to come out of solution slowly, resulting in gas bubble formation, and hence, "the bends" (gas in the joints) and "the staggers" (gas bubbles in the brain causing confusion and ataxia) and "the chokes" (probably PE).

It is also a concept that comes into play when talking about Ostwald and Bunsen coefficients....(more on that another time).



(Equations courtesy of Wikipedia)