Question Video: Changes of State in Cooling Air

The first step in the production of pure nitrogen is to cool air so that the components undergo changes of state. Water is the first component to condense as air is cooled. List the other components of air in the order that they condense or freeze as the temperature decreases.

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Video Transcript

The first step in the production of pure nitrogen is to cool air so that the components undergo changes of state. Water is the first component to condense as air is cooled. List the other components of air in the order that they condense or freeze as the temperature decreases.

What this process is after is pure nitrogen, N₂. Air is a great source of nitrogen because it’s about 78 percent nitrogen. But being only 78 percent nitrogen, it’s 22 percent other gases. So the air is cooled down. And all the components gradually turn into other states. For such a small molecule, water has unusually high melting and boiling points. So it’s the first thing to leave the gas phase as air is cooled. Our job is to look at the other components of air and list them in the order they’d leave the gas phase as air is cooled.

So our first step is to list all the components of air. Our second step is to rank them by boiling point, the temperature below which they will no longer be a gas. And the last step will be to write our ranked components as a list. So onto step one, I’m going to look at the composition of air and list each gas in order by its proportion. The gas that makes up most of air, as we’ve said already, is nitrogen. Nitrogen makes up 78.09 percent of dry air. Remember, we will not be including water vapour in our composition table because we’re looking at dry air where all the water has been removed.

The next largest component of air is oxygen, O₂. Oxygen makes up 20.95 percent of dry air. Next is argon at 0.93 percent. That’s less than one percent. And then we have carbon dioxide at 0.04 percent. Now, we could keep going looking at nitrogen dioxides and sulphur dioxides and other trace components of air. But these make up such a small proportion of air that we can ignore them. Now that we’ve looked up the proportion that each gas makes up of air, we need to look for the temperatures at which they cease to be a gas.

Now, the process of turning from a gas into a liquid is called condensation. But substances don’t have condensation points. They have boiling points which is the temperature at which the reverse happens, a substance turning from a liquid into a gas. But the boiling point is also the condensation point. So let’s look at the boiling point for each of these gases. The boiling point of nitrogen gas is minus 195.8 degrees Celsius. Remember that the negative sign means that it’s a very low temperature. Oxygen has a boiling point of minus 183.0 degrees Celsius. This means that oxygen has a boiling point that’s higher, more positive than the boiling point of nitrogen.

Meanwhile, argon has a boiling point of minus 185.8 degrees Celsius, that’s in between that of nitrogen and oxygen, while carbon dioxide has a relatively high boiling point at only minus 78.5 degrees Celsius. Now, if we’re being accurate, this is not the boiling point of carbon dioxide. It is, in fact, the sublimation point. Boiling is the process of a liquid turning into a gas, while subliming is a solid turning into a gas without becoming a liquid in between. The reverse of sublimation is deposition, a gas turning directly into a solid. That’s what CO₂ does at minus 78.5 degrees at one atmosphere.

Now that we have the temperatures at which all the components of dry air turn from a gas into a liquid or a solid, we can start ranking them. The question asks us to put the components of air in order as they change state from a gas as the temperature decreases. Therefore, we should rank them by the highest boiling point to the lowest boiling point. That puts carbon dioxide first, oxygen second, argon third, and nitrogen fourth. This means that, as we cool dry air, carbon dioxide will deposit first, followed by oxygen turning into a liquid, followed by argon turning into a liquid, followed by nitrogen turning into a liquid.

We include nitrogen here because we’ve been asked to list the other components of air besides water, not the other components of air besides nitrogen. The last step that remains is to list the components of air by their rank. For this, I’m going to use their full names, carbon dioxide, oxygen, argon, and finally nitrogen. In the commercial production of pure nitrogen, air is liquefied all the way. And then nitrogen is allowed to evaporate. Having the lowest boiling point, it evaporates first, leaving the carbon dioxide, oxygen, and argon behind. So our final answer is carbon dioxide, oxygen, argon, nitrogen.

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