Illustration by Shuqi Tang

How is separating mixtures a science when it is so intuitive? Even a child knows how to do it, with a vengeance, when they sieve out green peas from their egg fried rice.

But it is not always so straightforward. There is a mixture that, for ages, had been mistaken for a pure substance. It is everywhere around us, but we surprisingly understood very little about it.

This elusive mixture is air.

The nature of air was up in the air

Seeing is believing. We know that egg fried rice is a mixture because we see the mixing. The chef will stir-fry a copious amount of rice with eggs, rice, diced carrots, corns and peas. And we know that Uncle Roger will insist on adding MSG.

It is also heterogeneous. The components do not mix well into a uniform mash. Instead, we can see and distinguish the individual components.

However, air is a tricky mixture. It is homogeneous, whereby its mostly colourless components are uniformly mixed. We cannot easily isolate and identify the components. Neither can we see with our naked eyes the processes that add different gases into the air.

The difficulty in making observations meant that early scientists relied on their old textbooks and ancient ideas. They subscribed to the theory of Aristotle, an influential Greek thinker, that air is a single substance.

A neoclassical portrait of Marine-Anne Lavoisier and Antoine Lavoisier, featuring their glass vessels

A revolutionary idea: air as a mixture with variable composition

Things changed around 300 years ago. As new instruments like the vacuum pump and the mass balance were invented, scientists could conduct more experiments that eventually pointed to air as a mixture.

In the 1770s, French power couple Marie-Anne Lavoisier and Antoine Lavoisier were one of the first to convincingly argue that air is a mixture. They called one of its many components oxygen. The theory allowed them to explain why some kinds of air supported a burning flame for a longer duration. The greater the fraction of oxygen in the mixture of air, the longer the burning.

With the work of later scientists, we now know that air is a mixture of many substances. It contains elements like nitrogen and oxygen, as well as compounds like water and carbon dioxide.

Unlike compounds, the substances in a mixture are not chemically combined and hence do not have a fixed ratio. This is why the scientists could modify atmospheric air and increase its percentage of oxygen. On the other hand, since carbon dioxide is a compound, it will always contain twice as many oxygen as carbon. There is no way to change this ratio.

Illustration by Shuqi Tang

Separating mixtures by physical methods

Since mixtures are not chemically combined, we can separate them by physical methods. After all, we only need a fork to gently lift green peas out of fried rice.

But separating air is not as easy-peasy. It involves multiple physical methods like sublimation and fractional distillation. They happen at extremely low temperatures, hence requiring special equipment in modern factories.

Using the equipment, we cool air down in stages. Water vapour condenses first and is removed by an absorbent. Upon further cooling, carbon dioxide freezes at -79 °C, crashing out of the cold air. We do not stop here. We continue cooling the remaining oxygen and nitrogen to a numbing -200 °C, by which they become liquid.

Finally, we reheat the liquefied mixture of oxygen and nitrogen in a fractionating column to allow for its separation by fractional distillation.

The pure elements find many uses. Food companies use pure nitrogen in food packaging to prolong the shelf life. Hospitals and factories use pure oxygen in treatment and manufacturing.

More importantly, with the substances separated and purified, chemists can analyse them further. We can study their structure and bonding in detail, and relate them to their physical and chemical properties.

Distinguishing pure substances from mixtures by melting and boiling

Unlike the confused scientists of the past, we now know of a simple way to check if something is pure or not. We do it by melting or boiling it.

Pure substances melt and boil at specific temperatures. In contrast, mixtures change state over a range of temperatures.

For example, pure liquid nitrogen boils at -196 °C but liquefied air boils between -195 °C and -190 °C.

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