Compounds or Mixtures?

Published: 22 Jun, 2020 | Last modified: 15 Sep, 2020

In the last post, we have defined compounds as the substances whose molecules are composed of atoms of more than one elements, while mixtures are made of multiple substances.

You may wonder, "what is the difference between compounds and mixtures, and how do we decide whether something is a compound or a mixture?"

Law of definite proportions

Well, that's a very good question, the same question that Joseph Proust asked back in 1779. After some experiments, Joseph concluded that "a given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and method of preparation". (wikipedia)

This is now known as law of definite proportions.

Let's consider pure carbon dioxide (\( \ce{CO2} \)) gas and "pure air" for an example.

Pure carbon dioxide

Pure carbon dioxide gas is always composed of 27.29% carbon atoms by mass, and 72.71% oxygen atoms by mass.

This composition (27.29% carbon atoms and 72.71% oxygen atoms by mass) does not change with the place where you get the carbon dioxide gas from. You could collect it from our exhalation, or from a gas cylinder found in a chemical lab. In both situations, you'll get carbon dioxide with the same composition.

It doesn't change with how much carbon dioxide gas you get either. No matter whether you have just 10mL carbon dioxide gas, or a whole tank full of dry ice (solid carbon dioxide), you'll always find 27.29% carbon atoms and 72.71% oxygen atoms by mass.

Oh, you've noticed it, solid carbon dioxide has the same composition as carbon dioxide gas. Yes, that's right. The composition does not change with the state of the substance either.

That's why when they change states, they are still the same thing. So we say state changing is a physical change, it does not vary the composition of the compounds and hence their nature is not changed. When you are thirsty, you can eat a few ice cubes, or wait for them to melt and drink water later on, can't you?

"Pure" air

See the quotation mark on the pure for pure air? This is because pure air is not a pure substance at all. It is a mixture.

Air is composing of mainly oxygen gas (\( \ce{O2} \)) and nitrogen gas (\( \ce{N2} \)). The compositions of oxygen gas and nitrogen gas in air is not constant though.

When you walk into a forest, you feel refreshed. That's because there's a higher composition of oxygen gas. In a forest, oxygen gas is constantly produced by the plants via a process called "photosynthesis".

Global warming and carbon dioxide

In fact, the composition of air in the atmosphere has changed substantially since the Industrial Revolution in the 1760s. Human activities have released tremendous amount of carbon dioxide into the atomosphere since then. The carbon dioxide level has increased from about 280ppm in the 1750s to about 410ppm in Year 2020. (ppm: parts per million; 1ppm means 1 million gram or 1 ton of air contains 1 gram of carbon dioxide)

Carbon dioxide is able to absorb the heat coming from the sun, as well as prevent heat generated by human activities from releasing into the universe. As a result, excess carbon dioxide will raise the temperature over the years, giving rise to a series of climate problems and eventually seious issues with huge impacts on our society.

But when you stay in a closed room for a very long time, you may feel sleepy. This is because oxygen level in the room has dropped, although not very significantly. When we breathe in, we're taking in the oxygen gas in the air. While we exhale, we give out carbon dioxide. Since there's nothing to replenish the oxygen gas in the closed room, oxygen level would drop slightly.

As you may see, the composition of pure air could vary depending on a lot of factors, while the composition of carbon dioxide gas is not changing at all.

This is one of the most foundamental differences between compounds and mixtures. And it is described by the law of definite proportions.


Is a piece of paper a compound or a mixture? How about glass? And distilled water?

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Licensed under All Rights Reserved except otherwise stated. © 2020

Up Next: Electron Configuration »

Let's understand how electrons are allocated in the atom, as this is essential for chemists to predict the chemical properties of elements

« Read Again: Matter

Matter has mass, occupies some volume in space, and exists in different states

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