Chemguide: Core Chemistry 14 - 16

Burning elements in air or oxygen

This page takes a quick look at what happens when you heat a few metals in air or oxygen. In each case, the reaction in air is not as vigorous as it would be in pure oxygen. Oxygen only makes up about 20% of air, the rest being mainly nitrogen.

Some bits of video to start with

This short video shows what happens when four metals are heated in air and, in one case, in oxygen. In order of appearance, the metals are iron, calcium, sodium and magnesium.

And this one looks at a different form of iron (iron wool rather than iron filings) heated in air, but also keeps track of the changes in mass during the reaction.

This very short clip shows what happens when a piece of zinc foil is heated strongly in air. You may notice some traces of blue around the outside of the flame. That is typical of zinc burning, but notice that you have to keep heating constantly to get the reaction to happen.

This very, very short (12 seconds) clip shows what happens when you strongly heat copper foil. It just gets coated in black.

And finally, this beautiful clip from the excellent University of Nottingham Periodic Table series shows aluminium powder being blown into a flame.

What is happening?

In each case, the metal is either burning, or in some cases just reacting on the surface, to form an oxide. So magnesium burns to form white magnesium oxide. Copper doesn't burn, but its surface is converted to black copper(II) oxide .

In some cases, the oxide formed is more complicated than you might expect. So for example, the formula of the iron oxide produced is Fe₃O₄. It isn't a formula you could work out. The graphic at the end of the second video has the correct numbers of iron and oxygen atoms. That isn't something to worry about for now.

Sodium too is odd, because instead of the product being Na₂O, it is mainly sodium peroxide, Na₂O₂. At a higher level, you might be expected to know that - but, again, not now!

But the others all behave themselves - CaO, MgO, Al₂O₃, ZnO, CuO. You should be able to work out the formulae of all of these.

Can we get any useful measure of the reactivity of the metals from this?

The short answer is "No".

Think about iron, for example. How reactive is it? Iron filings sparkle when you shake them into a flame; iron wool also sparkles, but not as excitingly. But laboratory tongs are also made of iron. You can heat these to red heat for ages, and they won't burn in air. So is iron very reactive or not?

Think about aluminium as well. The powder blown into a flame burned immediately and fiercely. But a lot of saucepans are made of aluminium. If you heat a saucepan dry for a long time, it will probably melt, but it doesn't catch fire.

The reactivity often depends on whether the metal is finely divided (powder or filings, for example) or in a large chunk. There has to be a high surface area for the oxygen to get at.

The atoms in a chunk of iron, for example, will never be able to get in contact with oxygen molecules. The layer of iron oxide forming on the surface simply blocks the reaction.

If you want to sort the metals into reactivity order, you need a more subtle way of doing it.

The rest of this section of Chemguide is about putting the metals into a list with the more reactive at the top, and the least reactive at the bottom - the Reactivity Series. This is one of the most useful things you will come across at this level.