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Chemguide: Core Chemistry 14 - 16 Burning elements in air or oxygen This page explores what happens if you burn a selection of metals and non-metals in air or oxygen, and has a brief look at the oxides which are formed. General Anything which burns in air will burn more brightly in oxygen. Oxygen makes up only about 20% of the air, and any reaction between an element and oxygen in air is going to be diluted by the 80% or so of nitrogen present. When elements react with oxygen, they form oxides. Magnesium forms magnesium oxide, carbon forms carbon dioxide, hydrogen forms water (hydrogen oxide), and so on. Metals Whether a metal burns in air or oxygen depends on exactly what form it is in (a large chunk, or a fine powder, for example) and how reactive it is. The metal oxide formed is always a solid, and may simply form a layer of the oxide on the surface of the metal with no flame. Here is just a tiny selection of metals. You will meet others later on. Magnesium
You can see that the magnesium burns even more brightly when it is lowered into the oxygen. White magnesium oxide is formed.
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Note: If you watched this closely, you will have seen that a dark patch appears on the right-hand side of the gas jar, and you should have wondered why that was formed. It is obviously not white magnesium oxide, so what is it? In fact, the very hot magnesium also reacts with the glass to give silicon and possibly boron (depending on the type of glass). Trust your eyes when you are looking at reactions, and if something odd happens, question it! | |
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Sodium Sodium is a very soft metal usually stored under oil. If it is exposed to the air, it forms a white oxide layer very quickly. The next piece of video showing the burning of sodium in oxygen describes the oxides formed as "hygroscopic". That means that they pick up (and in this case react with) water from the atmosphere. At this level you are unlikely to need to know about the two different oxides of sodium formed. Iron Laboratory tongs are made of iron, and they obviously don't burn if you heat them either in air or oxygen. But if the iron is finely divided as iron filings or iron wool, it will burn. The black iron oxide formed is Fe3O4, known as triiron tetroxide. The name simply describes the formula. This is a formula you will have to learn - you can't easily work it out.
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Note: The red liquid at the bottom of the gas jar wasn't explained. I suspect it was an indicator of some sort, and is completely irrelevant to this reaction. We will have a bit more to say about this further down the page. | |
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Non-metals Carbon (and hydrogen) Two of the pure form of carbon are graphite and diamond. Both of these burn in oxygen to produce carbon dioxide. | |
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Note: If you have read the page about giant covalent structures you will know that these both contain carbon atoms in two different structural arrangements. | |
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The next bit of video is part of a Royal Institution Christmas lecture. It is a bit longer than most of the videos I am choosing (at almost 5 minutes) - but worth it.
Lime water is calcium hydroxide solution. It reacts with carbon dioxide to form a precipitate of solid white calcium carbonate, and this is used to test for carbon dioxide. A precipitate is simply a solid formed when you mix two liquids or a liquid and a gas (or even sometimes two gases). We usually say that carbon dioxide turns the lime water milky.
You may have noticed that the graphite and diamond were ignited with a hydrogen flame. This was burning with a blue flame and producing steam.
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Note: Why was a hydrogen flame used rather than, say, natural gas, methane, CH4? Because methane would burn to form carbon dioxide which would turn the lime water milky by itself. You couldn't then be sure that there was carbon dioxide coming from the burning carbon. | |
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Sulfur Sulfur burns with a tiny blue flame in air, but a much brighter blue flame in oxygen. Sulfur dioxide gas is produced. (You will find that the video calls this by an alternative name, sulfur(IV) oxide. At this level, the simpler sulfur dioxide is almost always used.)
Sulfur dioxide is a colourless poisonous gas, but when you watch the next video, you will see a sort of pale fog produced. That is formed as well as the sulfur dioxide. Sulfur has quite a low boiling point, and the heat of the reaction vaporises some of it. What you are seeing is a fog of tiny condensed particles of solid sulfur, as the vaporised sulfur turns back to solid again. Phosphorus It is very unlikely that you will need to know about burning phosphorus at this level, but it is an interesting reaction to see. Phosphorus catches fire spontaneously in air and is stored under water to stop air getting at it. Left on its own in air, it starts to smoulder and then catches fire. In the next video this is speeded up by dropping a piece of phosphorus onto warm sand in a flask of air. This isn't burning in pure oxygen, just air. The white smoke formed is a mixture of two solid phosphorus oxides, phosphorus(III) oxide and phosphorus(V) oxide. I'm not giving equations for these, because if I do, someone is bound to think that they have to learn them! Some comments about metal and non-metal oxides Metal oxides Most of the common metal oxides are ionic containing metal ions and oxide ions, O2-. That means that most metal oxides are high melting point solids. Most metal oxides are also insoluble in water. Those that are soluble react with water to make the metal hydroxide. For example, calcium oxide reacts with water to make calcium hydroxide.
The presence of hydroxide ions in solution makes the solution alkaline, and you can show this using indicators such as litmus or universal indicator. We will look at indicators properly later on in the course. The insoluble metal oxides have no effect at all on indicators. | |
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Note: I suspect the red liquid in the gas jar in the iron wool video was something like universal indicator solution being used as a part of a series of experiments burning a number of metals, some of whose oxides will react with the water to give alkaline solutions. | |
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Non-metal oxides Non-metal oxides are covalently bonded. If they are simple molecules like carbon dioxide or sulfur dioxide or water or the phosphorus oxides, then they are gases or liquids or low melting point solids. A few, like silicon dioxide, have giant covalent structures and are very high melting point solids. | |
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Note: If you aren't clear about the difference between the properties of different sorts of structure, you should explore the structure menu. | |
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Some non-metal oxides are neutral to indicators - water and carbon monoxide, for example. Some dissolve in, and react with, water to give acidic solutions. For example, sulfuric acid is formed iwhen sulfur trioxide reacts with water.
Carbon dioxide reacts to a very small extent of give carbonic acid, H2CO3, and so its solution is very slightly acidic. Most ot the carbon dioxide dissolved in water is there as simple carbon dioxide molecules, though.
© Jim Clark 2020 |
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