3 What are compounds?

Activity 1: Elements and compounds

Timing: 0 hours 10 minutes

Click on the video clip to watch Elements and Compounds, which focuses on water and its constituent elements.

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David Johnson

For those who know where to look and how to make things happen the world is a very rewarding place. And that’s true not just in business but in science as well. Here there is water everywhere and as you'll see it's got a lot to offer. But I want to start with something that comes from somewhere that is about as far and as different from that as you can get. Take a look at this. It's a mineral called caliche and it comes from the Atacama Desert in Northern Chile. Caliche is valued because it contains something called sodium nitrate but besides sodium nitrate there are other things in caliche such as salt and sand so our first job is to separate what we want, the sodium nitrate, from what we don’t want, the salt and sand, just as they do in Chile today. The sodium nitrate is used as a fertiliser. Now I don’t have to look very far for a hint on how to do it. Out there things are kept separate from sand because they dissolve in the sea. So I will just add water - but in a laboratory.

First, I will add generous amounts of caliche to hot water. The sodium nitrate and salt dissolve but things like sand and clay don’t. They remain in suspension. Now I will pour the hot mixture into a filter paper held in a funnel. The paper removes the clay and sand and what comes through is a solution containing dissolved sodium nitrate and salt.

How can I separate the sodium nitrate from the salt?

Well, if I cool this solution the salt stays put. But sodium nitrate is different. Hot water can hold a lot of it in solution but cold water only a little. So if I chill the solution most of the sodium nitrate will crystallise out.

I am just banking up the ice around it. Now it's a question of how long we have to wait.

I can see some coming out now.

That’s beautiful.

Now all I need do is to filter off those crystals, wash with a little ice cold water and dry them.

Here is the result. This purified sodium nitrate looks very different from the caliche from which it was first made.

Suppose I am a cook who has never seen Genoa cake before. I take it apart and look at the bits. It contains what looks like glace cherries, sultanas and bits of lemon peel. It tastes quite sweet so there must be sugar in it and there is also a matrix which I know from experience is made from flour and eggs. This method tells me not just the ingredients but also the proportions in which they are combined.

Now there is a way of checking that I have got it right. I put the cake back together again. I go out, buy the ingredients and make a cake from them. If I have got the ingredients and proportions right then it will taste just like Genoa cake. Mmm. Very good. This technique taking things apart and putting them back together again clearly works for cake so let's apply it to a chemical substance.

Here you can see water being taken apart. It's done by passing an electric current through it. Water isn't a very good conductor of electricity so I have boosted the conductivity with a trace of sulphuric acid. But you can take my word for it that it is water that is disappearing from the solution.

It's being taken apart into two gases. One gas appears at the carbon rod where the electric current leaves the liquid. To get there, the current has to pass through this U tube of solution from the other carbon rod where the current enters the liquid. It's at this other rod that the other gas is evolved. The gas is passed down side arms and into test tubes.

The left-hand gas has twice the volume of the other. It’s hydrogen. A mixture of hydrogen and air burns with a squeaky pop.

That’s a nice pop.

But I haven't shown you the test for the other gas, which is oxygen. Let me do that first with a gas jar of the stuff.

This is the standard test for oxygen gas. I light the splint, blow it out, put it into the jar of oxygen and it flares up instantly.

Now let’s check that this gas really is oxygen.

There we are. The splint flares up.

So, by using an electric current I have taken water apart into hydrogen and oxygen. Can I put it back together again? I will use this gas cylinder to pass hydrogen through a jet. When I light the jet the hydrogen burns and reacts with the oxygen in the air. Now I play the flame on to the surface of a flask full of ice. The product from the flame condenses on to the cold surface. It's dripping off the flask and it's quite safe to drink. It's water.

So water can be taken apart into hydrogen and oxygen gases. Chemists call water a compound of hydrogen and oxygen.

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Although there are about only 100 elements, there are many, many more than 100 substances in the world - not just thousands but millions of different substances. You could begin a list starting in your kitchen: water, salt, sugar, vinegar, bicarbonate of soda. None of the substances in this particular list are elements, so what are they? They are substances in which atoms of different elements are joined together. The proper chemical term for any such substance is chemical compound or just compound.

An important feature of compounds is that they are very different from the elements from which they are made. For example, water is made from hydrogen and oxygen, which are both colourless gases, whereas water is the wet liquid you drink that makes up 65% of your body. So, it is important to realise that a water molecule is quite different from the two types of atom from which it is formed. Water is not simply a mixture of hydrogen and oxygen; it contains hydrogen and oxygen atoms linked together in an ordered way. (You can make a house from Lego but you would not look at a pile of the separate blocks and say that is a house! In scientific terminology, the house is the molecule and the blocks from which it is built are the atoms.)

The next most common element in human bodies, after hydrogen and oxygen, is carbon. This, when linked to other atoms, forms most of the compounds of which plants and animals are made (apart, that is, from water). One very important category of compounds found in plants and animals is the proteins. Part of a protein molecule is shown in Figure 10. This very large molecule (it contains thousands of atoms!) is made up of only four different types of atom; note the complex way in which the atoms are put together.

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Figure 10 Part of the structure of a molecule of a protein. It is composed of four different types of atom

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Figure 10

Figure 10 Part of the structure of a molecule of a protein. It is composed of four different types of atom

Therefore, it is possible to have simple molecules such as water where only three atoms are bonded together to make a water molecule, and very complex molecules such as proteins where very large numbers of atoms are bonded.

Practise your understanding of elements and compounds by trying the following questions.