Making plastics
The sea is teeming with life. In the upper waters are billions of tiny plants and animals. When they die, their bodies sink to the sea floor and rot away. Over two hundred million years ago, in some seas, the bodies collected on the sea floor, but the conditions were not quite right for them to rot away. Instead these bodies became buried by sand which eventually turned into rock. The bodies were heated up by the rock pressing down on them and were cooked into a thick black liquid. Most of it remained trapped until the twentieth century when it was discovered that this liquid - oil - was rather useful.
The bodies of the sea life had turned into substances called hydrocarbons. Each hydrocarbon is made by two kinds of atoms - hydrogen atoms and carbon atoms. They are linked together to form molecules. In each hydrocarbon molecule the carbon atoms are arranged like beads in a necklace. A pair of hydrogen atoms is attached to each carbon atom. Oil is made from a variety of hydrocarbon molecules. Some have a chain of over twenty carbon atoms, while others have fewer than five.

The hydrocarbons are separated from each other at an oil refinery. There are two important processes which are used in the separation. They are evaporation and condensation. These two processes take place in a tall tower. The separation of the hydrocarbons begins by the heating of the oil. This causes most of the hydrocarbons to evaporate. Only hydrocarbons made from chains of over twenty carbon atoms remain as a liquid. They are drained out of the tower and used to make candle wax and bitumen. Inside the tower is a stack of trays with holes and the vapours rise through them. As the vapours rise they cool and begin to condense. The hydrocarbons with chains of about seventeen carbon atoms condense first. They are drawn off from the lower part of the tower and are used for making diesel fuel and oil for central heating systems. As the vapours rise higher, hydrocarbons with chains of about thirteen carbon atoms condense. They are drawn off to make fuel for jet engines called kerosene.

There are fewer vapours in the upper part of the tower but they keep on rising and condensing. When hydrocarbons with chains of about ten carbon atoms condense, they form a yellow liquid called naptha. This is the mixture of hydrocarbons from which plastics are made. Even higher in the column hydrocarbons with chains of about seven carbon atoms condense and form petrol. The shortest chains of hydrocarbons do not condense. They pass out of the top of the tower. These substances are called refinery gases. Some of them are also used to make plastics.

In the plastic-making process small groups of carbon atoms are linked together to make enormously long chains called polymers. Each polymer makes a different kind of plastic, such as polythene or polystyrene. Each kind of plastic has special properties, which make it useful in a particular way. For example, Formica is a tough heat resistant plastic, which is used for kitchen worktops. At first only a few plastics were made from oil but now there are many kinds. In fact if a special kind of plastic is needed, polymers can even be designed from which to make it.

Next time you pick up your plastic pen think about the black gooey oil that it came from, the millions of years that the oil stayed in the ground and the billions of tiny plants and animals that lived in those ancient seas.

Is oil being made today?
No. The special conditions, which prevented the bodies of the sea life from rotting, are not present in seas and oceans today. Oil is not a renewable material. There is only a certain amount of oil on the planet and we use it for many things. We must learn to use oil and its products wisely to make our supply of oil last as long as possible.

When the supply of oil runs out will we still be able to make plastics?
Probably. By then we may have developed special plants which produce substances we can make into plastics. Crops of plastic-making plants could be grown just as we grow crops of cereals today. The plants would be a renewable material for making plastic.

What causes the oil vapours to condense in the tower?
Each vapour has a boiling point, which is different from the others. For example, hydrocarbons with chains of about seventeen carbon atoms have a boiling point of around 300°C. When the rising vapour of that hydrocarbon cools to that temperature it condenses and changes into a liquid. This is a reversible change. No new substance has been made: the hydrocarbon has simply changed from the gas or gaseous state to the liquid state.

Why do the hydrocarbons condense at different heights in the tower?
The tower is hot at the bottom and cool at the top. As the hydrocarbons rise they move into cooler and cooler surroundings. When a hydrocarbon moves into surroundings which are the same temperature as its boiling point, the hydrocarbon condenses. For example in one part of the tower it is 200°C while a little above it the temperature is 120°C. Hydrocarbons with chains of about 13 carbon atoms have a boiling point of about 200°C and condense at the place in the tower which is at 200°C. Hydrocarbons with chains of about 10 carbon atoms have a boiling point of about 120°C. They do not condense in the place at 200°C but rise higher and condense in a place at 120°C.

Are polymers made by reversible changes?
No. Polymers are made by joining hydrocarbons together to make very long chains. The polymers cannot be broken up to make the original hydrocarbons. Polymers are made by irreversible changes.

Why do some plastics go soft when they get hot and others do not?
This is because of the way the polymers are arranged in the plastic. Plastics, such as polythene, which go soft when heated, have polymer chains, which simply lie next to each other. They are not linked together. When the plastic is heated the polymers just slide past each other. This makes the plastic soft. Plastics, such as melamine, which remain hard when they are heated, have polymers, which are linked to each other. When the plastic is heated the polymers cannot move so the plastic stays hard.