Automotive manufacturers are now required to indicate the precise level of carbon dioxide emissions for every new car. Small, low-emission cars seldom exceed 100 grams of carbon dioxide per kilometre.

Gas-guzzling luxury saloons or SUVs (large off-road vehicles or pickups for example) can emit more than three times these levels into the atmosphere. But carbon dioxide is a gas. How does one actually put a gas on a set of scales?

In actual fact, no one actually collects fumes emerging from a factory chimney or the exhaust pipe of a car to measure them. The carbon dioxide emission values are based on calculations. These calculations include fuel consumption, the carbon content of the fuel and the efficiency of the combustion process.

A similar calculation process is applied to emissions from power stations. The operators know that one kilogram of coal contains about 600 grams of the element carbon.

Assuming perfect combustion, all carbon atoms will combine with two oxygen atoms to form carbon dioxide. The combined weight of these two oxygen atoms is roughly three times more than one atom of carbon, weighing approximately 1600 grams. It follows from this that the weight of exhaust gas produced by one kilogram of coal amounts to 2.2 kilograms of carbon dioxide.

Dry ice in the school laboratory
Every school laboratory is in fact capable of weighing carbon dioxide. The simplest way is to use dry ice. Dry ice is pure, frozen carbon dioxide. It evaporates very rapidly at room temperature. The dry ice vapour can be collected in a balloon. The gas now needs to be transferred into a one-litre flask that has been pumped out and weighed precisely.

Then the flask, now filled, is weighed again. The difference in these two readings shows the weight of one litre of carbon dioxide at room temperature and under normal air pressure. This experimental result will be somewhere between 1.9 and 2 grams per litre.

One and a half full bathtubs of carbon dioxide for each kilometre driven

This result really becomes an eye-opener when it is extrapolated to the fuel consumption of a large saloon. Every time it completes one kilometre on the road, the vehicle emits 300 grams of carbon dioxide, which has a volume of 150 litres.

That would be enough to fill one and a half bathtubs. This volume, 150 litres of carbon dioxide gas, is roughly the amount that one tree can absorb in one hour. From which it follows that, if our large saloon travels 100 kilometres in one hour, it will emit about the same volume of carbon dioxide as 100 trees are capable of absorbing within the same period of time.