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Drinks that fizz: Ray Oliver investigates the science of pop

Joseph Priestley, famous for his work on oxygen, invented a way to carbonate water. His account, published in 1772, refers to "impregnating water with fixed air" (carbon dioxide). One of the first to exploit this was Jacob Schweppe, who sold carbonated waters in London in the 1790s. By the 1840s, shops sold sparkling soda water and ginger beer, prepared freshly on demand.

The gas was made by the reaction of chalk with an acid, and the gas stream was directed into casks containing plain or flavoured water. When the tap was opened, the gas pressure forced the foaming drink up to the shop counter. These soda water fountains must have been very hazardous to use.

In Italy and France, the list of certified health-enhancing ingredients on the labels of fizzy water is remarkable. It makes you wonder if it leaves room for the water. "Carbonated water" is made by adding carbon dioxide to the mineral water. In "naturally carbonated water", the gas has actually been removed at source and then put back again in the bottling plant. As for "fortified natural mineral water" - I just give up.

* The carbon dioxide used in fizzy drinks can be used to put out fires. To model a fire extinguisher, make some carbon dioxide in a tube by adding a seltzer tablet to water. Light a candle in a jar and direct the invisible gas on to the candle so that it goes out.

* Carbon dioxide is a very dense gas. Set up a gas balance using a ruler balanced in the centre. Fix two plastic cups, one on each end, using thread supports. Move the cups until they balance perfectly. Add carbon dioxide gas to one cup, prepared as above, and the balance will go down on that side.

* Try filling a jar with carbon dioxide as before. Blow some soap bubbles into the jar. The bubbles float, suspended on the dense invisible layer of gas beneath.

* What about gases dissolved in water? Try this: let the cold water tap run for a few moments, then fill a tall glass. Leave it to warm up slowly to room temperature. The inside of the glass becomes lined with tiny gas bubbles. Ask for an explanation. Children are often surprised to find that the hotter the water, the less gas can dissolve in it. This is the opposite to what happens with most solids dissolving - for example, sugar. Think about the implications for fish living in shallow ponds on hot days. Ask about air bubblers used in fish tanks and why they are so important.

Ray Oliver is a freelance writer and teaches science at St Albans Girls School

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