Pop your cork

17th October 2003 at 01:00
Ray Oliver presents some carbonated conundrums for the classroom

One of the great things about investigating fizzy drinks is the obligation to avoid waste by drinking any unused material. Depending on your budget, this may mean champagne or ginger beer. Since home-made ginger beer is infamous for its propensity to explode, champagne may be the safer choice. It makes a good teaching point to compare the weight, thickness and strength of champagne bottles with that of plastic cola bottles.

Many mineral waters are naturally effervescent. Choose your sample with care. What you want is carbonated water, fizzing with carbon dioxide. What you don't want is "sulphurreted hydrogen", bad-egg gas, present in some spring waters.

Fizzy drinks are big business and the technology dates back to the 18th century. It was soon realised that adding flavouring would be beneficial, even if some bottled waters made a virtue of having a vile taste. In Dickens's Pickwick Papers, Sam Weller compares the taste of spa water to that of a flat iron.

The flat drink problem

This can be tried at home, especially if your control experiment uses sparkling wine. Pour out half a glass of soda water and leave it for several hours. Taste it. Immediately afterwards pour out another sample from the fridge and compare the tastes. There is a more subtle difference than the presence or absence of bubbles. Carbon dioxide, when dissolved in water, gives a sharp, acidic taste. This is a major part of the taste and appeal of fizzy drinks. Once the gas has escaped, the taste changes, usually becoming sweeter.

Acid strength

On the pH scale, soda water generally gives a value of pH4. It is a weak acid. Test samples of fizzy drinks with a range of indicators such as universal (pH indicator), litmus and even home-made versions. Red cabbage and blackberry juice are both worth trying. Boil a sample of soda water for five minutes and test again.

The increase in temperature will drive out most of the dissolved carbon dioxide and reduce the acidity. The pH will rise towards neutrality, pH 7.

It is claimed, though it may be apocryphal, that one leading brand of fizzy cola can dissolve teeth. All you need is a redundant milk tooth and a glass of cola. Leave overnight and, hey presto, a tooth solution.

Bubble machines and sherbet too

You can make your own fizzy drinks using the machines sold in supermarkets.

The source of the fizz is usually a small metal tube filled with pressurised carbon dioxide. How much gas is inside? Children will realise that this could refer to the mass of gas or the volume of gas. If you are brave, the metal tube can be opened by tapping the point of a nail on to the seal. This is for demonstration only.

Weighing the full and empty tube gives the mass of carbon dioxide inside.

To find the volume needs a little more ingenuity. Fix a rubber hose over the end of the metal tube and push the nail point through the rubber. It is possible to break the seal and collect the gas that escapes rapidly from the hose.

A safer low-tech method can be used to make sherbet. Mix together the following dry powders: sodium bicarbonate (2 spoons), citric acid (4 spoons) and icing sugar (8 spoons). Children need to grind the powders together to give a uniform mixture of sherbet. When added to cold water, the sherbet drink fizzes vigorously. The acid and carbonate react to release bubbles of carbon dioxide.

A gas conundrum

If you let the gas escape from a 2-litre bottle of cola, does the weight of the bottle change? Many people are challenged by this question. One line of argument runs like this: "Gases are very light. The light gas escapes from the bottle. Therefore the bottle and drink inside will weigh more." This is complete nonsense. You can check it by weighing a large bottle of cola over a period of hours. The loss of weight is several grams as the dissolved gas escapes from the bottle.

The heaviness of carbon dioxide can have serious consequences. When beer and wine ferment, large amounts of the gas are produced. A layer of gas can form on the floor of a cellar and prove fatal to anyone who slips and falls.

Volcanoes release large volumes of gases, including carbon dioxide. In the 18th and 19th centuries, travellers to the Bay of Naples could visit the Grotto del Cane, the cave of the dog. The guide would lead a dog into the cave whereupon it would fall over. The carbon dioxide at the floor of the cave was responsible. Visitors breathing the air above were unaffected. The cave was so famous that the Boys' Playbook of Science published in 1880 gave instructions for the construction of a model cave - but not a model dog.

That sinking feeling

Children can investigate the high density of carbon dioxide by filling a balloon or bag and then releasing it. It is easier to use a polythene food bag as it needs less pressure than a balloon to inflate it. Open a bottle of soda water, put some petroleum jelly around the neck and attach the bag using an elastic band. It is essential to have a gas-tight seal.

Place the soda water bottle in a saucepan of very hot water. As the carbon dioxide bubbles out and fills the bag, it will swell and fall to one side.

If you seal the bag using cotton thread and release it, it falls to the floor.

Ray Oliver is teaches at St Albans Girls' School, Hertfordshire

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