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QIn an aeroplane at 30,000 feet, you can often water droplets, even thought the temperature is below freezing point. How can this be?

A Water can exist as a liquid below 0 Centigrade, and often does. There are two main reasons. Impurities in water reduce its freezing temperature, which is why salt is put on roads in winter. Adding salt can lower the freezing point as far as -13oC. Pure liquid water can also exist below 0oC. For water to freeze, molecules must form the correct structure. A container, such as a cup or bowl, offers plenty of chances for this structure to form. With a small amount of water, the structure is less likely. Hence drops of liquid water can exist in the atmosphere at temperatures as low as -40oC.

Q Smoke alarms often only last for five years. What stops them working after this time? Is the radioactive source used up?

A Smoke detectors use tiny amounts of a radioactive element called americium. An electronic sensor detects the amount of radiation coming from the americium. If that suddenly drops - because smoke has got in the way - the alarm sounds. Americium has a half-life of several thousand years, so running out of radioactivity isn't a problem. The reason you are advised to replace your smoke detectors every five years is that the electronic detector inside can go wrong.

Q Salt is made of crystals, as are diamonds. Why are diamonds much harder than sugar?

A Diamond is a macro-molecular structure of carbon atoms, where each carbon is bonded to four others by sharing electrons with them in a covalent bond. Salt consists of sodium and chloride ions, but these are bonded ionically, meaning one or more electrons from one atom (in this case sodium) are "given" to the other atom in the molecule. Because ionic bonds don't involve sharing electrons between atoms, they aren't as strong as covalent bonds. This is why diamonds are stronger than salt.

Q Why are modern British copper coins (1p and 2p pieces) magnetic, whereas old ones are not?

A In 1992, the metal used to make coppers was changed from bronze to copper-plated steel. This makes them magnetic.

Q Who invented ball bearings?

A The idea of using balls as anti-friction devices was first suggested by Leonardo da Vinci in the 16th century, but, as with many of da Vinci's ideas, it wasn't put to use for a long time. In 1794, Philip Vaughn from Carmarthen got the first patent on a bearing which resembled modern ball bearings; but it wasn't until steel became economically available in the late 19th century that steel ball-bearings, like those we use now, became popular.

Q Why do batteries which are running down recover slightly if you don't use them?

A In a copper-zinc battery, a copper plate and a zinc plate sit in a dilute sulphuric acid solution. When the battery is working, tiny bubbles of hydrogen gas gradually form around the copper plate and prevent the flow of charge. When the battery is disconnected, some of the hydrogen disperses slightly, allowing the flow of electrons again for a short while.

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