Tuesday 11 March 1997
Q How fast is the Earth spinning?
A It depends where you are. On the equator, where the circumference of the Earth is about 40,075 kilometres, the rotational speed with respect to an imaginary line through the Poles is about 1,670 km/h, or just over 1,000 mph. In the UK, where the distance travelled in 24 hours is only 26,000 km, we are travelling at 1,080 km/h, or 675 mph.
Q Which metal has the greatest density?
A The densest metals are those at the end of the periodic table, as their atomic nuclei have lots of protons and neutrons, but they don't take up that much more space than the nuclei of lighter atoms of elements. So there's more mass in about the same space. At room temperature and pressure Unniloctium (element 108) has an estimated density of 41g per cubic centimetre - but nobody has ever made that much. Of naturally occurring metals, osmium is the densest: 22.57g/cc at a temperature of 298 Kelvin. In comparison, mercury is just 13.53 g/cc.
Q How were ancient buildings constructed so well without benefit of modern building techniques?
A Many ancient buildings had religious significance - which often meant they merited the best materials and methods of construction then available. Despite this, many did not not survive well: the Victorians did a great deal of restoration work on many ancient buildings and would often build completely new structures, as they did with Canterbury Cathedral. A church founded in the 12th century is quite likely today not to have any original stones left in it. Westminster Abbey was built in 1065 - but none of the original materials remain.
Q What is white spirit made of?
A It is a mixture of spirits produced by heating petroleum - itself a mixture of hundreds of organic compounds with boiling points varying between 115C and 180C. In this temperature range, various compounds vaporise, and can be collected and used. Boiling a substance at different temperatures to give different products is called fractionation.
Q How do halogen lamps work?
A Normal light bulbs have a tungsten filament, which heats up when a current is passed through it in a partial vacuum. Halogen bulbs are filled with one of the halogen gases (such as argon) instead of a vacuum. Halogen gases are non-reactive, and the metal filament is also coated with a halide which means that it does not react with the gas - and can operate at a higher temperature for the same voltage. A process called the "halogen cycle" also lengthens the filament life by returning evaporated tungsten from the bulb's inner surface to the filament via the halide gas.
Q How much lead is there in lead crystal - and how is it that we can see through it?
A The amount of lead depends on the type of crystal: English lead crystal contains more than 30 per cent. But the lead is in the form of lead oxide, not the pure metal, just as ordinary glass is made from silica - an oxide of silicon, an opaque metal in its pure form. The lead oxide forms a random structure in the glass - and, like the silica, it lets light through.
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