Questions and answers provided by Science Line's Dial-a-Scientist on 0345 600444

Q These days bread isn't just made of flour and water. What else is added?

A Bakers often add their own mix of wheat and other grains to give the bread texture and flavour. Some add vitamins. In 1986 the manufacturers of the flour and wheat mixes also added a compound called creta preparate to some vitamin-enhanced breads, as a source of calcium and as a filler.

Q How do colour photocopiers work?

A The image is scanned four times - once for each of the three primary printing colours (cyan, magenta and yellow) and once for black. Using charge-coupled devices, or CCDs, which are electrical sensors that react very precisely to light, the copier assesses the brightness of the image for each colour. The paper then rolls around the drum four times as the correct amount of each coloured ink is applied.

Q What is tensile strength, and why is it important to know the tensile strength of materials?

A Tensile strength is the force needed to stretch a material until it breaks. Take bridge building: suspension bridges are built by hanging the bridge from a steel "rope" under tension. When designing the bridge it is important to know under what force the steel breaks at, to be sure the bridge will hold up.

Q Cows eat green grass. So why is their milk white?

A The colour of the food animals eat doesn't really determine the colour of what comes out - and this is especially true in a cow, where the grass is completely broken down as it passes through its four stomachs, and no longer has a colour. So the question becomes, why is milk white? This is because it's an emulsion - one liquid completely suspended in another. Milk is a fine dispersion of calcium caseinate suspended in liquid.

Q What are parachutes made of, and how long do they last?

A Silk hasn't been used in the manufacture of parachutes for 50 years. Nowadays they use rip-stop nylon, which is made of lots of tiny squares which prevent a rip from propagating along its length. A parachute will last about 10-12 years, or 100 jumps (whichever comes first) before it should be replaced. Of course, it can be damaged earlier - usually on landing. A standard person-carrying parachute requires about 100 square metres of material.

Q Last week's answer about halogen bulbs was wrong; how do they really work?

A That's true - the error was introduced during editing. Here's the right answer. A conventional light bulb, filled with an inert gas such as argon, is limited to a certain temperature: the tungsten filament gradually evaporates and condenses on the cooler glass envelope, and a very bright bulb would wear out too quickly. A halogen bulb enables the filament to run at a higher temperature, hence is brighter. Halogens - fluorine, chlorine, bromine, iodine and astatine - are highly reactive. A tungsten halide (usually tungsten iodide) is the key: it is stable at low temperature, but unstable when very hot. When the tungsten evaporates from the hot filament it forms a vapour which cools as it nears the outer glass, and reacts with the iodine to form tungsten iodide. This itself is stable until it comes into contact with the very hot filament, where it dissociates into its elements. Tungsten is redeposited on the filament, releasing iodine vapour to repeat the cycle.

You can also visit the technoquest World Wide Web site at http:// Questions for this column can be sent by e-mail to

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