1
Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format. New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address. Our latest collection - serious enquiry, brilliant insight and the hilariously unexpected Available from booksellers and at www.newscientist.com/ polarbears Do Polar Bears Get Lonely? THE LAST WORD No solids I made a sauce for lamb from blueberries and cassis, but made too much so put some in the freezer. It wouldn’t set solid and spent a month being malleable at -7 °C. It tasted OK when we used it later. Why wouldn’t it freeze solid? n Don’t know the answer, but have you got the recipe? Alice Collins Washington DC, US n The blueberry and cassis sauce probably contained a lot of sugar (natural and added). Dissolved substances lower the freezing point of a liquid. For instance, a 42 per cent sucrose solution – the highest concentration for which I have data – doesn’t start to freeze until the temperature gets down to -4.45 °C. And the zero point of the Fahrenheit temperature scale was, according to some accounts, determined by the temperature at which a mixture of ice and salt froze. This is close to -18 °C. This lowering of the freezing point aside, there is also the possibility that the sauce was so thick that ice could not form. As for why your sauce kept so well for a month despite not freezing, the low temperature would have slowed any decomposition, as it would whether a substance freezes or not, while the high sugar content has a preservative effect. Eric Kvaalen La Courneuve, France n There would have been a lot of alcohol (ethanol) and a lot of sugar (sucrose) in the sauce. Both are very effective at lowering the freezing point. In years past, ethanol was the basis of commercial antifreezes, for example, and I always put some methylated spirit (which in spite of its name is ethanol) in the windscreen washer of my car to prevent it freezing up. Guy Cox Electron Microscope Unit University of Sydney Australia n A temperature of -7 °C is not very cold for a freezer and not adequate for long-term food storage. I would reduce the temperature via the controls if possible, or replace the freezer. John Kinross School of Life Sciences Edinburgh Napier University, UK Ripple effect This glass seems to have no ripples in it when viewed from the side, but lots when viewed from above (see photos). Why is this? n The “ripples” are caused by multiple reflections as light from the base of the glass bounces between the inner and outer surface of the glass on its way up. At each reflection a little light escapes and enters the eye, producing the observed “ring” at that height. If you look down the outside of the glass from above, you will see ripples once again. Keith Thompson Bendigo, Victoria, Australia n The effect is caused by internal reflection, just as happens inside an optical fibre. To see a neat demonstration of this, cover the sides and one end of a glass tube with black plastic tape, then punch a hole about 1 millimetre across in the sealed end. Now look down the small hole and you will see concentric rings caused by internal reflections. If you look through the unsealed end you get some pretty interesting views too. I have used this effect as a little test when interviewing prospective physics students at Oxford. Mike Glazer Clarendon Laboratory University of Oxford, UK This week' s questions A DUSTING DOWN Shelves and window sills seem to collect dust evenly, but dust on floors seems to collect in clumps. Why is there a difference? Alison Denham Accrington, Lancashire, UK BLANK LOOK My digital TV has a menu containing a guide to several days’ worth of forthcoming programmes on each channel. When I first bring up the guide, there are lots of blanks in the schedule. Over the next few minutes these gaps are gradually filled, seemingly at random. How does this happen and what determines the order in which the holes in the schedule are filled? Alex Andrews Newton Abbott, Devon, UK ª Dissolved solids, especially salts or sugars such as sucrose, lower the freezing point of a solution” Last words past and present, plus questions, at www.last-word.com
