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Footloose
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THE LAST WORD
This week’s questions
Furless midriff My Year 7 class left brine in evaporating dishes next to a sunny window. Several pupils ended up with dishes like the one pictured. We could explain the salt crystals at the bottom of the dish, but why is there no deposit around the middle of the dish when the rim is thickly furred with salt?
n There is actually a thin layer of salt deposited in the mid-band of the dish. The real question is why a much thicker layer of crystals formed further away from the liquid. This is because of the combined effects of capillary suction and evaporation. Around the edge of the liquid, surface tension causes the solution to rise up the wall slightly. This layer is thin and has a high surface area to volume ratio, so experiences a high rate of evaporation that causes salt crystals to precipitate around
“Crystals make the surface rough and capillary forces suck solution further up the walls” the edge. These make the surface rougher, enabling capillary forces to suck solution further up the dish’s walls. The further it moves from the main reservoir, the more exposed it is to evaporation and the less opportunity there is for the high salt concentration to diffuse back into the bulk. The solution thus becomes supersaturated more rapidly at the rim, and the process of
The writers of answers that are published in the magazine will receive a cheque for £25 (or US$ equivalent). Answers should be concise. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and an email address if you have one. New Scientist retains total editorial control over the published content. Reed Business Information Ltd reserves all rights to reuse all question and answer material that has been
Footloose
I sometimes notice that one of my shoes is loose, although the other is fine. When I tighten the laces on the loose shoe, the other shoe then feels loose. Is there an explanation for this peculiar effect? Andrew Brittain Sutton, Surrey, UK Clouding the issue
precipitation accelerates. Eventually, the liquid is sucked so far from the bulk that the rate of evaporation equals the maximum capillary flow rate, at which point most of the salt crystallises out and forms the large deposits you can see. In some situations with very low evaporation rates, I have seen salt deposits continue to grow over the rim, down the outside of the dish and onto the bench. Simon Iveson Callaghan, New South Wales, Australia n If some of the evaporation dishes are slightly porous, the dissolved salt could be migrating through the clay. The water evaporates preferentially at the highest point, leaving behind the precipitated salt on the rim. The middle is too dilute
submitted by readers in any medium or in any format and at any time in the future. Send questions and answers to The Last Word, New Scientist, 110 High Holborn, London WC1V 6EU, UK, by email to [email protected] or visit www.newscientist.com/topic/lastword (please include a postal address in order to receive payment for answers). Unanswered questions can also be found at this URL.
and has too low a concentration of solution to form crystals. As a potter, I see this when I reclaim clay slop in porous bowls, in which dissolved salts in the clay precipitate at the top lip only. Egyptians used salt migration to make self-glazing pots. Salts would migrate to the outside of the clay, and would then combine with the silica in the clay to form a glaze when firing. Today, potters add soluble forms of sodium to trap carbon in Shino glazes to achieve a charcoalgrey spotting pattern. The sodium migrates with the water from the glaze to the outside and top of the pot. The sodium compounds melt early in firing and, if there is a lot of soot in the kiln chamber at that temperature, the soot will get trapped in the glaze, appearing as black areas on the finished pot. Jennifer Assinck Newmarket, Ontario, Canada
Are there any wavelengths at which the sun still casts a shadow when the sky is full of clouds? Could I make a sundial that would work on a cloudy day? Stephen Parish London, UK Whorl class
Do our fingerprints change? When I was a child, I was taught that they never would. However, when I provided my biometrics for a visa application, it was pointed out that my fingerprints did not match those that were taken five years earlier. Is this possible, or is it more likely to be an error with the records or software? Chisato Kobayashi North Yorkshire, UK Sound barrier
Is sound sensitive to the composition of the atmosphere through which it propagates? John O’Hara Mount Waverley, Victoria, Australia
Question Everything The latest book of science questions: unpredictable and entertaining. Expect the unexpected Available from booksellers and at newscientist.com/questioneverything
THE LAST WORD
This week’s questions
Furless midriff My Year 7 class left brine in evaporating dishes next to a sunny window. Several pupils ended up with dishes like the one pictured. We could explain the salt crystals at the bottom of the dish, but why is there no deposit around the middle of the dish when the rim is thickly furred with salt?
n There is actually a thin layer of salt deposited in the mid-band of the dish. The real question is why a much thicker layer of crystals formed further away from the liquid. This is because of the combined effects of capillary suction and evaporation. Around the edge of the liquid, surface tension causes the solution to rise up the wall slightly. This layer is thin and has a high surface area to volume ratio, so experiences a high rate of evaporation that causes salt crystals to precipitate around
“Crystals make the surface rough and capillary forces suck solution further up the walls” the edge. These make the surface rougher, enabling capillary forces to suck solution further up the dish’s walls. The further it moves from the main reservoir, the more exposed it is to evaporation and the less opportunity there is for the high salt concentration to diffuse back into the bulk. The solution thus becomes supersaturated more rapidly at the rim, and the process of
The writers of answers that are published in the magazine will receive a cheque for £25 (or US$ equivalent). Answers should be concise. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and an email address if you have one. New Scientist retains total editorial control over the published content. Reed Business Information Ltd reserves all rights to reuse all question and answer material that has been
Footloose
I sometimes notice that one of my shoes is loose, although the other is fine. When I tighten the laces on the loose shoe, the other shoe then feels loose. Is there an explanation for this peculiar effect? Andrew Brittain Sutton, Surrey, UK Clouding the issue
precipitation accelerates. Eventually, the liquid is sucked so far from the bulk that the rate of evaporation equals the maximum capillary flow rate, at which point most of the salt crystallises out and forms the large deposits you can see. In some situations with very low evaporation rates, I have seen salt deposits continue to grow over the rim, down the outside of the dish and onto the bench. Simon Iveson Callaghan, New South Wales, Australia n If some of the evaporation dishes are slightly porous, the dissolved salt could be migrating through the clay. The water evaporates preferentially at the highest point, leaving behind the precipitated salt on the rim. The middle is too dilute
submitted by readers in any medium or in any format and at any time in the future. Send questions and answers to The Last Word, New Scientist, 110 High Holborn, London WC1V 6EU, UK, by email to [email protected] or visit www.newscientist.com/topic/lastword (please include a postal address in order to receive payment for answers). Unanswered questions can also be found at this URL.
and has too low a concentration of solution to form crystals. As a potter, I see this when I reclaim clay slop in porous bowls, in which dissolved salts in the clay precipitate at the top lip only. Egyptians used salt migration to make self-glazing pots. Salts would migrate to the outside of the clay, and would then combine with the silica in the clay to form a glaze when firing. Today, potters add soluble forms of sodium to trap carbon in Shino glazes to achieve a charcoalgrey spotting pattern. The sodium migrates with the water from the glaze to the outside and top of the pot. The sodium compounds melt early in firing and, if there is a lot of soot in the kiln chamber at that temperature, the soot will get trapped in the glaze, appearing as black areas on the finished pot. Jennifer Assinck Newmarket, Ontario, Canada
Are there any wavelengths at which the sun still casts a shadow when the sky is full of clouds? Could I make a sundial that would work on a cloudy day? Stephen Parish London, UK Whorl class
Do our fingerprints change? When I was a child, I was taught that they never would. However, when I provided my biometrics for a visa application, it was pointed out that my fingerprints did not match those that were taken five years earlier. Is this possible, or is it more likely to be an error with the records or software? Chisato Kobayashi North Yorkshire, UK Sound barrier
Is sound sensitive to the composition of the atmosphere through which it propagates? John O’Hara Mount Waverley, Victoria, Australia
Question Everything The latest book of science questions: unpredictable and entertaining. Expect the unexpected Available from booksellers and at newscientist.com/questioneverything