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The spring is sprung
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THE SPRING IS SPRUNG The mineral water in my local shop has a label telling me it is from a 3000-year-old source, yet there is still a “best before end” date on it approximately two years in the future. If the water has been in its aquifer for 3000 years, why should it go off in a sealed bottle?
Mineral water has passed through layers of rock that have different effects on the water. Some minerals dissolve in the water, supposedly improving both its taste and health-giving properties, hence the demand for it. The small pore size of the rocks the water passes through acts as a filtration system, improving the purity of the water by removing larger molecules such as biological contaminants. As soon as the water emerges it is vulnerable to contamination again. The “best before” dates are based on the amount of time the bottler believes the water will remain without measurable levels of contamination due to the lack of completely sterile conditions in their bottling plants. If the water is stored in a plastic bottle the date might also relate to contamination from the constituents of the plastic, which may change the taste of the water. John Thompson London, UK The reason for the best before date on bottled spring water is not the contents but the container. Most mineral or spring water seems to be Questions and answers should be kept as concise as possible. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and email address if you have one. Questions should be restricted to scientific enquiries about everyday phenomena. The writers of all published answers will receive a cheque for £25 (or the US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by
packed in polyethylene terephthalate (PET) bottles. During the manufacture of the bottles traces of catalyst or plasticiser, which may include antimony, remain in the plastic and are leached out into the water over time. To avoid this glass bottles, which have stood the test of time, are preferable. Rob Davids St Ives, New South Wales, Australia “Pure” water does not decompose or suddenly go off. However, manufacturers of foods and beverages have to give “best before” dates to cover their backs. If the bottle sat around for long enough the plastic might decompose or the seal might degrade, allowing bacteria to enter and contaminate it. As for the water being 3000 years old, most of the water we drink has probably been in existence as water molecules for millions of years. What is important is the purity of the water, not its age. 3000 years in an underground aquifer may have filtered out all the organic matter, but readers in any medium or format. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK (fax +44 (0) 20 7611 1280), by email to [email protected] or visit www. newscientist.com/lastword.ns (please include a postal address in order to receive payment for answers). If you would like a list of all unanswered questions please send an SAE to LWQlist at the above address.
Memorable answer? The Last Word and New Scientist have teamed up with Crucial Technology (www.crucial.com/uk) and will be awarding each successful author a 512MB Gizmo! overdrive.
there might still be harmful dissolved chemicals such as arsenic. Simon Iveson Mayfield, New South Wales, Australia
FROZEN IMAGES In January I dropped some bricks into my pond, which is a metre deep. In March the pond froze over and an image of the bricks appeared like a hologram in the ice (see Photo). What caused this?
Pond water contains a certain amount of dissolved gas, including oxygen. Because of the physical properties of water, the colder it is the less gas per unit volume it can hold. Water is at its densest at a temperature of about 4 °C. As the water temperature in the pond drops, cooled by the colder air above, the surface water sinks to the bottom by convection. Once the whole body is at 4 °C this convection stops because further cooling makes the surface water less dense. The surface
starts to freeze and the coldest water begins to release its dissolved gas. Some of this would bubble upwards, but much more would diffuse down, still remaining in solution, until eventually the water surrounding the bricks becomes supersaturated. The rough surface of the bricks, particularly around the edges and corners, provides nucleation sites for dissolved gases. Gas molecules collect preferentially around the edges of the bricks, eventually producing bubbles. As these reach a critical size they break away and float straight upwards in the still water. Because there is a layer of ice on the surface, the bubbles become trapped and frozen into it. As the ice layer thickens and bubbles continue to rise from the brick, the 3D shape develops. The rate of bubbling was probably very slow, as was the rate of freezing, so the very detailed effect was able to form. David Jackson Liverpool, UK
THIS WEEK’S QUESTIONS Water bomb How big is the biggest possible raindrop? Michael Leonard Stoke on Trent, Staffordshire, UK Landlubber Where on our planet is the furthest point from any sea? I’m hoping it is the middle of Asia somewhere because I’m travelling that way soon, and want to stand at that point in swimming trunks, snorkel and mask. Hugh Jones Slapton, Northamptonshire, UK
WHY DON’T PENGUINS’ FEET FREEZE? The latest collection from The Last Word, answering some of the world’s most baffling questions Available in bookstores and online at www.newscientist.com/lastword3.ns
Last Words past and present, plus a full list of unanswered questions, are available at www.newscientist.com
THE SPRING IS SPRUNG The mineral water in my local shop has a label telling me it is from a 3000-year-old source, yet there is still a “best before end” date on it approximately two years in the future. If the water has been in its aquifer for 3000 years, why should it go off in a sealed bottle?
Mineral water has passed through layers of rock that have different effects on the water. Some minerals dissolve in the water, supposedly improving both its taste and health-giving properties, hence the demand for it. The small pore size of the rocks the water passes through acts as a filtration system, improving the purity of the water by removing larger molecules such as biological contaminants. As soon as the water emerges it is vulnerable to contamination again. The “best before” dates are based on the amount of time the bottler believes the water will remain without measurable levels of contamination due to the lack of completely sterile conditions in their bottling plants. If the water is stored in a plastic bottle the date might also relate to contamination from the constituents of the plastic, which may change the taste of the water. John Thompson London, UK The reason for the best before date on bottled spring water is not the contents but the container. Most mineral or spring water seems to be Questions and answers should be kept as concise as possible. We reserve the right to edit items for clarity and style. Please include a daytime telephone number and email address if you have one. Questions should be restricted to scientific enquiries about everyday phenomena. The writers of all published answers will receive a cheque for £25 (or the US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by
packed in polyethylene terephthalate (PET) bottles. During the manufacture of the bottles traces of catalyst or plasticiser, which may include antimony, remain in the plastic and are leached out into the water over time. To avoid this glass bottles, which have stood the test of time, are preferable. Rob Davids St Ives, New South Wales, Australia “Pure” water does not decompose or suddenly go off. However, manufacturers of foods and beverages have to give “best before” dates to cover their backs. If the bottle sat around for long enough the plastic might decompose or the seal might degrade, allowing bacteria to enter and contaminate it. As for the water being 3000 years old, most of the water we drink has probably been in existence as water molecules for millions of years. What is important is the purity of the water, not its age. 3000 years in an underground aquifer may have filtered out all the organic matter, but readers in any medium or format. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK (fax +44 (0) 20 7611 1280), by email to [email protected] or visit www. newscientist.com/lastword.ns (please include a postal address in order to receive payment for answers). If you would like a list of all unanswered questions please send an SAE to LWQlist at the above address.
Memorable answer? The Last Word and New Scientist have teamed up with Crucial Technology (www.crucial.com/uk) and will be awarding each successful author a 512MB Gizmo! overdrive.
there might still be harmful dissolved chemicals such as arsenic. Simon Iveson Mayfield, New South Wales, Australia
FROZEN IMAGES In January I dropped some bricks into my pond, which is a metre deep. In March the pond froze over and an image of the bricks appeared like a hologram in the ice (see Photo). What caused this?
Pond water contains a certain amount of dissolved gas, including oxygen. Because of the physical properties of water, the colder it is the less gas per unit volume it can hold. Water is at its densest at a temperature of about 4 °C. As the water temperature in the pond drops, cooled by the colder air above, the surface water sinks to the bottom by convection. Once the whole body is at 4 °C this convection stops because further cooling makes the surface water less dense. The surface
starts to freeze and the coldest water begins to release its dissolved gas. Some of this would bubble upwards, but much more would diffuse down, still remaining in solution, until eventually the water surrounding the bricks becomes supersaturated. The rough surface of the bricks, particularly around the edges and corners, provides nucleation sites for dissolved gases. Gas molecules collect preferentially around the edges of the bricks, eventually producing bubbles. As these reach a critical size they break away and float straight upwards in the still water. Because there is a layer of ice on the surface, the bubbles become trapped and frozen into it. As the ice layer thickens and bubbles continue to rise from the brick, the 3D shape develops. The rate of bubbling was probably very slow, as was the rate of freezing, so the very detailed effect was able to form. David Jackson Liverpool, UK
THIS WEEK’S QUESTIONS Water bomb How big is the biggest possible raindrop? Michael Leonard Stoke on Trent, Staffordshire, UK Landlubber Where on our planet is the furthest point from any sea? I’m hoping it is the middle of Asia somewhere because I’m travelling that way soon, and want to stand at that point in swimming trunks, snorkel and mask. Hugh Jones Slapton, Northamptonshire, UK
WHY DON’T PENGUINS’ FEET FREEZE? The latest collection from The Last Word, answering some of the world’s most baffling questions Available in bookstores and online at www.newscientist.com/lastword3.ns