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THE LAST WORD Mix and match Is it possible for two (or more) ingredients, when mixed, to weigh more than they do separately? If so, what and why? When I make porridge in the morning it certainly seems as if this is the case. If so, have I stumbled on a potential dieting gold mine? n The simple answer is no. One of the fundamental laws of physics is the conservation of mass and energy. Normal chemical processes do nothing to alter matter, they just rearrange atoms in different ways. Nuclear reactions can convert matter to energy, but as long as this energy is not lost from the system, the system will still weigh the same. There is certainly no way for a system to gain mass unless there is a flow of energy or matter into it from outside. For instance, a rusting nail weighs more than the original because it has reacted with oxygen from the air. In the case of porridge, the questioner is seeing a swelling of the oats as they absorb water. The contents of the pot may even swell to exceed their original volume, due to the expansion of gases trapped inside the oat grains during the cooking process. However, the mass of the porridge cannot exceed the original mass of the oats and water before they were mixed together. If anything, there will be a loss of mass due to evaporation of some of the water. Be careful not to confuse volume with mass. Simon Iveson Chemical Engineering Discipline The University of Newcastle Callaghan, New South Wales, Australia n Theoretically, because E=mc 2 , the mass would increase very slightly if there is an increase in energy. But usually, mixing two substances decreases the energy, otherwise they wouldn’t mix – like oil and water do not. There is one explanation for an apparent increase in the weight, though. When you weigh something, you are actually measuring its true weight minus the buoyancy due to the volume of air it displaces. And weight is different from mass. So if mixing two ingredients results in a smaller volume, then there will be less buoyancy and it will seem heavier. For instance, mixing soda (sodium carbonate) and water will show this effect, but it’s a very small one. I suspect that the enquirer is fooled by the density. The mixture of water and, say, oats is denser than the bulk density before mixing because of all the air that was previously between the dry oats, so it looks heavier even though it’s not. Eric Kvaalen Les Essarts-le-Roi, France n It is possible for two substances to weigh more after they are mixed than they did separately. Alcohol and water love to mix and form hydrogen bonds. A 50:50 mixture of ethanol and water takes up about 96 per cent of the volume of the separate liquids. Thus a litre of alcohol mixed with a litre of water contracts by about 80 millilitres and thereby displaces 80 millilitres less air than the separate liquids do. And 80 millilitres of air weighs about 0.1 grams. So the mixed liquids are heavier by 0.1 grams because less displaced air means less buoyancy from the weight of displaced air. David Emanuel Tulsa, Oklahoma, US This week’s questions CRYSTAL CRISIS This tub of plasticine (see photo) has been sitting in a box at the back of my study. I don’t think it has been subjected to any particularly extreme environmental events, yet it looks like it is covered in white crystals. Any ideas how this has happened? Matt Robinson By email, no address supplied IN A SPIN I have been watching TV coverage of international cricket matches this summer. The channel uses a new device that measures how quickly bowlers spin the ball. The results seem amazing. Even slow spinners manage more than 1000 revolutions per minute, and some spin the ball faster than 3000 rpm, which equates to more than 50 revolutions per second. Is this correct? And if it is, how do they spin it so quickly? David Mushens London, UK “There is no way for a system to gain mass unless there is a flow of energy or matter into it from outside” Last words past and present at newscientist.com/topic/lastword Don’t miss the winning answers to last month’s question on page 25 Answer our next question and you could win £100 Turn to page 25 to see the latest question or visit www.newscientist.com/topic/energy, where you will also ﬁnd the terms and conditions THE LAST WORD ON ENERGY sponsored by The writers of answers 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. Include a daytime telephone number and email address if you have one. We are pleased to acknowledge financial support from Statoil in producing The Last Word. New Scientist retains total editorial control over the content. 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