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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. THE LAST WORD Beach circles I took these photos at Shelly Beach in Queensland. Can any reader explain how these almost symmetrical shapes were formed? n These small mud mounds, typically found in tidal flats in colder regions, are sometimes called monroes. They generally occur in groups and have a conical shape, occasionally with a “nipple” at the top. Up to 20 centimetres tall, they usually develop under ice sheets that build up on beaches in autumn. The sheets put pressure on the underlying mud, expelling air and water trapped beneath soft layers close to the surface. Icy conditions are not common in Queensland, however, so the examples shown were probably formed by “dewatering”. This is a much simpler process involving mud at shallow depths with an unusually high water content. Under the weight of the overlying sediment, the mud sporadically spews water up to the surface. The dewatering can be exacerbated by the presence of shallow gas, which can create pressure in the sediment. The heightened pressure can even form mud volcanoes, which may be more than 10 metres tall. The most notorious example is probably Lusi in Indonesia (New Scientist, 3 February 2007, p 8). Such volcanoes are common in uncompacted muddy sediments in South-east Asia, where rotting vegetation may produce gas. Monroes are also seen in eastern Canada, as well as preserved in ancient rocks. A geologist with a fertile imagination named them after the actress Marilyn Monroe (Journal of Sedimentary Research, vol 43, p 848), making them the only geological structure named after a celebrity (unless someone knows better). Jon Noad Calgary, Alberta, Canada Spray it again Rain comes from evaporated water, much of which comes from the sea. So why isn’t rain salty? n Recall the classic school science demonstration. Saltwater is heated or left to stand. The water then boils or evaporates, leaving salt crystals in the container. Thomas Parker Barry, Vale of Glamorgan, UK n A liquid boils when its vapour pressure – the pressure exerted by molecules escaping the liquid into gaseous form – is equal to atmospheric pressure. Pure water boils at 100 °C, molten sodium chloride at 1413 °C. Evaporation can happen well below boiling point because molecules are constantly escaping from the surface of liquids (and even solids). The reason water evaporates more quickly on a hot day than on a cold day is because its molecules move faster at higher temperatures, so more escape from the surface. In other words, vapour pressure rises with temperature. Spill some salt on the table and it does not evaporate, however. That’s because its vapour pressure is negligible at room temperature. So no salt will vaporise from the sea, and rain is therefore not salty. Vapour pressure is affected by the strength of attraction between molecules in a liquid. The attraction between water molecules is due to hydrogen bonding – involving a hydrogen atom in one molecule being attracted to the oxygen atom in another. This effect is relatively weak, so at a given temperature, water has a high vapour pressure compared with salt (sodium chloride), in which there is a strong electrostatic attraction between sodium and chloride ions. That said, water molecules attract each other more strongly than, say, methane molecules do – hence, liquid methane will boil at a chilly -164 °C. The sodium and chloride ions in seawater do attract water molecules, making it harder for them to escape and reducing vapour pressure. So salty water boils at a slightly higher temperature than pure water, the size of the effect depending on the salt concentration. Peter Borrows Amersham, Buckinghamshire, UK This week’s question FAT CHECK I understand that men lose weight more easily than women do. What are the reasons for this? Paula Melcroft Leicester, UK “They are the only geological structure named after a celebrity – Marilyn Monroe” Last words past and present, plus questions, at last-word.com The new book out now: packed full of wit, knowledge and extraordinary discovery Available from booksellers and at newscientist.com/dolphin Will we ever speak dolphin?
