![]() ![]() Katie: OK, so when acids meet alkalis, the chemical reaction produces water and a salt of some kind, Miss? Some salts are odd colours and even poisonous. Nothing mixed in a science lab should be eaten. Katie: Salt… As in the salt that you put on your chips? And the reaction always produces two things - water and salt. ![]() So, what happens when acids and alkalis meet? Is there a reaction? And the alkalis are on the right of the pH scale, ranging from 8 to 14, where 8 is the least alkaline and 14 is the most alkaline. OK, so we've got acids on the left there and they have a pH range from 0 to 6, where 0 is the most acidic and 6 is the least acidic. Miss Fong: Well, first up let's remind ourselves about the properties of acids and alkalis. Mason et al.Miss Fong: Today we're talking about chemical reactions and, in particular, what happens when acids and alkalis mix with each other. A water droplet’s dive can be seen for comparison at the bottom. SHARP BLAST Just before it ignites in water, an alkali metal (Na/K, an alloy of sodium and potassium) loses electrons and shoots spikes from its surface, which can be seen at the top of this slow-motion footage. ![]() It’s a perfect example of taking a long-held assumption, questioning it, and coming up with a better understanding, Sachleben says. Like Jungwirth, Sachleben hopes that the new finding makes its way into chemistry classrooms in addition to laboratories. “It makes sense,” says organic chemist Rick Sachleben of Momenta Pharmaceuticals in Cambridge, Mass. As the metal continually unloads electrons, the heat needed to ignite the hydrogen gas builds up before steam can stifle the explosion.Ĭomputer simulations of the reaction backed up the chemists’ explanation. These newly exposed atoms then lose electrons, creating more positive atoms that form spikes upon spikes. This creates gaps in the surface, exposing underlying atoms to the water. Once electrons abandon the surface of the metal, they leave behind positively charged atoms, which repel each other and create spikes as they leap away. The spikes that shoot from the surface are pieces of positively charged metal, the chemists say. This detail gave the chemists the answer they were looking for. But just before that - at 0.35 milliseconds - something weird happened: the metal’s smooth surface became spiky, like an angry hedgehog. At 0.5 milliseconds after the metal touched the water, the explosion was in full swing. With a camera rolling, the researchers held a 100 milligram glob of the alloy a meter above a pool of water, then let it drop. To catch the fiery events, the researchers started with a cheap camera that takes 500 images per second before turning to a camera capable of snapping 30,000 images per second. A liquid at room temperature, this blend of alkali metals offers a predictable blast, scene after scene - unlike pure alkali metals, which can produce inexplicably varied explosions. Mason and Jungwirth decided to film the explosion of an alloy of sodium and potassium. The chemical reaction, Mason reasoned, should smother itself. A blanket of vapor on the metal would block fleeing electrons and halt the heat-up. But Mason theorized that before enough heat could build up to ignite the hydrogen gas, the extra warmth would also create steam from the surrounding water. In order for electrons to jump ship, the metal and water have to be in direct contact. “I’ve been doing this sodium explosion for years,” he told Jungwirth, “and I still don’t understand how it works.” In his spare time, he set off sodium explosions on his balcony over and over again. Philip Mason, a chemist working with Jungwirth, wasn’t satisfied with that explanation. For comparison, images on the right show what happens when a water droplet plunges into water. READY FOR ITS CLOSE-UP High-speed images taken just before an alkali metal explodes in water (left column) show spikes forming on the metal’s surface 0.35 milliseconds after it hits the surface. The gas then ignites in that newly-generated heat. Once afloat, these liberated electrons attack water molecules, breaking off hydrogen atoms to form explosive hydrogen gas. In a splash of water, the metal jettisons electrons, which generates heat. Alkali metals, a group of elements including potassium and sodium, are highly reactive. In textbooks, chemists describe the reaction in simple terms.
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