普特英语听力练习材料：致命的氯

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Killer Chlorine

If you’ve ever seen movies about World War I, you know that the soldiers in the trenches often wore bulbous gas masks that made them look like human insects. The masks were necessary to protect them from chemical weapons such as chlorine gas, a noxious substance that could almost instantly suffocate unprotected victims.

Pool Pests
If chlorine is so dangerous, why the do we dump it in swimming pools?

As you probably already know, chlorine is used to disinfect swimming pools. It also gives most pools their distinctive odor. Chlorine comes in pellet, liquid, and gas forms, and when it reacts with pool water it immediately breaks down into different chemicals, including hypochlorous acid and hypochlorite ion. Both chemicals attack microorganisms in the water by breaking through their cell walls and scrambling the inner workings of the cells, rendering them harmless.

Parts Per Million
So why don’t these killer chemicals destroy human cells as well? They would, if not for the very small amounts of chlorine needed to cleanse the typical pool. For example, the concentration of chlorine used to sanitize an Olympic-size pool is three parts per million, meaning that for every three molecules of chlorine there are one million water molecules. Such low concentrations may cause slight irritation to the eyes and skin, but there’s not enough chlorine to cause any real harm to swimmers.

Chlorine is not the only chemical used to keep pools clean—bromine and ozone are becoming increasingly popular in pool maintenance facilities. But chlorine is still the pool cleaner’s mainstay.
致命的氯
如果你看过关于一战的电影的话，你肯定知道战壕内的士兵们通常会配戴着球茎形状的防毒面罩，看起来很像人形昆虫。这种面具可以保护士兵不被化学武器攻击，如氯气----一种可以快速使人窒息的有毒气体。

水池里的有害微生物。
如果氯算危险物质的话，那么为什么我们要把它倒进游泳池里呢？
你可能已经知道，氯可以用于消毒游泳池。同时，大多数加氯的游泳池也会有一种独特的味道。氯可以以固、液、气三种形式存在。氯遇到水会立刻分解成不同的化学物质，如

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