英语听力文摘 为什么盐会结块？
为什么盐会结块?
You probably noticed that sometimes the salt gets clumpy. If it’s really humid outside–there may even be condensation on top of the salt shaker.
The reason this happens is that salt is hygroscopic. Hygroscopic means that salt absorbs water vapor from the surrounding air.
Eventually, the salt attracts enough water vapor that the gas changes into liquid, and the salt partially dissolves and starts to clump together. To combat this tendency, manufacturers put additives in salt that keep it from liquefying.
Sugar is hygroscopic, too. This is partly the reason cake doesn’t go stale as quickly as bread. You see, when baked goods such as bread or cake lose water, the starch molecules in them undergo a chemical change that make them hard and stale.
But because cake contains more sugar than bread, and sugar is hygroscopic, it retains water better and stays fresh a little longer.
This also explains why saltines and cookies get soft when they go stale instead of hard–because they’re absorbing water from the air around them.
生活中，你可能发现盐有时会结块。如果室内很潮湿的话，在盐瓶上甚至会产生液滴。
产生这种现象的原因是：盐吸湿。吸湿的意思就是盐从周围空气中吸收水蒸气。
最终，盐吸收了足够多的水蒸气后气体就会变成液体，盐就会局部溶解并结成一团。为了避免这种情况，生产商在盐中加了添加剂防止其溶解。
糖也吸湿。这也是蛋糕比面包更难保鲜的部分原因。要知道，焙烤食品像面包或者蛋糕失去水分后，其淀粉分子就会发生一种化学变化使起变得坚硬，不新鲜。
但是因为蛋糕中含有的糖分比面包多，糖又吸湿，蛋糕就保持了充足的水分，所以保鲜的时间长些。
盐薄饼干和饼干不新鲜了就会变软儿不是变硬。因为它们吸收了周围空气里的水分。
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