酥脆的饼干在空气中放置一段时间会变软是什么原因?

解题思路：空气中各成分的体积分数分别是：氮气大约占78%、氧气大约占21%、稀有气体大约占0.94%、二氧化碳大约占0.03%、水蒸气和其它气体和杂质大约占0.03%；酥脆的饼干在打开密封包装敞口放置一段时间后会变软，说明空气中含有水蒸气；举例说明：从冰箱中取出干的玻璃杯，一会儿，表面出现水雾．

氮气大约占78%、氧气大约占21%、稀有气体大约占0.94%、二氧化碳大约占0.03%、水蒸气和其它气体和杂质大约占0.03%；酥脆的饼干在打开密封包装敞口放置一段时间后会变软，说明空气中含有水蒸气；举例说明：从冰箱中取出干的玻璃杯，一会儿，表面出现水雾，故答案为：水蒸气；从冰箱中取出干的玻璃杯，一会儿，表面出现水雾；

点评：
本题考点： 空气的成分及各成分的体积分数．

考点点评： 本考点考查了空气中各种气体的含量，同学们要加强记忆有关的知识点，在理解的基础上加以应用，本考点基础性比较强，主要出现在选择题和填空题中．

有水蒸气.
因为酥脆的饼干水分少,所以遇水会变软.

1,氧气 2.水
燃烧需要氧气,变软了说明食物潮了,所以有水

说明空气中含有 水蒸气

C

D

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Osmanthus tea cake with a small but fine, thin and crisp, crunchy and sweet, fragrant and beautiful and so on
Lushan mist tea, with fragrant Xin, Wei Hou, color Tsui, Tang Qing of the characteristics of

解题思路：A、根据氧气的检验方法考虑；
B、酥脆的饼干在空气中变软是因为空气中含水蒸气；
C、根据氧气的化学性质分析判断；
D、根据氧气、氮气的沸点不同分析．

A、常用带火星的木条来检验纯O2，空气中的氧气不能使带火星的木条复燃，故A错误；B、空气中敞口放置饼干变软是因为空气中含水蒸气是正确的，正确；C、氧气的化学性质比较活泼，不具有可燃性，不属于可燃物，故C错误...

点评：
本题考点： 氧气的化学性质；空气的成分及各成分的体积分数；氧气的工业制法．

考点点评： 本题考查了空气中成分的相关性质，掌握其性质才能正确解答．

因为空气中存在水蒸气
酥脆的食品吸收水蒸气,所以变软
氧气一般是使食品氧化变质,而不是变软

（1）根据取少量样品，向其中加入稀盐酸，产生大量气泡并且取适量样品放人试管加热，产生可以使澄清石灰水变浑浊的气体这两个信息知使面包、饼干形成小孔的气体应该是二氧化碳，你猜测产生这种气体的原因有碳酸氢钠与酸反应和碳酸氢钠受热分解；
（2）碳酸氢钠能使食品膨松的主要原因是碳酸氢钠受热分解产生CO ₂ 气体；
（3）碳酸氢钠的物理性质是白色粉末状固体，能（易）溶于水；
（4）碳酸氢钠可与盐酸反应，所以它可用于治疗胃酸过多，受热分解能产生二氧化碳，所以可用作干粉灭火器的药品．
故答案为：
（1）碱，CO ₂ （或二氧化碳），碳酸氢钠与酸反应和碳酸氢钠受热分解；
（2）碳酸氢钠受热分解产生CO ₂ 气体
（3）白色粉末状的固体，能（易）溶于水
（4）作干粉灭火器中的药品等

解题思路：空气中各成分的体积分数分别是：氮气大约占78%、氧气大约占21%、稀有气体大约占0.94%、二氧化碳大约占0.03%、水蒸气和其它气体和杂质大约占0.03%；酥脆的饼干在打开密封包装敞口放置一段时间后会变软，说明空气中含有水蒸气；举例说明：从冰箱中取出干的玻璃杯，一会儿，表面出现水雾．

氮气大约占78%、氧气大约占21%、稀有气体大约占0.94%、二氧化碳大约占0.03%、水蒸气和其它气体和杂质大约占0.03%；酥脆的饼干在打开密封包装敞口放置一段时间后会变软，说明空气中含有水蒸气；举例说明：从冰箱中取出干的玻璃杯，一会儿，表面出现水雾，故答案为：水蒸气；从冰箱中取出干的玻璃杯，一会儿，表面出现水雾；

点评：
本题考点： 空气的成分及各成分的体积分数．

考点点评： 本考点考查了空气中各种气体的含量，同学们要加强记忆有关的知识点，在理解的基础上加以应用，本考点基础性比较强，主要出现在选择题和填空题中．

解题思路：运用水蒸气会使食品变软解答．

水蒸气会使食品变软，酥脆的饼干放置在空气会变软．
故选D．

点评：
本题考点： 空气的成分及各成分的体积分数．

考点点评： 要熟练掌握空气的成分及其体积分数即可解答这类问题．

水分

A、酥脆的饼干放置在空气中一段时间会吸水变软，说明空气中含有水蒸气；故A正确；
B、夏天吃剩下的食物要及时放入冰箱，因为食物缓慢氧化后会变质；故B正确；
C、填充气球时，常用氢气代替氦气，因为氢气密度比氦气小；故C错误；
D、人们经常饮用矿泉水，因为矿泉水含有多种对人体有益的矿物质．故D正确；
故选C．

h2o co2

酸：sour
甜：sweet
苦：bitter
辣：hot ,spicy
咸：salty
油腻：oily

1,O2
2,h2o
3,CO2

水
冬天窗户上出现露珠

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