自来水是用适量氯气杀菌消毒过的．不法商贩常用自来水冒充纯净水谋取暴利，可用化学试剂鉴别真伪，该试剂可以是（　　）

下列化石可以证明鸟类可能起源于古代爬行类的是（    ）

在标准状况下，将aLNH₃完全溶于水得到VmL氨水，溶液的密度为ρg·cm^－3，溶质的质量分数为ω，溶质的物质的量浓度为C mol/L。下列叙述中正确的是
①              ②
③上述溶液中再加入VmL水后，所得溶液的质量分数大于0.5ω
④上述溶液中再加入1．5VmL同浓度稀盐酸，充分反应后溶液中离子浓度大小关系为：
c（Cl^－） ＞ c（NH₄⁺） ＞ c（H⁺） ＞ c（OH^－）

分类、类比是学习化学常用的方法．

合成材料的使用打打的方便了人们的生活．请你以使用塑料的利与弊对下列论点进行分类．①塑料容易被制成不同的形状；②塑料的制造的成本低，而且耐用、防水；③有些塑料容易燃烧，燃烧时产生有害气体；④有些塑料制品使用不当会对人体健康造成危害；⑤塑料是由石油炼制的产品制成的，而石油资源是有限的；⑥回收利用废弃塑料时分类十分困难，而且经济上不合算；⑦塑料一般不导电、不导热，是良好的绝缘体；⑧大部分塑料不会腐烂，也不会被细菌分解，容易造成白色污染．

      Everyone likes living in a clean and comfortable environment. If the envionment(环境) is bad, it will affect（影响）our bodies, and make us not feel well. Sometimes we may be terribly ill. At that time we don't want to work, and we have to stay in bed and rest at home. So the envrionment is very important to us.
      It's germ that makes us ill. There are germs everywhere, They are very small and you can't find them with your own eyes, but you can see them with a microscope（显微镜）. They are very small and there may be hundreds of them on a very small thing, Germs can always be found in the dirty water. When we look at dirty water under the microscope, we shall see them in it. Germs can also be found in air and dust（灰尘）. If you cut your finger, some of the dust from the floor may go into it, and you will have much pain in it. Sometimes the germs will go into all of your boby, and you will have pain everywhere.
     To keep us healthy, we should try to our best to make our environment become cleaner and tidier. This needs us to act together.