问题 问答题

鉴别物质时通常先用物理方法,后用化学方法.现需鉴别三包失去标签的白色固体粉末,它们可能是CaCO3、Na2CO3和Na2SO4,请按下表完成实验设计

  实验步骤 实验现象和结论
 步骤一:

(物理方法)

____________
步骤二:

(化学方法)

____________

答案

碳酸钙难溶于水,加水溶解后,可以首先将碳酸钙鉴别出来,碳酸钠遇到盐酸会产生二氧化碳气体,而硫酸钠不会与盐酸生成气体,故答案为:

  实验步骤 实验现象和结论
 步骤一:

(物理方法)

 分别取样,加入足量的水,振荡, 不溶解的是碳酸钙
步骤二:

(化学方法)

 分别向所得的溶液中滴加盐酸 有气泡产生的是碳酸钠,

无明显现象的是硫酸钠

多项选择题
阅读理解

阅读理解。

    Animals are natural resources that people have wasted all through our history.

    Animals have been killed for their fur and feathers, for food, for sport, and simply because they were in

the way. Thousands of kinds of animals have disappeared from the world forever. Hundreds more are on

the danger list today. About 170 kinds in the United States alone are considered in danger.

    Why should people care? Because we need animals. And because once they are gone, there will never

be any more.

     Animals are more than just beautiful and interesting. They are more than just a source of food. Every

animal has its place in the balance of nature (生态平衡). Destroying one kind of animal can create many

problems.

     For example, when farmers killed large numbers of hawks (老鹰), the farmers stores of corn and grain

were destroyed by rats and mice. Why? Because hawks eat rats and mice. With no hawks to keep down

their numbers, the rats and mice multiplied (繁殖) quickly.

     Luckily, some people are working to help save the animals. Some groups raise money to let people know

about the problem. And they try to get the governments to pass laws protecting animals in danger.

     Quite a few countries have passed laws. These laws forbid (禁止) the killing of any animals or plants

on the danger list. Slowly, the number of some animals in danger is growing.

1. Animals are important to us mainly because ____. [ ]

A. they give us a source of food

B. they are beautiful and lovely

C. they keep the balance of nature

D. they give us a lot of pleasure

2. What has happened to the animals on the earth? [ ]

A. Hundreds of kinds of animals have disappeared forever.

B. Many kinds of animals have died out.

C. About 170 kinds of animals have disappeared forever.

D. All kinds of animals are in danger.

3. Which of the following is NOT true? [ ]

A. People care much about animals because they need them.

B. Once a certain kind of animals is gone forever, there will never be any more.

C. Killing all rats and mice may cause some problems.

D. People must not kill any animals.

4. What can we infer from the fact that quite a few countries have passed laws protecting animals in danger? [ ]

A. Every person will know the importance of protecting wild animals.

B. Animals in danger will not be killed any more.

C. The number of some animals in danger will increase.

D. Animals in danger will be kept away from people.