问题 填空题

在联欢会上,有“魔术师”称号的小谦同学表演了一组化学魔术:将三种无色溶液加入到A、B、C三只玻璃杯中,分别向其中加入碳酸钠溶液,可观察到下列现象:A杯变为“雪碧”(产生大量气泡);B杯变为“牛奶”(出现白色浑浊);C杯变为“红酒”(生成红色溶液).请你帮助他揭开魔术秘密:

(1)A杯中产生的气体是______.

(2)B杯中可能发生反应的化学方程式是______(任写一个).

(3)C杯中,在加入碳酸钠溶液之前的无色溶液是______,溶液的颜色发生变化的原因是______.

答案

(1)A杯变为“雪碧”(产生大量气泡),碳酸钠与酸反应产生二氧化碳气体,产生大量气泡,故气体为二氧化碳;

(2)碳酸钠与氢氧化钙反应产生白色沉淀,故B杯中发生反应的化学方程式可能为Ca(OH)2+Na2CO3═CaCO3↓+2NaOH

(3)碳酸钠溶液呈碱性,能使酚酞溶液变红.故C杯中无色溶液是酚酞.

故答案为:(1)二氧化碳(CO2).

(2)Ca(OH)2+Na2CO3═CaCO3↓+2NaOH.

(3)酚酞溶液,碳酸钠溶液显碱性,使酚酞溶液变红.

阅读理解

阅读理解。

     American people like to say "Thank you" when others help them or say something kind to them.People

of many countries do so, too.It is a very good habit.

     You should say "Thank you" when someone passes you the salt on the table, when someone walking

ahead of you keeps the door open for you, when someone says you have done your work well, or you have

bought a nice thing, or your city is very beautiful. "Thank you" is used not only between friends, but also

between parents and children, brothers and sisters.

     "Excuse me" is another short sentence they use.When you hear someone say so behind you, you know

that somebody wants to walk past you without touching you.It's not polite to break others when they are

talking. If you want to speak to one of them, say "Excuse me" first, and then begin talking.You should also

do so when you begin to cough or make any noise before others.

     Let's learn to say "Thank you" and "Excuse me".

1. You should say "Thank you" when _______.

A. you say something kind to others

B. you help others

C. someone helps you

D. you need others to help you

2. From the passage we know "Thank you" is _______.

A. widely used in the world

B. used more often than "Excuse me"

C. used only by Americans

D. used only between friends

3. You should say "Excuse me" if you want to _______.

A. cough

B. make some noise

C. go first

D. all of the above

4. When you are going to ask someone to tell you the way, you should say _______.

A. Thank you

B. That's very kind of you

C. Excuse me

D. I'm sorry

5. This passage mainly tells us the way _______.

A. to be happy

B. to be polite

C. to help others

D. to learn from Americans

单项选择题