问题 选择题

若要检验一种溶液是否是该温度下的饱和溶液,应选择的可靠而简便的方法是……( )

A.降低10℃观察有无晶体析出

B.加热蒸发掉10g水,再冷却到原温度,观察有无晶体析出

C.用化学方法测定溶液的量

D.投入一小粒少量溶质晶体,观察它是否溶解或减少

答案

答案:D

A、若溶质的溶解度随温度的升高而降低,则将其饱和溶液降低温度不会析出晶体;

B、加热蒸发掉10g水后,有两种可能,一是溶液仍未达饱和,不会析出晶体,二是析出晶体也不能说明原溶液就是饱和溶液,因为可能蒸发掉少于10g水就已经可以析出晶体了;

C、用化学法较繁,且只测溶液的量不够,还需侧溶质的量

阅读理解

阅读理解。

    Miss Gogers taught physics in a New York school. Last month she explained to one of her classes about

sound, and she decided to test them to see how successful she had been in her explanation. She said to them,

"Now I have a brother in Los Angeles. If I was calling him on the phone and at the same time you were 75

feet away, listening to me from across the street, which of you would hear what I said earlier, my brother or

you and for what reason?"

    Tom at once answered, "Your brother. Because electricity travels faster than sound waves." "That's every

good," Miss Gogers answered; but then one of the girls raised her hand, and Miss Gogers said. "Yes, Kate."

    "I disagree," Kate said. "Your brother would hear you earlier because when it's 11 o'clock here it's only

8 o'clock in Los Angeles."

1. Miss Gogers was teaching her class _____. [ ]

A. how to telephone

B. about electricity

C. about time zone (时区)

D. about sound

2. Miss Gogers raised this question because she wanted to know whether _____. [ ]

A. it was easy to phone to Los Angeles   

B. her student could hear her from 75 feet away

C. her students had grasped her lesson   

D. sound waves were slower than electricity

3. Tom thought that electricity was _____. [ ]

A. slower than sound waves  

B. faster than sound waves

C. not so fast as sound waves   

D. as fast as sound waves

4. Whose answer do you think is correct according to the law of physics? [ ]

A. Tom's  

B. Kate's   

C. Bath A and B   

D. Neither A nor B

判断题