问题 问答题

某同学设计了测定空气中氧气含量实验,该同学的实验步骤如下:

①将图中的集气瓶分为5等份,并作好标记.

②在带橡皮塞和导管的燃烧匙内装入足量的红磷,将导管上的止水夹夹紧,在酒精灯上点燃红磷,并立即伸入集气瓶内,塞紧橡皮塞.

③充分反应后,待集气瓶冷却至室温,打开止水夹.

请回答下列问题:

(1)该实验中红磷需稍过量,目的是______.         

(2)步骤③中打开止水夹后观察到的现象是______,由此可得出空气中氧气的体积分数约为______,

(3)水被倒吸入集气瓶不足1/5的原因写两点:______,______.

答案

(1)实验的目的是测定空气中氧气含量,原理是消耗氧气使装置内外产生压强差,水被吸入占据氧气的空间,从而达到测定氧气的目的.红磷过量,可使集气瓶中的氧气完全消耗.

(2)红磷与氧气反应,生成了白烟五氧化二磷固体,所以会导致集气瓶内压强减小,水被压入集气瓶,进入的水量正好占据集气瓶内氧气的空间,约为集气瓶体积的

1
5

(3)①红磷的量不足,使瓶内氧气未耗尽; ②瓶塞未塞紧,使外界空气进入瓶内; ③未冷却至室温就打开止水夹,使进入瓶内水的体积减少;④烧杯中的水不够等.都会使进入的水少于五分之一.

故答案为:(1)充分反应掉集气瓶中的氧气;(2)烧杯中的水倒流至集气瓶,液面上升在一等分刻度处;

1
5
;(3)红磷的量不足,使瓶内氧气未耗尽;未冷却至室温就打开止水夹,使进入瓶内水的体积减少.

选择题
阅读理解

One of the most wonderful things about having a positive attitude is that it can touch many people, and it often exists in our everyday life.

Recently, I stopped by a store to get a newspaper and a bottle of drink. The young woman at the check-out counter said, “That’ll be three dollars in all please.” She then glanced down at the paper I was buying and said. “I’m sick and tired of all this negative stuff-on the front pages. I want to read some good news for a change.” She then said, “In fact, I think soemone should just publish a Good News newspaper--- a paper with wonderful, inspirational stories about people over-coming difficulty and doing good things for others. I’d buy one every day!” she then thanked me for coming in and said, “Maybe we’ll get lucky tomorrow; maybe we’ll get some good news.” And she laughed. She made my day.

The following day after my business appointments. I dropped by the same store again to pick up bottled water and a piece of newspaper, but a different young lady was behind the counter. As I checked out I said “Good afternoon” and handed her my money. She said nothing, not a word, not a smile or not a gesture. She just handed me my change and in a negative tone ordered “Next!”

It hit me right between the eyes: Two people, the same age; one made me feel great, and the other, well, made me feel that I had inconvenienced her by showing up.

Every morning you should ask yourself this important question. “Who do I want to be today? The Grouch(不高兴的人) or ‘The Good News Girl?’” Your answer will go a long way toward determining the joy and happiness that you will experience in your life.

53. What does the first salesgirl dislike according to her words?

A. Those who have bad manners.         

B. The bad news in the newspaper.

C. The advertisements in the newspaper.   

D. The bad news who break the laws.

54. After he had bought a newspaper on the first day the author _________.

A. was very lucky and excited that day    

B. always remembered the girl’s word

C. read the news in the newspaper carefully 

D. was in a pleasant mood the whole day

55. We learn from the passage that the salesgirl the author met the next day was ________.

A. humorous         B. enthusiastic       C. very cool         D. positive

56. What does the underlined sentence in Paragraph 4 mean?

A. I was very surprised.                  B. I was especially angry.

C. I felt very unhappy.                   D. It hurt my heart badly.