问题 问答题

设计方案证明空气是混合物.

(1)要证明空气是混合物,至少需要证明空气中含有______种物质;

(2)在空气中纸张的温度达到其着火点时发生燃烧,证明空气中含有______;

(3)证明空气中含有水蒸气可选用的方法是______;

(4)敞口久置在空气中的石灰水表面形成白膜,说明空气中含有CO2,其反应可用化学方程式表示为______.

答案

空气中各成分及体积分数为:氮气:78%、氧气:21%、稀有气体:0.94%、二氧化碳0.03%、水蒸气和杂质:0.03%.

(1)混合物是由两种或两种以上的物质组成,显然空气是混合物;

(2)氧气具有助燃性,在空气中纸张的温度达到其着火点时发生燃烧,可以证明空气中含有氧气;

(3)酥脆的饼干放在空气中变软,说明饼干吸收了空气中的水蒸气,可以证明空气中有水蒸气;

(4)敞口久置在空气中的石灰水表面形成白膜,说明空气中含有CO2,生成的白膜是碳酸钙,其反应可用化学方程式表示为:CO2+Ca(OH)2=CaCO3↓+H2O;

故答案为:

(1)两;

(2)氧气;

(3)酥脆的饼干放在空气中变软;

(4)CO2+Ca(OH)2=CaCO3↓+H2O.

单项选择题
单项选择题

Of all the areas of learning the most important is the development of attitudes. Emotional reactions as well as logical thought processes affect the behavior of most people.
"The burnt child fears the fire" is one instance; another is the rise of despots like Hitler. Both examples also point up the fact that attitudes stem from experience. In one case the experience was direct and impressive; in the other it was indirect and cumulative. The Nazis were indoctrinated largely by the speeches they heard and the books they read.
The classroom teacher in the elementary school is in a strategic position to influence attitudes. This is true partly because children acquire attitudes from those adults whose word they respect.
Another reason it is true is that pupils often delve somewhat deeply into a subject in school that has only been touched upon at home or has possibly never occurred to them before. To a child who had previously acquired little knowledge of Mexico, his teacher’s method of handling such a unit would greatly affect his attitude toward Mexicans.
The media through which the teacher can develop wholesome attitudes are innumerable. Social studies (with special reference to races, creeds and nationalities), science matters of health and safety, the very atmosphere of the classroom... these are a few of the fertile fields for the inculcation of proper emotional reactions.
However, when children come to school with undesirable attitudes, it is unwise for the teacher to attempt to change their feelings by cajoling or scolding them. She can achieve the proper effect by helping them obtain constructive experiences.
To illustrate, first grade pupils afraid of policemen will probably alter their attitudes after a classroom chat with the neighborhood officer in which he explains how he protects them. In the same way, a class of older children can develop attitudes through discussion, research, outside reading and all-day trips.
Finally, a teacher must constantly evaluate her own attitudes, because her influence can be deleterious if she has personal prejudices. This is especially true in respect to controversial issues and questions on which children should be encouraged to reach their own decisions as a result of objective analysis of all the facts.

The example of the pupils’ 1 earning about Mexico shows that ______.

A.a child usually learns the right things from their teachers

B.a teacher can correct a pupil’s wrong ideas

C.a teacher’s attitude can influence a child’s attitude by teaching

D.a child’s attitude is very changeable