题干

下列各组物质中,前者属于混合物,后者属于纯净物的是(   )

A:水银  液氧

B:钢铁  汽油

C:洁净的空气  蒸馏水

D:甲烷 塑料

上一题 下一题 0.0难度 选择题 更新时间:2014-05-26 10:48:56

答案(点此获取答案解析)

C

同类题2

阅读下面文段,完成小题。

应有格物致知精神
丁肇中

我是研究科学的人,所以重视实验精神在科学上的重要性。
科学发展的历史告诉我们,新的知识只能通过实地实验而得到,不是由自我检讨或哲理的清谈就可求到的。
实验的过程不是消极的观察,而是积极的探测。比如,我们要知道竹予的性质,就要特地栽种竹予,以研究它生长的过程,要把叶子切下来拿到显微镜下去观察,绝不是袖手旁观就可以得到知识的。
实验不是毫无选择地测量,它需要有细致具体的计划。特别重要的,是要有一个适当的目标,以作为整个探索过程的向导。至于这目标怎样选定,就要靠实验者的判断力和灵感。一个成功的实验需要的是眼光、勇气和毅力。
由此我们可以了解,为什么基本知识上的突破是不常有的事情。我们也可以了解,为什么历史上学术的进展只靠很少数人关键性的发现。
时至今天,王阳明的思想还在继续支配着一些中国读书人的头脑。因为这个文化背景,中国学生大都偏向于理论而轻视实验,偏向于抽象的思维而不愿动手。中国学生往往念功课成绩很好,考试都得近一百分,但是在研究工作中需要拿主意时,就常常不知所措了。
在这方面,我有个人的经验为证。我是受传统教育长大的。到美国大学念物理的时候,起先以为只劓艮“用功”,什么都遵照老师的指导,就可以一帆风顺了,但是事实并不是这样。一开始做研究便马上发现不能光靠教师,需要自己做主张、出主意。当时因为事先没有准备,不知吃了多少苦。最使我彷徨恐慌的,是当时的惟一办法——以埋头读书应付一切,对于实际的需要毫无帮助。
我觉得真正的格物致知精神,不但研究学术不可缺少,而且对应付今天的世界环境也是不可少的。我们需要培养实验的精神,就是说,不论是研究自然科学,研究人文科学,还是在个人行动上,我们都要保留一个怀疑求真的态度,要靠实践来发现事物的真相。现在世界和社会的环境变化很快,世界上不同文化的交流也越来越密切。我们不能盲目地接受过去认定的真理,也不能等待“学术权威”的指示。我们要自己有判断力。在环境激变的今天,我们应该重新体会几千年前经书里说的格物致知的真正意义。这意义有两个方面:第一,寻求真理的惟一途径是对事物客观的探索;第二,探索应该有想像力、有计划,不能消极地袖手旁观。希望我们这一代对于格物和致知有新的认识和思考,使得实验精神真正变成中国文化的一部分。
(节选自(应有格物致知精神》)

同类题3

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    Today, many people use plants from other places to design their yards. Well, do they have any influence on animals living nearby? As we know, plants supply food for things like insects and plant-eating animals. In turn, birds and meat-eating animals feed on these insects and plant-eating animals. It is easy to see how important plants are.

    Although foreign plants may look beautiful, they can cause problems when there are too many of them. I live in Virginia, USA, and sometimes see a plant—commonly known as Kudzu—that seems to cover everything in the area, even climbing over whole trees and houses.

    Kudzu is an unbelievable plant since it grows very fast. It was first brought to the United States from south-east Asia around the 1870's as a crop that farmers could grow. However, Kudzu quickly came to be considered harmful throughout the southern United States.

    Since Kudzu grows fast in many different environments, it can completely cover areas of land quickly. The plant is also not eaten by any insects or birds in the United States so it can grow unchecked. Kudzu can cover trees, bushes, and even houses. It costs the United States 500 million dollars a year to just keep Kudzu from growing too fast. The plant is a perfect example of what scientists call an invasive (侵略性的) species, which grows more quickly than other native plants.

    All foreign plants have the possibility to spread quickly. Not only are they costly, but planting them in gardens actually takes away food from insects. If everyone filled their garden with plants native to where they live, many lovely butterflies and other native creatures would be attracted to their backyards! So, take a look out of your window — how does your garden grow?