问题 问答题 简答题

简述有机溶剂沉析的原理。

答案

参考答案:

1.降低了溶质的介电常数,使溶质之间的静电引力增加,从而出现聚集现象,导致沉淀。

2.由于有机溶剂的水合作用,降低了自由水的浓度,降低了亲水溶质表面水化层的厚度,降低了亲水性,导致脱水凝聚。

乙醇:沉析作用强,挥发性适中,无毒常用于蛋白质、核酸、多糖等生物大分子的沉析;

丙酮:沉析作用更强,用量省,但毒性大,应用范围不广;

特点:

(1)介电常数小,60%乙醇的介电常数是48,丙酮的介电常数是22

(2)容易获取

阅读理解

阅读短文,根据要求完成下面各小题。

In the past twenty years, riding bicycles has become more and more popular among people of all ages. 83 Bicycles have long played an important part in the liyes of American young people. But today an even greater number of adults cycle along roadsides and bicycle paths. These new riders are often trying to become fit or lose weight. They have found that bicycling is a pleasant way to stay healthy

This is certainly good for health. But it has also brought some problems. With all these added people on the road, bicycle safety has become a matter of real importance.

Bicycle safety is mostly a matter of common sense. People should ride only on bicycle paths where they will not be badly hurt. There are also a number of rules that riders should follow, especially when riding on streets or highways. Bicyclists should never ride against the traffic,nor should they carry other people on their bicycles. They are also supposed to ride in single line on busy streets and to "walk bicycles across the busy intersections (交叉路口) . 84By following these rules, bicycle riders will not only stay healthy but also be safe.

In one recent year, the National Safety Council reported that 34 thousand bicycle riders were hurt in highway accidents. In that same year, another six hundred riders were killed. Since the introduction of crash helmets (头盔) in 1980, the number of injuries (伤者) and deaths from bicycling accidents had gone down. Today, more and more riders realize the dangers of bicycling. They are wearing helmets and paying closer attention to cycling rules.

根据短文内容回答下列问题。

小题1:How many rules should the riders follow? Give one example.

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小题2: According to the National Safety Council’s report, what’s the total number of injuries and deaths in one recent year?

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B. 将短文中划线部分译成汉语。

小题3:_______________________________________________________________

小题4:________________________________________________________________

C. 请给短文拟一个适当的英文标题。

小题5:________________________________________

单项选择题