题干

下列有关实验操作的“先”与“后”的说法中,正确的是(   )

A:制取气体时,先装药品,后检查装置的气密性

B:加热KClO3并用排水法收集O2实验结束时,先熄灭酒精灯,后移出导管

C:稀释浓硫酸时,先把浓硫酸倒入烧杯中,后沿器壁缓慢注入水,边加边搅拌

D:加热试管时,先使试管底部均匀受热,后用酒精灯的外焰固定加热

上一题 下一题 0.0难度 选择题 更新时间:2011-03-31 04:24:33

答案(点此获取答案解析)

D

同类题5

   A ban on smoking in most public places goes into effect(效果) on Sunday in China - home to a third of the world's smokers. That means around 300 million smokers will no longer be allowed to light their cigarettes in the "enclosed public places." These include hotels, restaurants, theaters and public transport waiting rooms. The ban does not cover offices or factories.
   The campaign is aimed at controlling(控制) the number of deaths from smoking-related illnesses, running at a million a year. According to the World Health Organization, more than 3,000 people die every day in China because of smoking. Health experts also hope it will help Chinese to raise the knowledge of the dangers of smoking.
   But the new rules have been criticized because they do not include punishments(处罚) for those who choose not to obey them.Most people worry that Chinese smoking ban may have little effect.
   Business owners often hate rules that force(强迫) them to ban smoking on their restaurants, bar, cafe..., because many customers(顾客) do not like the rules and complain. Often you find people smoking at the next table while you are eating your meal or having a drink in a bar.
It appears that many Chinese people don't know the dangers of smoking. Research suggests only one in four knows the harm(害处) cigarettes or second-hand smoke can cause.
   At the same time the government makes a lot of money from the sales of cigarettes by the state-owned firms that make and sell all tobacco products throughout the country.