题干

为了除去粗盐中的Ca2+、Mg2+、SO42及泥沙,得到纯净的NaCl,可将粗盐溶于水,然后进行,①过滤;②加过量NaOH溶液;③加适量盐酸;④加过量Na2CO3溶液;⑤加过量BaCl2溶液.这些操作中正确的顺序(  )

A:④①②⑤③

B:②⑤④①③

C:①②④⑤③

D:④②⑤①③

上一题 下一题 0.0难度 选择题 更新时间:2016-06-29 06:44:42

答案(点此获取答案解析)

B

同类题4

阅读下面短文,从短文后各题所给的四个选项(A、 B 、C 、D)中,选出可以填入空白处的最佳选项。

    One month ago, my daughter started kindergarten. As usual, I wished her success. I was telling a lie. What I actually wish for her is 1. I believe in the power of failure.

    Success is 2 in a sense. Success is proving that you can do something that you already know you can do, or doing something correctly the first time, which can 3 be a problematic victory. First-time success is usually a fluke (侥幸). First-time failure, 4, is expected; it is the natural order of things.

    Failure is how we learn. I have been told of an African phrase 5 a good cook as “she who has broken many pots”. If you have spent enough time in the 6 to have broken a lot of pots, probably you know a fair amount about 7. I once had a late dinner with a group of chefs, and they spent time 8 knife wounds and burn scars. They knew how much credibility (可信度) their 9 gave them.

    I earn my 10 by writing a daily newspaper column. Each week I am 11 that one column is going to be the worst column of the week. I don't just set out to write it; I try my best every day. 12, every week, one column is inferior (较差的) to the others, sometimes extremely so.

    I have learned to 13 that column. A successful column usually means that I am treading (踏) on 14 ground, going with tricks that work, or dressing up popular ideas in fancy words. Often in my inferior columns, I am trying to 15 something I've never done before, something that I'm not even sure can be done.

    My daughter is a perfectionist. She will feel her failures, and I will want to 16 her. But I will also, I hope, 17 her of what she learned, and how she can do 18 next time. I probably won't tell her that failure is a good thing, because that's not a(n) 19 you can learn when you're five. I hope I can tell her, though, that it's not the end of the world. Indeed, with luck, it is the 20.