题干

关于某溶液中所含离子的检验,下列判断正确的是(   )

A:加入BaCl2溶液,生成白色沉淀,加稀硝酸沉淀不消失,则原溶液中一定含SO42﹣

B:用洁净的铂丝蘸取溶液在火焰上灼烧,产生黄色的火焰,则原溶液中一定不含K+

C:向某溶液中加入2滴KSCN溶液,溶液不显红色,再向溶液中滴几滴新制的氯水,溶液变为红色,该溶液中一定含有Fe2+

D:取少量晶体溶于水,加入少量NaOH溶液,加热,在试管口用湿润的蓝色石蕊试纸检验,若变红,则可证明晶体的成分中含有NH4+

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答案(点此获取答案解析)

C

同类题2

根据短文内容,从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。

    Homework is a major part of going to school: It's your teachers' way of evaluating(评价) how much you understand of what's going on in class. ____ Luckily, you can do a few things to make homework less work.

    Be sure you understand the homework.

    Write your homework down in your notebook or day planner if you need to. ____ It's much easier to take a minute to ask the teacher during or after class than to struggle to remember later that night!

    Use any extra time in school.

    Many schools have study halls that are specifically designed to allow students to study. It's tempting (诱惑) to hang out with friends during study periods or unstructured (松散的) time. ____  

    Take a break.

    ____ So take some breaks while doing your homework. Sitting for too long without stretching or relaxing will make you less productive than if you stop every so often. Taking a 15-minute break every hour is a good idea for most people.

    ____

    If you don't finish your homework during school, think about how much you have left and what else is going on that day, and then budget your time. Most high-school students have between 1 and 3 hours of homework a night. If it's a heavy homework day and it seems like you've got an assignment in every subject but gym and lunch, you'll need to devote more time to homework. It's a good idea to come up with some kind of homework schedule, especially if you are involved in sports or activities or have an after-school job.

A. Pace yourself.

B. Settle down to do homework.

C. And it helps digest important concepts (概念).

D. Most people's attention spans (注意力持续时间) aren't very long.

E. Don't be afraid to ask questions about what's expected.

F. Once your homework is done, you can check over it if you have extra time.

G. But the more work you can get done in school, the less you'll have to do that night.