题干

下列实验操作不能达到实验目的是(   )

选项

实验目的

实验操作

A

若配制480mL与240mL溶液

应分别用500mL容量瓶和250mL容量瓶

B

证明NH4Cl溶液能促进Mg(OH)2固体的溶解

向两份等量Mg(OH)2固体中分别加入等体积蒸馏水和NH4Cl溶液

C

证明Al2O3的熔点比Al高

把砂纸打磨过的铝箔在酒精灯上加热到熔化

D

证明胶体的分散质能透过滤纸

将Fe(OH)3胶体过滤后所得滤液置于暗处,再用激光笔照射滤液

A:A  

B:B  

C:C  

D:D

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答案(点此获取答案解析)

C

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    For most of my 20 years as a teacher, summer vacation was my time to relax. So why was I standing in the schoolyard of an unfamiliar school, wearing myself out for a summer teaching job? The extra paychecks were nice, but I lacked the energy of my younger colleagues. Like Stella. She was in her early twenties, and made keeping up with the kids look effortless. She reminded me of myself, back when I was a bright-eyed student teacher at Ramona Elementary…

    I'd never forget my first day. I was too full of energy. Nervous energy. My supervising teacher was watching, and I wanted to make a good impression. I asked my third graders to take out their crayons for the day's lesson. All of them obeyed. Except one. A girl with two long, dark braids(辫子). Everyone called her Estrellita, or “little star.” Why was she unprepared for class? I demanded to know.

    “My sister has my crayons,” she said.

    “You should each have your own crayons,” I told her. “That's no excuse.”

    “There are 10 children in my family,” Estrellita said quietly, her big brown eyes never leaving my face. “We have to take turns.”

    I was taken aback. I'd completely misjudged the situation. All day Estrellita's words played on my mind. The next morning I bought a pack of crayons to leave on Estrellita's desk. She was so happy! That experience taught me an important lesson. Every student had a unique set of challenges—it was my mission to help my students overcome them. That mission used to energize(激励 )me.

    Rest was almost over. Stella turned to me and we started chatting. “How long have you been teaching?” she asked. I told her I'd started twenty years before, at Ramona Elementary.

    “I went to school there twenty years ago!” Stella said. I looked at her again, this time really seeing her. Those big brown eyes. That long, dark hair...

    “Did you use to have two long braids?” I said. “We called you Estrellita… ”

    Stella shouted. “ You ! You gave me the crayons!”

She'd become a teacher. To help students like I did. Even doing something that small mattered.

    That summer, I threw myself into teaching with a renewed sense of purpose. Estrellita had taught me a lesson once again.