题干

某研究小组用下图所示装置,探究能否用镁条代替红磷测定空气中氧气含量。

实验前,小组成员提出了不同的猜想。

【猜想与假设】

猜想1:可以代替,因为镁条与氧气反应后不会产生气体

猜想2:不能代替,查阅资料知金属镁很活泼,可能会与空气中的氮气反应

【进行实验】

步骤

实验操作

实验现象

组装好装置后,打开止水夹,手捂集气瓶,一段时间后,松开手

烧杯中导管口有气泡冒出,松手后,烧杯中导管末端有一段液柱

关闭止水夹,点燃镁条,迅速伸入集气瓶中,塞好塞子

镁条燃烧,放出大量热,发出耀眼白光,生成大量白烟,剩余固体为白色带状物

冷却至室温,打开止水夹

烧杯中水从导管进入集气瓶中,体积约为集气瓶体积的2/5


【解释与结论】

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完形填空

    Powell was walking home one dark night. As he1the city bridge, he noticed something 2--a person was sitting on the rail, legs hanging over a3 about 100 feet below, at this time.

    “At first I thought he was just4 But as I got closer, I heard him whispering.5I clearly heard ‘I'm just going to jump,'” said Powell.

    “Hey, buddy, what are you doing?” Powell asked. He kept his6, standing about six feet away not to 7the guy.

    “I'm going to jump,” the guy said.

    “His8had pain in it, but I could9he didn't really want to do this. He10felt there wasn't any other way.” said Powell.

    Powell sought to11him. “You have any kids?”

    Without turning to look at Powell, the stranger only showed a12of his daughter on his cell phone. She looked to be two years old. “Think about how13her dad at a young age will affect her.” Powell14 Over the next ten minutes they talked, the stranger gradually opened his heart.

    “I'm having a15time.” the stranger said. “I can't make any money and I'm hungry.”

    Thankfully, Powell's calm, sincere persuasion caused the stranger to16turn and look at him. Powell, now about two feet from the stranger, held out his hand and17it there. “I felt I could catch him18if he jumped, but I also wanted him to know that I was there for him.” he said. To his19 the stranger took his hand and climbed down.

    Powell is great! It's always good to offer help and show your care if you find someone in your life20with disappointment.