小明用一个半径为5cm，面积为15πcm²的扇形纸片，制作成一个圆锥的侧面（接缝处不重叠），那么这个圆锥的底面半径为(         )

用科学方法认识物质

掌握了物质的组成、性质才能合理地应用物质，研究物质组成、性质的基本方法主要有：观察法、实验法等．

（1）用观察法认识空气

观察法是一种有计划、有目的地用感官（眼、鼻、耳、手）考察研究对象的方法．

用观察法研究空气的性质，请你写出一种观察方法，该方法得出的空气的一条性质：观察方法是____ ，得出的性质是____ ；

（2）用实验法认识空气的组成及其成分的性质

实验法是指通过实验来验证对物质性质的预测或探究物质的组成、未知性质的方法．

①用实验法认识空气的组成

实验室用如图所示装置来测定空气中氧气的含量．请据图和所学知识回答下列问题．

实验中发生反应的化学方程式是____ ，该实验的原理是____，已知图中玻璃管的容积为50mL，实验前注射器活塞对应的示数是100mL：实验后当气球回复实验前状态时，注射器活塞对应的示数是70mL，则氧气约占空气体积的____ ．

②用实验法认识二氧化碳的性质

小明设计了如图所示的三组实验来探究二氧化碳的性质：

在实验一中，将一瓶二氧化碳沿烧杯内壁慢慢倒入小烧杯中，可以观察到的现象是____ ，由此可以得出二氧化碳的两条性质分别是____ 、____ ；

实验二中，向集满CO₂的试管内倒入约占试管容积三分之一的澄清石灰水，旋紧胶塞，持续振荡一定时间，试管中发生反应的化学方程式是　____ ，充分振荡后，试管内压强（填“＞”或“=”或“＜”）____ 试管外大气压强；

实验三中，先分别向两支盛有蒸馏水的试管中滴加几滴紫色石蕊试液，如实验（Ⅰ），然后向其中一支试管中通入二氧化碳，如实验（Ⅱ）．在实验（Ⅱ）中溶液变____ 色，反应的化学方程式为____ ．

    Australian Comedy---COSI(5th Round)

    Date: April 12-16---7:30 pm

    Venue: Beijing Poly Theatre

    Price: 800-1,500 yuan

    Cosi is a 1996 Australian comedy film directed by Mark Joffe. In the film, Lewis Riley wants to get a job as a director at a mental (精神病的)hospital. He gets the job and finds himself directing a production of the Mozart opera Cosi fan tutte, an opera in Italian. And it is going to be performed by actors and actresses that he must select from among the patients, who only speak English.

    Twelfth Night

    By TNT Theatre Britain

    Date: April 13-15---7:30 pm

    Venue: 9 Theatre Beijing

    Price: 60-380 yuan

    Twelfth Night, or What You Will is a comedy by William Shakespeare, believed to have been written around 1601-1602. The play focuses on the twins Viola and Sebastian, who are separated in a ship accident at sea. Viola(who pretends to be a boy) falls in love with Orsino. But Orsino in turn is in love with Olivia when meeting Viola falls in love with her, thinking she is a man.

    Jazz Weekend---Thomas Enhco Piano Recital

    Date: May 6---7:30 pm

    Venue: Beijing Concert Hall

    Price: 80-599 yuan

    Thomas Enhco was born in Paris, 1988. He starts playing the violin and piano at an early age, and studies classical music and jazz. At 12, Thomas works with a French jazz violinist Didier Lockwood. He is spotted by Peter Erskine, who offers to help with Thomas Enhco's first CD. In 2004-2005, Thomas creates and records his first album, Esquisse, which is released in 2006. He receives the Young Talent Prize at the Montauban Jazz Festival in 2006.