一、证明气体的密度比空气小检验某气体密度比空气小时,可将气体充入气球中,用细线将气球口扎紧.若气球上升,证明气体的密度比空气小.二、起反冲作用做“白磷燃烧前后质量的测定实验”时,可在锥形瓶口的橡皮塞上安装一根玻璃管,在玻璃管上端系牢一个瘪气球,使玻璃管下端能与白磷接触.在此实验过程中,发现白磷燃烧,小气球是先变大后变小.此时,小气球起了反冲作用,即防止白磷燃烧使瓶内的气体膨胀而散逸到瓶外. First, to prove that the density of gas than air Small test of a gas density than the air hour, the gas can be filled in the balloon, the balloon with a thin wire tight mouth .If the balloon rises, to prove that the gas density is less than the air. Red role in the “white phosphorus combustion before and after the determination of the quality of experiments,” the rubber plug in the tapered bottle mouth to install a glass tube, fasten a deflated balloon at the top of the glass tube so that the lower end of the glass tube can contact with white phosphorus In the course of this experiment, it was found that when the white phosphorus is burned, the small balloon first becomes bigger and then smaller, at this time, the small balloon plays the role of recoil, that is, the white phosphorus is prevented from burning and the gas in the bottle is expanded and dissipated outside the bottle.