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引言

如果你曾在户外不小心放飞了一个充满氦气的气球,那么你就会知道有些气体的密度轻于其他气体。以你的氦气球为例,它很有可能在你逮住之前就飞走了,因为它比我们所处环境中的气体轻得多。我们通常认为气体没有密度,但事实是它们确实有。在这项试验中你将会发现一些常见气体所拥有的不同密度,其中包括你呼出的气体。

背景

我们每天见到的所有物质,从牙膏到餐盘,都是由各种不同类型的原子组成的。原子是保持它们化学元素特性的最小物质单位。这些不同原子的种类和排列,解释了它们在环境中是以固体、液体还是气体形式存在的不同特性。

这些特征包括密度、质量和体积。密度体现了某种物质质量(重量)和它的体积(所占据的空间)之间的关系。物质的原子质量、它们的大小及排列形式共同决定了它的密度。如果我们知道一个物体的质量和体积,那我们可以算出它的密度。它的公式是:密度=质量/体积

从这个等式我们可以得知,如果两个体积相同的物体,其中一个比另一个重,那么这两个物体的密度会不一样。如果你有一个塑料制成的模型和一个体积相同却是用铅制成的模型,那么用铅制成的模型会感觉重一些。因为这两个模型的体积相同,而我们知道铅的密度大于塑料。

第 2 段(可获 1.78 积分)

Differences in density have to do with the size of the atoms as well as how tightly they are packed together. The atoms that comprise metal are generally heavier than those of plastic, and are packed more closely together. In this activity you will be observing the density of different gases—and how differences in density affect how an object behaves.

Materials

  • Yardstick
  • Two balloons
  • Four tablespoons baking soda
  • Stopwatch or timer
  • One cup white vinegar
  • Clean 16-ounce plastic water or soda bottle
  • Small plastic funnel (If unavailable, use tinfoil or parchment paper to make a temporary funnel.)
  • A partner to help
  • A sink
  • Pencil or pen
  • Sheet of paper
第 3 段(可获 1.38 积分)

Preparation

  • Create a table on your sheet of paper with three columns and three rows.
  • Label the left column: “Balloon.” Write: “Balloon A” in the middle box and “Balloon B” in the bottom box. Label the middle column: “3-Foot Drop”; label the right column: “6-Foot Drop.”

Procedure

  • Carefully pour the vinegar into the water bottle.
  • Carefully pour the baking soda into one of the balloons using your funnel. Do not tie it closed. Hold it carefully so that the baking soda does not spill out.
  • Secure the baking soda–filled balloon to the top of the water bottle. Avoid letting the baking soda fall into the bottle. To do this, hold the mouth of the balloon and shake the baking soda down to the bottom. Keep holding it this way while you gently cover the top of the bottle with the mouth of the balloon, not allowing the contents of the balloon to drop into the bottle.
  • Ensure that the mouth of the balloon covers the bottle top as completely and securely as possible.
  • Place the bottle in the sink. Note the size of the balloon and the appearance of the vinegar in the bottle.
  • Carefully tip the balloon vertically so the baking soda spills into the bottle. What happens when the baking soda contacts the vinegar in the bottle? Is the liquid in the bottle changing? What happens to the balloon? What do you think is causing the balloon to change shape?
  • When the reaction slows, you can gently shake the bottle and tap the balloon, to ensure that no baking soda is stuck in the balloon or on the sides of the bottle.
  • When the reaction is complete, ask your partner to help you remove the balloon from the bottle without allowing gas to escape the balloon. While the balloon is still attached, have your partner tightly squeeze the balloon closed just above where its mouth meets the bottle. With the balloon held closed, you can gently remove it from the bottle. (Don’t be surprised if a puff of gas escapes from the bottle when you remove the balloon!)
  • Tie off the balloon. This is “Balloon A.”
  • Take the second balloon and blow it up to the same size as Balloon A and tie it off. This second balloon is “Balloon B.”
  • Have your partner hold the yardstick vertically, resting the end on the floor. They should also hold your stopwatch or timer.
  • Stand next to your partner and hold Balloon A at the top of the yardstick, so that it is exactly three feet off the floor.
  • Drop the balloon and at the same time have your partner start the timer.
  • Note how long it takes the balloon to fall to the ground. Record this time in your table.
  • Repeat the balloon-drop steps with Balloon B. Which balloon took longer to drop to the ground?
  • Have your partner use the yardstick to measure six feet from the ground. (It’s easier if you do this against a wall.)
  • Repeat the balloon-drop steps, dropping each balloon from six feet. Record your results in your table.
  • Extra: Test to see the maximum distance that you can throw each balloon. Is one easier to throw than the other?Why do you think that is?
第 4 段(可获 6.81 积分)

Observations and results

In this activity you created the gas carbon dioxide (CO2) by combining baking soda and vinegar. Both are known as the reactants—in this reaction because they undergo a change while taking part in the reaction. Vinegar is weakly acidic whereas baking soda is a bicarbonate. When they are combined, a two-step reaction takes place. The first step of the reaction is an acid–base reaction, and the second step is a decomposition reaction. When both steps are complete, the final products are CO2 and water (H2O). When you added the baking soda to the vinegar, you should have observed bubbling and foaming in the bottle. This was the CO2 gas being produced and released. The reaction produced too much CO2 for the bottle to hold, however. As a result, the CO2 gas escaped into Balloon A and the balloon expanded. Once all of the baking soda mixed with the vinegar, the contents of the bottle were CO2 and H2O. When there was nothing left to react, the reaction ended.

第 5 段(可获 2.16 积分)

这项活动的下一步是测量两个不同气球掉落地面的速度,正如你所知道的,气球A含二氧化碳,来自苏打粉和醋反的反应。相比之下,气球B包含你吹它时呼出的空气。我们呼出空气的主要成分是氮和氧,二氧化碳大约只占4%到5%。因此你正在测试装有接近纯二氧化碳的气球A,是否比装有接近纯氮氧的气球B下降得更快。

你应该已经发现气球A比气球B更早落到地面上,你可能注意到气球B在降落过程中遭遇的空气阻力更大。气球B并不是直线下落的,而是下落时左右摇晃。因为两个气球的大小相同(体积),但一个比另一个下降得更快,我们的结果告诉我们气球A中的CO2气体比气球B中发现的气体的密度更高。

第 6 段(可获 2.25 积分)

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