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Let's have a look at the experiment again. A rubber balloon which was inflated with helium gas was immersed in water. The helium gas and water are next to each other, separated by the thin rubber membrane of the balloon. The helium molecules inside the balloon bounce about violently hitting the balloon's rubber membrane. In the same way, water molecules around the outside of the balloon move about violently colliding with the rubber membrane. |
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If you recall, the result of the experiment was that helium gas inside the balloon came out and formed lots of bubbles of helium gas on and around the balloon. This happens because helium gas molecules (which cannot be seen with the naked eye) pass through the rubber membrane to the outside. However, there aren't any open holes even when you carefully examine or touch it. In fact in the same way that helium consists of small molecular particles the rubber membrane is made of a collection of rubber molecules as well. Try to think of small particles (spherical in shape) joining together to make a membrane. Can you arrange balls together without leaving any holes or gaps? Probably not. For some reason small cracks or holes are present. In this way, the rubber membrane of balloons will contain small hole openings which are invisible to the eye. |
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I guess you are getting the picture now. The helium molecules escape through these small holes to the outside. |
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In this way the helium molecules which slowly go through the balloon build up sufficiently to form bubbles which can be seen with the eye. Why is it then, that despite there being hole openings in the rubber membrane, water does not enter the inside of the balloon? |
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Let's imagine the relationship between helium molecules and the rubber membrane as a relationship between a small ball (say a garden pea) and a garden sifter (filter). |
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Let's collect peas of various sizes and glass balls which are much smaller than peas and put them into the sifter. |
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Balls which are smaller than the holes in the sifter will drop through and larger balls will not. Small helium molecules which were smaller than the holes in the rubber membrane (the sifter) could pass through it. |
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Water molecules larger than the holes in the rubber membrane could not pass through it. That is why water could not enter inside the balloon. |
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By the way, bubbles were not on the balloon which contained air, were they? In other words, since the air wasn't able to go through the balloon membrane it must be different from helium gas. So how is it different?
Air is actually composed of nitrogen, oxygen and other gases. The molecules of these substances are much larger than the holes in the rubber membrane of the balloons so they could not pass through it. As molecules are very small things, some are able to pass through the invisible holes in rubber in the same way helium does. However, air (nitrogen and oxygen) despite being a similar gas to helium was not able to pass through rubber. In other words, despite molecules being very small it is clear that there are different sizes among them. |
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Now recall the experiment in which steam passed through cloth. Water vapor was able to pass through the small holes in a special cloth. However, think about this carefully. Why couldn't water (in this case hot water) pass through the holes? |
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Actually, water and steam are composed of the same molecules. However, they are in different forms. Water is a liquid and steam is a gas. The molecules in liquids such as water will stick to like molecules and congeal together. However, the molecules in gas bodies like water vapor move around freely. Thus water could not pass through holes that steam was able to pass through.
Incidentally, what happens if the molecules bind together even more strongly than they do for water? Well, you get ice! Depending on temperature water changes its form from ice, to water, to steam. The strength of the bonding between like molecules is very much related to temperature. |
Copyright (c) 2000 Science Museum(Japan),All rights reserved
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