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addict | 13:59 Sun 27th Apr 2008 | Science
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A u-shaped tube containing water with porous pot on the right hand side. A beaker containing hydrogen is place over the pot. The water levels in the tube quickly change. (Water goes down on the side of the pot). Is this because:1. more gas molecules are moving into the porous pot than are moving out or because 2.fewer molecules are moving into the porous pot that are moving out? Also what happens to the water level in the left hand side of the u-tube is the beaker of hydrogen is removed? Explain answer in terms of the movement of the molecules. Desperate!

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The key to this is diffusion. You want to compare the relative rates of diffusion of hydrogen and air (nitrogen / oxygen).

Graham's law of diffusion.
Under the same conditions of temperature and pressure, the rates of diffusion of two gases vary inversely as the square roots of their molecular masses.
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That is way over my head. Only doing key stage 3 science!
OK then. Hydrogen has a much smaller molecular mas than either nitrogen or oxygen (air) and so hydrogen will pass through the porous pot much faster than the air will.

Will that make the volume in the U-tube greater or smaller? What will that do to the liqid level in the manometer?

Now think what is in that arm of the U-tube. What will happen when the beaker of hydrogen is removed? Remember Hydrogen diffuses faster than air. If the volume of gas on the right-hand side changes, what will that do to the level on the left side of the manometer?


Unfortunately, living in Scotland, I have no idea what Key Stage 3 is, but hopefully that should be at about the right level for you. Is Key Stage 3 around Secondary Year 2 (13-14 yr olds)?
gen2, the Key Stages are as follows in England and Wales;

Key Stage Age of Child School Years

One 5 - 7 1 + 2
Two 7 - 11 3 - 6
Three 11 - 14 7 - 9
Four 14 - 16 10 + 11

Oh for the simplicity of the Scottish system!
The table formatting gremlin is on duty tonight. Let's try again.

Key Stage One is taken by children aged between five and seven in years one and two in primary school.

Key Stage Two is taken by children aged between seven and eleven in years three to six in primary school.

Key Stage Three is taken by children aged between eleven and fourteen in years seven to nine in secondary school.

Key Stage Four is taken by children aged between fourteen and sixteen in years ten and eleven in secondary school.

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