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Thistle tube and Pressure

  1. Dec 27, 2004 #1
    This might seem like a chemistry question, but the principle behind it is mainly physics.

    How does keeping the thistle tube below the acid level prevent gas from seeping through it in a acid and metal reaction? In case you are wondering the thistle tube is the tube with a bulb-like opening at the top in the attachment. In the diagram, the thistle tube is above the acid level, but that's not the point, since it's just to show you what it looks like. Please explain this. Thanks.

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  2. jcsd
  3. Dec 28, 2004 #2

    Andrew Mason

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    Where is the metal?

    The metal reacting with the acid will create gas bubbles which will rise vertically through the acid to the surface and add to the pressure in the stoppered bottle. If the thistle tube end is below the surface of the acid and to the side of the metal, no gas will enter the tube. If it is above the metal or if the metal is a powder distributed throughtout the acid, some portion of the gas being emitted would enter the tube. I would think that this proportion would be: area of tube opening/area of metal if the metal is a surface below the tube. If the metal is mixed with the acid, the proportion would be: (volume of acid below tube end)/(volume of total liquid/metal) .

    So if you make the diameter small compared to the surface of the metal, and, in the case of powdered metal in suspension in the acid, if you lower the end to a point near the bottom, not much gas will get through.


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