Pressure and Thistle tubes

**apchemstudent** · Dec27-04, 11:28 PM

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.

**Integral** · Dec28-04, 12:15 AM

If the picture provided is not the set up you are using, how can we help? Please provide a better description of how your apparutus is set up. Also a better description of the experiment would not hurt.

**apchemstudent** · Dec28-04, 12:29 AM

Originally Posted by Integral

If the picture provided is not the set up you are using, how can we help? Please provide a better description of how your apparutus is set up. Also a better description of the experiment would not hurt.

A better of how my apparatus is setup:

Well, we have a beaker filled with water and inverted in a tub of water, where the gas is collected in.

After that, we have a stopper with 2 holes. One is for a tube that directs the gas into the beaker. The other one is for the "Thistle tube". After a piece of active metal is placed into the flask and with the stopper in place as well, we add acid through the thistle tube to start the reaction.

A better picture, as you can see the only difference is that the thistle tube is submerged in the acid, which is the proper method. I still don't know why this is needed to prevent the gas from seeping through the thistle tube.

**Integral** · Dec28-04, 04:16 AM

Clearly, if your thistle tube were not below the surface of the liquid in the beaker, any gas generated by your reaction would simply exit the beaker through the open tube to the atmosphere. With the Thistle tube below the surface of the acid, once the pressure in the beaker is greater then atmospheric, the gas will displace water in the collection beaker. There will be some gas generated inside the thistle tube but only a very small amount.

I assume that initially the reaction beaker is completely filled with acid. If there is any air above the acid any gas collected will be mixed with what was initially in the reaction beaker.

There will be some of the reacting material forced up the thistle tube by the same pressure which forces the evolving gasses into the collection beaker. But since the density of the reacting materials is much greater then that of the gas being evolved, the column in the thistle tube should not reach the level of the collection bottle.