How does it work ?
First of all, ensure the outside end of the siphoning glass tubing is close. Then, pour some dilute hydrochloric acid through the funnel into the jas until the acid level covers the marble slabs. When the slabs come into contact with the acid a chemical reaction begins to kick in, releasing carbon dioxide gas:
CaCO3 + 2HCl --> CaCl2 + H2O + CO2
The slabs gradually corrode by the acid and dissolves as calcium chloride (CaCl2). Open the gas tap to release the gas for use. When the gas is no longer for use, close the tap. The gas, no longer able to escape, traps in the jar and pressure begins to build inside the jar. Very quickly, the gas pushes the acid back into the funnel and the acid level in the jar drops until it is just out of contact with the marble slabs.
Diagram below shows the effect of pressure.
At this stage, the chemical reaction stops and the pressure built up within the jar maintains the acid in the funnel. If the tap is opened, the gas is released and pressure drops so that the acid goes back into the jar. Reaction is started once again to produce carbon dioxide. This procedure can be repeated many times until the acid is depleted. Use of large marble slabs means that the carbon dioxide stock can be used for a long time. The cover can therefore seal as there is no need to replace the carbonate.
Large marble sizes also ensure the slabs do not fall to the bottom of the jar. The use of small marble chips should be avoided. This is becuase the chips can be corroded into smaller pieces by the acid which can easily fall to the bottom of the jar. It is more difficult to control the reaction when this occurs.