Fermentation experiment?

I'm doing a fermentation lab tomorrow and in my pre-lab write up, I noticed that the delivery tube (egress for carbon dioxide) is set to enter a test tube with barium hydroxide in it. I'm just wondering if anyone knows why they wouldn't just set it up to enter a test tube with water or something? I know that when the yeast and sugar are fermenting to ethanol, carbon dioxide is given off...but what is the significance of the barium hydroxide?