In the laboratory preparation of CO$_2$, why must the end of the thistle funnel be kept submerged in water?

Question

Accepted Answer

to prevent CO$_2$ gas from escaping back through the funnel

Answer

to cool the flask

Answer

to dissolve the carbonate

Answer

to absorb HCl

In the laboratory preparation of CO2_22​, why must the end of the thistle funnel be kept submerged in water?