What phase change is happening on the outside of the dry ice? How do you know?

#"Sublimation"# describes the transition solid to gas. Carbon dioxide, dry ice, is unusual, because it is one of the few materials that sublime at standard pressure.

i.e., #"sublimation"# #CO_2(s) rarr CO_2(g)#

#"And (ii) Condensation of water...."#,.

But because dry ice is cold (its sublimation point is #-78.5# #""^@C#), often, when you expose a block of dry ice to the atmosphere, atmospheric water vapour condenses on the cold block.

#H_2O(g) rarr H_2O(l)rarrH_2O(s)#.

Often when you brush a block of card ice with your fingers, you brush off water ice, which melts on your fingers to give liquid water.

So how do you know? Good question. Well, when you put a block of dry ice in a bag, upon sublimation there is no solid material in the bag. Clearly the solid mass has gone somewhere, and sublimation is a likely hypothesis.

Here is a simple experiment that could test this proposition. Weigh a mass (say #5.0*g#) of dry ice with a balloon. Put the dry ice pellet in a balloon, seal the balloon, and allow the balloon and its contents to warm to room temperature. Of course the mass of the balloon and its contents will remain close to #5*g#. What will occur to the volume of the balloon?