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

Question

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

Peyton Adams · Accepted Answer

Two phase changes occur:

#"(i) Sublimation...."#,.

#"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)#. Water ice melts on your fingers when you brush a block of card ice with your fingers, producing liquid water. You ask, "How do you know?" Well, when you sublimate a block of dry ice in a bag, nothing solid remains in the bag. This indicates that the solid mass has moved, and sublimation is a plausible explanation. 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?

Natalie Anton · Answer

undefined As dry ice changes directly from a solid to a gas without going through the liquid phase, it is obvious that sublimation is the phase change occurring on the exterior of the ice.