Sublimation may be the most complicated change of matter to discuss with second-grade students. This is because sublimation -- the changing of water from a solid to a gas -- is unfamiliar and rarely witnessed. However, by using dry ice, you can illustrate this concept for your second-grade class. You can make a dramatic representation by building a volcano around a pot of hot water. Drop the dry ice into the water and the volcano will smoke, as the solid ice sublimates into gas. Do not touch the dry ice or let students touch it as it can cause burns.
Some second-grade students may be more familiar with evaporation, but it can still be confusing. An interesting way to illustrate the rate of evaporation is to create puddles on the pavement -- the school parking lot may be ideal -- by pouring water on the ground. Have students trace around the puddles with chalk to illustrate the original size. Return to the puddles hourly to see the progress and have students trace the new puddle shapes to track the rate of evaporation. You can also place 2 cups of water on a table, one covered and one uncovered. The water from the uncovered cup will evaporate because of the lower pressure, while the water in the covered cup will remain in its original state of matter.
Illustrating freezing is easy -- you can just put ice cube trays filled with water into a freezer. A more entertaining and enlightening project for your second-grade students would be one that illustrates how you can alter the freezing point of water. You can do this by floating an ice cube in a glass of water and challenging students to lift it out using only a match. After a few minutes, take the match, bend it at a 90-degree angle, and place it on top of the ice cube with one side pointing upward. Pour salt on top of the match; this will lower the freezing point of the water, turning some ice back into liquid. Once the heat dissipates, the water will refreeze from liquid to solid, attaching the match to the cube; grab the match and pull the ice cube out of the water.
Melting, like freezing, is a relatively simple concept to illustrate. For an entertaining way to show that different substances have different melting temperatures, you can use a hair dryer to test the relative melting temperatures of several substances, such as ice, chocolate, marshmallows and ice cream. The best part is that your second-grade class may have no qualms about helping you clean up this experiment!