“Face changes?! I know what those are!” one student exclaimed as he rapidly transitioned between an impressive array of different facial expressions. This theatrical display halted as the volunteers broke the news that the lesson for the week was not in fact focused on how to cross one’s eyes while licking their nose, but, rather, was centered on the concept of phase changes. Though this concept was met with slightly fewer giggles, the excitement in the room did not diminish, especially when we revealed that we would be using ice cream to model these changes.

To begin our lesson, the volunteers did a brief intro on the concept of phase changes. It was clear that the students had been exposed to the idea before the lesson as they were able to rattle of and correctly apply terms such as solid, liquid, gas, evaporating, melting and freezing. Building upon these understandings, the volunteers drew molecular diagrams of the transitions to ground their knowledge atomically. Students were also asked to hypothesize about whether or not mass and the number of molecules is conserved through these changes. Most students understood that the number of molecules would not decrease, and, once we explained that these molecules were where the mass came from, they were able to grasp the concept of conservation of mass.

With a solid scientific foundation in place, we began the process of making ice cream. To do this, each student put a half cup of half and half, sugar and vanilla in a plastic bag. They then weighed the bag and recorded the initial mass of the liquid in a data table. Once this was complete, the students put their bag into a larger bag that was filled with salt and ice. As they mashed bags to freeze the ice cream, we continued discussion of the process of freezing. Additionally, we talked about the role of ice in making the system cold. They immediately related this concept to the “salt and ice challenge” which is apparently one of their favorite pastimes. The lesson went without hitch, except for the fact that the ice cream took a long time to freeze and was , in many cases, softer than ideal. When the ice cream finally solidified, the students re-massed their bag and were able to see the truth behind the conservation of mass.

To garnish the ice cream, the students made a chocolate sauce (which allowed them to explore the concept of melting) and whipped cream. We also brought fruit and nuts to top the sundays and, much to the volunteers amazement and excitement, the kids almost universally gravitated to the fruit.

Throughout the course of the lesson the kids expressions transitioned from excitement over ice cream, to awe and curiosity about conservation of mass to pure joy from eating their desserts. In this regard, the eager student at the beginning of the lesson was right; perhaps phase changes and face changes aren’t all that different after all.

Written by Rebecca Fairchild ’18

Pictures by Jilian Banner ’17