The Child's Ideas for 3. Water to Ice

The Challenges of Learning about Freezing/Melting

Children initially think that when something freezes or melts, its weight should change as well.

Compared to the perceptual changes that occur when reshaping, crushing or grinding materials, the changes that occur with freezing and melting are much more radical. When water freezes, it goes from being something fluid, wet, and drinkable to something that is rigid hard, and chewable; when butter melts, it goes from being soft, rectangular, and yellow, to runny, formless, and clear. Further, children have limited experience with things that freeze or melt in their everyday life—water, butter, ice, and snow being their prime examples. This gives rise to a host of questions: Can you melt or freeze all materials? If so, what happens when something melts or freezes? Is it still the same kind of stuff? How do you know? What actually remains the “same” across these transformations?

Children are not always sure that it is still the same kind of material. After all, water and ice have distinct names. If being wet and fluid are central features of being water, how can ice be water? Calling ice “frozen water” suggests that there might be a deeper connection—but what exactly is that connection? How can they think about what is the same about the stuff across this transformation?

Children initially think that when something freezes or melts, its weight should change as well. Generally, they expect that harder things are heavier, so they think that ice should be heavier than water or that solid butter would be heavier than melted butter. (Appreciating that weight stays the same across melting/freezing is thus more difficult for children than appreciating the invariance of weight across shape change.) Of course, children cannot tell whether or not the weight has changed by simply hefting, so they need to engage in careful measurement to see what is happening. Thus, there are opportunities for continued discussion about issues of weight measurement and measurement error in the context of these investigations.

Investigating freezing and melting exposes new puzzles that cry out for deeper explanation. When water freezes (unlike what happens when you pour it from one shaped container to another), its volume actually increases (as can be demonstrated dramatically when bottles of water “explode” when put in the freezer). How can this be, especially if its weight (and mass) remains the same?

Ultimately, satisfying answers to all these questions call for constructing a particulate model of matter. Although there are many challenges in teaching students about the particulate nature of matter—convincing them there are pre-existing particles of material of fixed size and weight, that these particles are separated by empty space and are in constant motion, that the particles are the matter itself—the (explanatory) benefits are immense. One interesting feature of this model is that it explains what exactly remains the same across freezing/melting. The individual molecules in ice and water are fundamentally the same, just in a different arrangement. Ice molecules are not hard and water molecules are not wet. Appearances can be deceiving indeed.

—Carol L. Smith