The Scientist's Essay for Grade 5, 3. Water to Ice
What's important about freezing and melting?
In the solid the molecules are held tightly in position in a highly ordered structure, rather like the musicians in a well-trained marching band. In the liquid the same molecules are positioned at random, like the musicians hanging around after the game — they're about as densely packed, but not systematically.
Familiar as they are, freezing and melting are really quite remarkable phenomena. Over a wide range of temperatures, ice is just ice and water is just water, and their properties don't really change a whole lot. Then at one very specific temperature (32°F or 0°C) a dramatic transformation occurs — the material changes from liquid to solid (or vice versa) and its density changes noticeably. Even more remarkably, the whole process is reversible: You can take the same water from liquid to solid and back again as many times as you want, and the water at the end will be exactly the same as it was before you started.
The physics of phase transitions, including freezing/melting, is pretty complicated. But there are some interesting observations to make, questions to ask, and connections to draw to other kinds of transformation, like crushing and reshaping. For example: Is water the only materials that melts and freezes, or is it a more general phenomenon? Are there properties that don't change when the material melts or freezes? The weight? The volume? Are the solid and the liquid different materials, or the same? (Raising the question: What do we mean by "the same material"? What kind of evidence is appropriate for answering the question?)
On a microscopic level, we ultimately understand the liquid and the solid as comprising the same molecules, but organized differently. In the solid the molecules are held tightly in position in a highly ordered structure, rather like the musicians in a well-trained marching band. In the liquid the same molecules are positioned at random, like the musicians hanging around after the game — they're about as densely packed, but not systematically. In order to move into the disordered state, the bonds holding the molecules in their array must be partially broken by the thermal energy of the molecules — that's why heat is required to melt the material, and why you can go back and forth between the two states. This is only a partial explanation. It doesn't explain why the transition occurs all at once at a specific temperature, rather than gradually as the temperature increases. But it's a start, and it has the right basic ideas.
—Roger Tobin
