Melting point

While digging through Cole Blaq's photostream to find an appropriate spray can model for my previous blog post, I once again enjoyed looking at his Enter the Brick series of sculptures based on the basic LEGO brick. One that deserves some highlight here is his melting point. The melting point is the temperature at which a solid changes into a liquid form. When I'm teaching about melting point, I often use LEGO bricks as an analogy (or just basic brick walls). A solid crystal is like a wall built of overlapping LEGO bricks. Each molecule is interacting with the molecules around it via a series of forces. As temperature is added, these molecules vibrate more and more. At some point, the force of the molecules vibrating is greater than the force holding them together, and so they start to slide past each other--i.e. go from the solid phase to the liquid phase. In a pure sample, each 'brick' is in a completely identical situation, and so each 'brick' is held to its neighbors by identical forces. Because of this, it takes the exact same amount of heat energy to break apart each of the bricks, and so there is a sharp melting point (that is, everything melts within a couple of degree range). On the other hand, if you have a mixture of different types of bricks, they don't fit together as neatly or as regularly. Therefore it takes less heat energy to cause melting (this is why, btw, you dump salt on the ice on your sidewalk), and, since each brick is in a slightly different situation and is therefore held to its neighbors by a different amount of force, the melting happens over a much broader range.