Rise and Fall of the KMT

rise and fall Ben watching for change in hot (red) water and cold (blue) water.

In one of the elegant labs from TOP Science, called “Rise and Fall” you simply fill one baby food jar with cold water (I colored it blue to make it easy to identify) and fill another baby food jar with warm or hot water (which I colored redo to make it easy to see and identify.) Students place the warm water on top (see picture above) and watch … and watch … no change. The war red water stays inside the top jar and the cool blue water stays in the bottom jar even though there is no barrier between the two jars.

Do the same thing, only place the blue (cold) water on top and the red (warm) water on the bottom, and removing the barrier between the jars lets the cold blue water on top fall down into the lower jar and the warm red water on the bottom swirl up into the top jar. The result is two jars full of a homogeneous purple colored water.

This makes a great demonstration: clear visual events that look initially intuitive (the waters stay in each jar and do not mix) but then make you wonder – the second arrangement mixes. Why is there a difference?

We just recently completed a unit on density and now we are studying the “KMT” (kinetic molecular theory). After looking at the basic properties of three states: solid, liquid, gas we related their state to the KMT. Energy causes movement; temperature is a measure of kinetic energy. Now we are linking prior knowledge (density) to the KMT (more energy increases molecular motion so a given material will be less dense when it is heated.

This is why I love teaching: doing fun stuff, making a mess, using experience to learn about how cool the material world is!

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