by Janice VanCleave
as the air in the atmosphere shifts and changes, warms and cools, so does
the water that fills oceans, lakes, rivers, and streams. In spring, frozen
lakes begin to thaw. Do students think that water moves differently depending
on its temperature? Tell them that water molecules in water contract,
or bunch up, when it turns cold. When these molecules become cold enough,
they are packed so closely together that the water turns into ice. When
the ice warms and melts, the molecules expand, or move farther apart.
Water's behavior also alters with smaller shifts in temperature. The movement
of water due to temperature differences is called convection currents.
These currents occur in oceans and lakes but can also be observed in small
amounts of liquid. Have your intermediate students try the following experiment
to determine how temperature affects the movement of water. Here's how
they can do it.
1. Direct youngsters
to use a pencil point to punch four small and evenly-spaced holes around
the bottom of the paper cup.
Colored water leaves the cup, curling as it flows downward.
When the warm water enters the cup, it is cooled by the ice cubes. The cold water in the cup contracts, and the warm water in the jar expands. The contraction of the cold water makes it denser than the warm water. For this reason, the cold water escaping through the holes sinks through the warm water in the rest of the jar.