What will usually happen to matter that is cooled?

What will usually happen to matter that is cooled? It contracts. In most substances, cooling lowers the average energy of atoms or molecules, so they vibrate and move less vigorously. The particles generally occupy slightly less space, reducing the material’s length, area, or volume. This everyday behavior is called thermal contraction, the reverse of thermal expansion.

The effect is important in engineering. Bridges and railway tracks include expansion gaps because materials expand in heat and contract in cold. Metal lids can also loosen when warmed because the lid expands slightly faster than the glass jar. Uneven cooling can create thermal stress, which is why hot glass may crack when placed in cold water.

“Usually” matters because water behaves unusually near its freezing point. Water reaches its greatest density at about 4 °C; cooling it below that temperature makes it expand until it freezes. Ice is less dense than liquid water, so it floats. This exception helps lakes freeze from the surface rather than solidifying completely from the bottom.