Learning Lesson: Crunch Time

 Overview

Pressure is not only a matter of altitude but also the temperature. If the volume of a space doesn't change, then as the temperature increases, so does the pressure. The molecules and atoms that comprise the air gain energy as they absorb heat. That increase in energy results in faster moving atoms, which we observe as an increase in pressure.

The opposite occurs when the temperature decreases. As the molecules lose energy, their motion is decreased, and we observe a decrease in pressure. The students will see a plastic 2-liter bottle crushed by the normal atmospheric pressure in the room due to localized decreasing temperature.

Procedure

1. Place two cups of hot tap water into each two 2-liter bottle.
2. Place your thumb over each bottle opening and shake the bottles. This ensures the air inside the bottle is warmed.
3. Pour the water out of each bottle and quickly screw a bottle cap on only one of the two bottles.
4. Stand both bottles side-by-side and observe over the next five minutes.

Discussion

As air cools inside the uncapped bottle, the molecules loose their energy, leading to a lowering of the pressure inside of the bottle. This lowered pressure pulls outside air through the bottle opening, immediately equalizing the pressure.

As with the uncapped bottle, air inside the capped bottle also cools as molecules loose their energy. However, with no replacement air able to enter, the cooling temperature results in lower air pressure inside compared to outside the bottle. The higher outside pressure begins to slowly crush the capped bottle, changing the volume until the inside and outside pressures are equal.