### Targeted Learning Goal:

For any single state of matter, changes in temperature typically change the average distance between atoms or molecules. Most substances or mixtures of substances expand when heated and contract when cooled. (from Benchmarks for Science Literacy 4D/M3b)

### What Students are Expected to Know

• As the temperature of a substance increases, the average distance between the atoms/molecules of the substance typically increases, causing the substance to expand.
• As the temperature of a substance decreases, the average distance between the atoms/molecules typically decreases, causing the substance to contract.
• Thermal expansion or contraction can happen to solids, liquids, and gases.
• Expansion or contraction due to changes in temperature can also happen to mixtures of substances.
• Expansion or contraction due to changes in temperature is not permanent (e.g., objects that expand when heated then contract when cooled).
• The number of atoms and the mass of the atoms do not change with changes in temperature.
• Different substances expand and contract differently.

### Example Phenomena

Thermal Expansion and Contraction of a Solid
 A thermal expansion kit consisting of a metal ball and a metal ring, both mounted on wooden handles is used to illustrate the idea that solids expand when heated and contract when cooled. Students should observe that the ball fits through the ring when both are at room temperature, but the ball does not fit through the ring when the ball is heated. Students should also observe that after the ball has cooled back down to room temperature, it fits through the ring again. Students need to infer that when the ball can no longer pass through the ring, it is because the ball has increased in size and that when the ball cools back to room temperature and can fit through the ring, the ball's size has decreased, even though it may look the same to the students. A second, unheated ball could be used to demonstrate that the ring has not changed in size.

Thermal Expansion of a Liquid in a Thermometer
 To appreciate that liquids expand when heated and contract when cooled, students are shown a mercury thermometer and observe the level of mercury rising as the thermometer is heated and falling as the thermometer is cooled. Students need to infer that when the level rises, the liquid in the bulb is expanding, and when the level falls, the liquid in the bulb is contracting. Note that the glass of the thermometer is also expanding. However, the mercury expands 10 times more than the glass for a given temperature change. Students should not handle mercury thermometers since mercury is highly poisonous. Students may use alcohol thermometers for hands-on activities.
Gas Contracting in a Mylar Balloon
 Mylar balloon is filled with helium gas at room temperature and sealed. Mylar  balloons work for this phenomenon because they do not stretch like rubber balloons. Students should observe that when the balloon is cooled, it looks deflated and when the balloon returns to room temperature, it looks inflated. Students must infer that the balloon looks deflated because the gas in it contracted  when cooled and the balloon re-inflates when it returns to room temperature is  because the gas in it expanded. The thermal expansion of gases is much more noticeable than solids and liquids.  One liter of gas expands 34 mL for a temperature increase of 10ÂșC. For all phenomena dealing with the expansion and contraction of gases, use a pliable  container to allow the gas to expand and contract safely.