Key Idea: For any single state of matter, increasing the temperature typically increases the distance between atoms or molecules. Therefore, most substances expand when heated.

Students are expected to know that:

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

Boundaries:

  1. Students are not expected to know the details of the relationship between the speed of the atoms or molecules and thermal expansion.
  2. They are also not expected to know the substances that violate this rule and shrink when heated or that water will shrink when heated anywhere between 0°C and 4°C.
  3. Students are not expected to know or apply gas law equations.
  4. Because the definition of the size of an atom is varied and complex, we only expect students to know that the size of an atom or molecule does not decrease when the temperature increases and that the size does not increase when temperature decreases.
Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

Grades
6–8

Grades
9–12

AMM089

Solid substances do not expand or contract with changes in temperature (Herrmann-Abell & DeBoer, 2007, 2008).

34%

25%

AMM096

The mass of the atoms or molecules of a substance increases when the temperature increases and decreases when the temperature decreases (AAAS Project 2061, n.d.).

25%

22%

AMM122

The average distance between the atoms or molecules of a substance remains the same when the temperature of the substance changes (AAAS Project 2061, n.d.).

25%

18%

AMM038

Heat is made of "heat molecules" (Berkheimer et al., 1988).

24%

16%

AMM086

The number of atoms or molecules of a substance increases when the temperature increases and decreases when the temperature decreases (Herrmann-Abell & DeBoer, 2008).

21%

13%

AMM019

Water molecules break down when heated (Griffiths et al., 1992).

19%

14%

AMM091

The mass of the atoms or molecules of a substance increases when the temperature decreases and decreases when the temperature increases (Herrmann-Abell & DeBoer, 2007, 2008).

19%

15%

AMM129

The molecules of air break down when the air is cooled (AAAS Project 2061, n.d.).

19%

12%

AMM037

Substances shrink when heated (especially solids) (Lee et al., 1993).

15%

12%

AMM087

The number of atoms or molecules of a substance increases when the temperature decreases and decreases when the temperature increases (Herrmann-Abell & DeBoer, 2007).

16%

9%

AMM076

The size of the atoms or molecules of a substance increases when the temperature decreases and decreases when the temperature increases (AAAS Project 2061, n.d.).

14%

11%

Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.