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:
- 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.
- This 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.
Boundaries:
- Students are not expected to know the details of the relationship between the speed of the atoms or molecules and thermal expansion.
- 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.
- Students are not expected to know or apply gas law equations.
- 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.
| Item ID Number |
Knowledge Being Assessed | Grades 6–8 |
Grades 9–12 |
Select This Item for My Item Bank |
|---|---|---|---|---|
50% |
65% |
|
||
The molecules of hot air are farther apart than the molecules of cold air. | 49% |
60% |
|
|
The molecules of cold water are closer together than the molecules of hot water. | 46% |
59% |
|
|
When an iron frying pan cools, the distance between the iron atoms decreases. | 45% |
59% |
|
|
39% |
59% |
|
||
The distance between water molecules increases as a cup of water is warmed. | 38% |
58% |
|
|
39% |
58% |
|
||
30% |
43% |
|
||
27% |
47% |
|
||
26% |
33% |
|
Misconception |
Student Misconception |
Grades |
Grades |
|---|---|---|---|
34% |
25% | ||
25% |
22% | ||
25% |
18% | ||
24% |
16% | ||
21% |
13% | ||
Water molecules break down when heated (Griffiths et al., 1992). | 19% |
14% | |
19% |
15% | ||
The molecules of air break down when the air is cooled (AAAS Project 2061, n.d.). | 19% |
12% | |
Substances shrink when heated (especially solids) (Lee et al., 1993). | 15% |
12% | |
16% |
9% | ||
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.
No NGSS statements are associated with this idea in the selected project.