Key Idea: When heated, solids can change into liquids and liquids can change into gases. When cooled, gases can change into liquids and liquids can change into solids. These changes of state can be explained in terms of changes in the proximity, motion, and interaction of atoms and molecules.

Students are expected to know that:

  1. When the temperature of a liquid decreases, the average speed of the atoms or molecules decreases and, as a result, the pull that exists between the atoms or molecules is strong enough to link them together as a solid.
  2. When the temperature of a solid increases, the average speed of the atoms or molecules increases and the pull between the atoms or molecules is no longer strong enough to hold them together as a solid; the atoms or molecules are now more loosely connected as a liquid.
  3. When the temperature of a gas decreases, the average speed of the atoms or molecules decreases and, as a result, the pull that exists between the atoms or molecules is strong enough to loosely connect them together as a liquid.
  4. When the temperature of a liquid increases, the average speed of the atoms or molecules increases and the pull between the atoms or molecules is no longer strong enough to hold them together as a liquid. In this case, the pull between atoms or molecules is so weak that they are no longer connected to each other, but rather they exist as a gas.
  5. At the boiling point and freezing point, atoms or molecules from anywhere in the substance can enter the gas state and solid state, respectively. Because of this, the bubbles that form when the substance is boiling are atoms or molecules of that substance in the gaseous state.
  6. Evaporation or condensation can also occur independent of temperature; i.e., at any temperature there are some atoms or molecules that may move from one state to another at the surface of a substance. This also includes atoms or molecules on the surface of a solid that can enter the gas state.
  7. A substance is made up of the same type of atom or molecule regardless of whether it is in the solid, liquid, or gas state. There is no change in the identity of the atoms or molecules during a change of state; only the arrangement, motion, and interaction of the atoms or molecules change.
  8. Atoms or molecules are not destroyed during a change of state.
  9. Any change of state is reversible.

Boundaries:

  1. Although changes of state can be caused by changes in either temperature or pressure, students are only expected to know the effects of changes in temperature.
Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

Grades
6–8

Grades
9–12

AMM061

The molecules of a substance break down into individual atoms when the substance evaporates. During evaporation, water breaks down into hydrogen and oxygen (Bar et al., 1991; Bar et al., 1994).

39%

37%

AMM049

The molecules of a substance break down into individual atoms when the substance boils. For example, molecules of water become atoms of hydrogen and oxygen when water boils (Osborne et al., 1983; Renstrom et al., 1990).

36%

34%

AMM051

When water boils, the bubbles formed during boiling contain air, not water in the gas state (Osborne et al., 1983; Renstrom et al., 1990; Bar et al., 1991; Johnson, 1998a; Chang, 1999).

29%

26%

AMM038

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

25%

17%

AMM043

Molecules change shape during a phase change (Novak et al., 1991; Griffiths et al., 1992).

19%

19%

AMM015

Observable properties of the state are attributed to the individual molecules (e.g., molecules in a solid are hard; molecules move in gases and liquids, but not in solids; or the molecules of the substance change from soft to hard when a liquid freezes) (Lee et al., 1993).

23%

12%

AMM047

The identity of the molecules of a substance changes during a phase change (Lee et al., 1993).

21%

14%

AMM045

Molecules change weight/mass during a phase change (Griffiths et al., 1992).

19%

14%

AMM133

Matter is destroyed during melting (AAAS Project 2061, n.d.).

15%

10%

AMM044

Molecules change size during a phase change (Novak et al., 1991; Griffiths et al., 1992).

11%

9%

AMM132

Matter is destroyed during boiling (AAAS Project 2061, n.d.).

11%

7%

AMM131

Matter is destroyed during evaporation (Bar & Galili, 1994; Bar & Travis, 1991; Lee et al., 1993; Osborne & Cosgrove, 1983)

11%

7%

AMM063

When water evaporates from an object, that water is absorbed into the object (Osborne et al., 1983; Bar et al., 1991; Bar et al., 1994).

7%

7%

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.