Key Idea: Whenever substances within a closed system interact with one another, the total mass of the system remains the same.

Students are expected to know that:

  1. When substances mix, undergo chemical reactions, change state, or dissolve, or when objects are cut or broken into smaller pieces, the total mass of all the matter will always remain the same.
  2. Regardless of the form that the products of these processes may take (for example, when a sugar cube dissolves in water or a chemical reaction produces a gas), the mass will always stay the same.
  3. If it appears that the mass has changed, it is because some material has not been accounted for.
  4. Because light is not matter, its presence or absence does not affect the mass of the matter.

Boundaries:

  1. Students are not expected to know that mass is not conserved in energy-mass conversions such as nuclear reactions or other subatomic interactions.
  2. Note: The words “weight,” “weigh,” and “mass” are used appropriately in the assessment items. The students are not expected to know the difference between “weight” and “mass.”
Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

Grades
6–8

Grades
9–12

SCM029

Mass is not conserved during processes in which gases take part (Mas et al., 1987; Berkheimer et al., 1988; Hesse et al., 1992). If a gas is produced during a chemical reaction that takes place in a closed system, the total mass decreases (Ozmen et al., 2003).

55%

50%

SCM032

In a closed system, the total mass increases during a precipitation reaction (Barker et al., 1999; Ozmen et al., 2003).

54%

49%

SCM064

When a liquid in a closed container is heated, the mass of the liquid increases as the liquid expands (AAAS Project 2061, n.d.).

48%

34%

SCM041

During biological decomposition in a closed system, the total mass of the system decreases (Mitchell et al., 1984).

41%

32%

SCM028

In a closed system, mass decreases after a solid dissolves in a liquid (Stavy, 1990b; Ozmen et al., 2003).

27%

20%

SCM027

When a chemical reaction occurs, matter just disappears. For example, gasoline is used up in the car and disappears (Andersson, 1986).

16%

11%

SCM051

Mass increases after a solid dissolves in a liquid (Stavy, 1990b).

15%

13%

SCM010

Matter can disappear with repeated division, dissolving, evaporation, or chemical change (Smith et al., 2004).

10%

10%

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.