Key Idea: Whenever atoms interact with each other, regardless of how they are arranged or rearranged, the total mass stays the same.

Students are expected to know that:

  1. Atoms are not created or destroyed when substances mix, undergo chemical reactions, change state, or dissolve, or when objects are cut or broken into smaller pieces.
  2. The total number of each kind of atom always remains the same regardless of what happens to the matter (mixing, chemical reactions, changes of state, dissolving, or objects being cut or broken into smaller pieces).
  3. The mass of an atom does not change.
  4. If the measured mass has changed, it is because some atoms have not been accounted for.

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.
Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

Grades
6–8
Grades
9–12

SCM086

Atoms can become lighter during a chemical reaction (AAAS Project 2061, n.d.).

See the Original Project

SCM085

Atoms can become heavier during a chemical reaction (AAAS Project 2061, n.d.).

See the Original Project

SCM083

New atoms are created during chemical reactions (AAAS Project 2061, n.d.).

See the Original Project

SCM075

Atoms can be destroyed during a chemical reaction (AAAS Project 2061, n.d.).

See the Original Project

SCM068

The mass of a closed system will increase if a new kind of molecule is formed in the system (AAAS Project 2061, n.d.).

See the Original Project

SCM067

The mass of a closed system will decrease if two substances combine to form one substance (AAAS Project 2061, n.d.).

See the Original Project

SCM066

The number of different kinds of molecules, not the number of each kind of atom, is always conserved (AAAS Project 2061, n.d.).

See the Original Project

SCM065

The total number of atoms, not the number of each kind of atom, is always conserved (AAAS Project 2061, n.d.).

See the Original Project

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).

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SCM005

The number of molecules is always conserved (Mitchell et al., 1984).

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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.