Science Assessment ~ Ideas ~ A pure substance has characteristic properties, such as density, a boiling point, and solubility, all of which are independent of the amount of the substance and can be used to identify it.

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

Sub-Ideas
Items & Student Performance
Misconceptions
NGSS

Students are expected to know that:

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.
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).
The mass of an atom does not change.
If the measured mass has changed, it is because some atoms have not been accounted for.

Boundaries:

Students are not expected to know that mass is not conserved in energy-mass conversions such as nuclear reactions or other subatomic interactions.

Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID Number	Knowledge Being Assessed	Grades 6–8	Grades 9–12
SC043005	When baking soda and lemon juice react in a sealed plastic bag, the weight will not change because the number of each kind of atom does not change.	27%	46%
SC102002	The number of each kind of atom stays the same during a chemical reaction. (This item uses circles to represent atoms.)	52%	58%
SC089005	The number of each kind of atom stays the same during the combustion of propane. (This item uses circles to represent atoms.)	45%	59%
SC090003	The number of each kind of atom stays the same during the reaction between copper and oxygen. (This item uses circles to represent atoms.)	37%	54%
SC045004	Two liquids undergo a chemical reaction in an open jar and bubbles form. The mass of the liquids is less after the reaction because some atoms went into the air.	39%	54%
SC092004	When a liquid changes to a gas in a sealed container and the number of atoms stays the same, the mass of the jar and everything in it also stays the same.	34%	43%

Frequency of selecting a misconception
Misconception ID Number	Student Misconception	Grades 6–8	Grades 9–12
SCM083	New atoms are created during chemical reactions (AAAS Project 2061, n.d.).	49%	34%
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).	30%	29%
SCM075	Atoms can be destroyed during a chemical reaction (AAAS Project 2061, n.d.).	34%	26%
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.).	30%	19%
SCM066	The number of different kinds of molecules, not the number of each kind of atom, is always conserved (AAAS Project 2061, n.d.).	20%	18%
SCM086	Atoms can become lighter during a chemical reaction (AAAS Project 2061, n.d.).	19%	13%
SCM065	The total number of atoms, not the number of each kind of atom, is always conserved (AAAS Project 2061, n.d.).	19%	13%
SCM067	The mass of a closed system will decrease if two substances combine to form one substance (AAAS Project 2061, n.d.).	16%	14%
SCM085	Atoms can become heavier during a chemical reaction (AAAS Project 2061, n.d.).	15%	13%
SCM005	The number of molecules is always conserved (Mitchell et al., 1984).	12%	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.

NGSS Statements
Code	Statement
PS1.B MS	Substances react chemically in characteristic ways. In a chemical process, the atoms that make up the original substances are regrouped into different molecules, and these new substances have different properties from those of the reactants.
PS1.B MS	The total number of each type of atom is conserved, and thus the mass does not change.