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

Sub-Ideas
Items & Student Performance
Misconceptions
NGSS

Students are expected to know that:

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.
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.
If it appears that the mass has changed, it is because some material has not been accounted for.
Because light is not matter, its presence or absence does not affect the mass of the matter.

Boundaries:

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

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
SC078003	Mass is conserved when a stick of butter is cut into pieces.	70%	73%
SC056004	The weight of a jar containing water and sugar stays the same after some of the sugar dissolves.	37%	53%
SC094004	Mass is conserved when a plant dies in a sealed jar.	36%	49%
SC059004	When a chemical reaction occurs in a sealed container, the mass of the materials in the container stays the same.	31%	48%
SC084004	As a thermometer is heated and the level of liquid in the thermometer rises, the mass of the liquid stays the same.	29%	44%
SC077005	If a chemical reaction occurs between two liquids in a sealed jar, the mass will not change if a gas is formed, and it will not change if a solid is formed.	24%	34%

Frequency of selecting a misconception
Misconception ID Number	Student Misconception	Grades 6–8	Grades 9–12
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%
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%
SCM041	During biological decomposition in a closed system, the total mass of the system decreases (Mitchell et al., 1984).	41%	32%
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%
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.

NGSS Statements
Code	Statement
PS1.B Grade 5	No matter what reaction or change in properties occurs, the total weight of the substances does not change. (Boundary: Mass and weight are not distinguished at this grade level.)