CTS Guide: Conservation of Matter, pp 154-155- Section IV Research Summaries
Concept of Conservation of Matter and Conservation Reasoning
Learning chemistry vocabulary that has both scientific and everyday meanings, known as dual meaning vocabulary (DMV), can be challenging for many students. This may explain why some students do not understand what conservation of mass means. (Song and Carheden 2014).
Liu and Lesniak (2006) examined students’ conceptual development of matter from elementary to high school using released TIMSS (Trends in International Mathematics and Science Study) data and found that students’ understanding of the four big core ideas about matter (conservation of matter, physical properties and change, chemical properties and change, and structure and composition) are not learned separately but are interrelated. They found that on average, pertaining to conservation of matter, 3rd and 4th graders could not reach a competence level for learning conservation; 7th graders could reach a competence level for learning conservation and physical properties and change aspects of matter; 8th and 12th graders could reach a competence level for learning all the matter aspects except for structure and composition; and 12th-grade students with a science specialization could reach a competence level for learning all four aspects.
Studies have found that even after studying chemistry, many students have a difficult time connecting ideas about atoms to observable phenomena (Treagust, Chittleborough, and Mamiala 2003). Even if students know that matter is conserved, they may fail to use the idea of atoms in their explanation.
Many high school students will still use sensory reasoning when thinking about conservation of matter, despite being well advanced in thinking logically and using mathematics (Barker 2000).
Stavy and Tirosh (2000) investigated intuitive rules used by students of all ages to explain conservation problems. One of these rules is called “Less A- Less B.” If it looks like there is less material present after a change, they may reason that the weight or mass would be less. Alternatively, the rule “More A- More B” is used when students perceive something as being larger or taking up more space after a change as having more weight or mass.
One interesting finding is that children who recognize conservation believe more firmly in their answers on conservation tasks when paired with non-conservers as partners, and they are able to offer multiple explanations and are more likely to manipulate the task materials to prove their point than non-conservers (Miller and Brownell 1977).
Conservation reasoning was a hallmark of Piaget’s studies of children’s cognitive development (Piaget and Inhelder 1974).
Conserving Matter (or Mass) During a Change in State
Results of a study by Adeniz and Kotowski (2012) suggest that middle and high school science teachers need to place a greater and explicit emphasis on students’ understanding of the law of conservation of mass during phase changes, the fact that phase changes are physical changes and do not result in a change in chemical composition, and that the observed changes in the physical appearances of molecules during melting are the result of weakening intermolecular forces. Unless teachers keep these principles in mind when planning and enacting instruction, students may develop misconceptions about the particulate nature of matter.
Student understanding of conservation of matter begins with qualitative notions. In a study conducted by Stavy (1990), by fifth grade, students qualitatively understood matter was conserved when it changed from a solid to a liquid but they were just beginning to understand the change quantitatively (AAAS 1993).
Several studies have shown that the way students perceive a physical or chemical change determines whether they recognize the material is conserved during the change (Driver et al. 1994).
In a study by Stavy (1987), students were shown two samples of ice that had the same weight. One sample was melted and then students were asked to compare the melted sample with the unmelted ice. The percentage of students who used conservation reasoning increased with age: 5% ages 5 and 6, 50% age 7, and 75% age 10.
A study by Osborne and Cosgrove (1983) found that some children regard the liquid form of a material as differing in weight from the same amount of mass as its solid form.
Conserving Matter (or Mass) During Dissolving
Barker and Millar (1999) reported a study in which 250 students were asked what they thought the mass of a solution of salt (sodium chloride) would be compared with the mass of the solute and solvent before they are combined. About 57% of 16-year-olds thought the masses would have the same value, 16% thought that a gas would be released when the salt dissolves, and 7% thought that mass was lost during the dissolving process. By the age of 18, the percentage giving the correct answer was 62%, yet 15% still thought a gas was produced and about 4% thought mass was lost. These data suggest that some students may think dissolving is a chemical reaction, and that release of a gas changes the mass.
Students’ ideas about what happens to sugar as it dissolves frequently fail to include the conservation of mass. The gap between the proportion of students who conserved substance but not mass widened between ages 9 and 11 but narrowed in later school years. After age 12, many, but not all, students begin to develop a conception of weight and mass and begin to conserve mass of the solute (Driver et al. 1994).
A study of middle school students showed that at the macroscopic level, some students did not understand conservation of matter during dissolving. Some thought the sugar “kind of evaporated” from the water or it melted away. Others thought that since you couldn’t see the sugar in water after it dissolved that it no longer existed. Some described it as dissolving into nothing (Lee et al. 1993).
Older students may believe that a solution is lighter than the sum of the weights of a solute and solvent because the solute becomes smaller and smaller until it disappears (Stavy 1990).
Several researchers have investigated students’ conservation ideas in the context of dissolving. Discrepancies have been found between students who conserve a substance but fail to conserve weight. Holding (1987) found a high percentage of eight-year-olds believed that a solute was somehow present in some form when it was dissolved. When he probed students about the weight of the solution, however, only 50% of those who thought the sugar was still there felt it had weight. One reason for this appears to be that students thought the dissolved sugar was in a “suspended” state; thus it was not pressing down on the container as “weight”.
Driver (1985) described three different types of reasoning students use in determining whether matter is concerned during dissolving: (1) solute disappears into “nothing” when dissolved, (2) mass and volume are confused, and (3) solute is still present in the solution but is lighter.
Piaget’s early studies showed that young children think that sugar “disappears” when dissolved in water, and they fail to use conservation reasoning to account for the sugar. They are content with the notion that the weight of water would not change, because the substance added to it no longer exists (Piaget and Inhelder 1974).
Conserving Matter (or Mass) During a Chemical Reaction
Several studies have shown that the way students perceive a physical or chemical change determines whether they recognize the material is conserved during the change. For chemical reactions that evolve gas, mass conservation is more difficult for students to understand. If a chemical reaction results in the apparent disappearance of some materials, some students do not recognize that mass is conserved. More than half of a group of 15-year-olds considered to have “above average ability” predicted loss of mass on the combustion of a sample of iron wool. Another difficulty is that many students do not recognize the quantitative aspects of a chemical change and the conservation of overall mass. (Driver et al. 1994).
Research shows that some students have a difficult time conserving matter in a closed container when a gas is involved and the volume of the container changes such as a balloon in a cold room. They confuse volume with quantity (Sere 1985).
Conservation of Matter in Other Contexts Such as Life and Earth Science
Research indicates that few students view conservation of matter in different contexts such as photosynthesis, assimilation of food, and respiration. This may also be related to difficulties students have in attributing growth, such as a seedling growing from a seed, to food being transformed to make up new material (Driver et al. 1994).
Leach et al. (1992) found that even at age 16, few students have a view of matter that involves conservation in a life science context.
Volume, Mass and Weight
Young children’s ideas about weight are strongly associated with how heavy something feels (Snir, Smith, and Raz 2003).
Students’ confusion in distinguishing between weight and mass is often because the two terms are used interchangeably. Hewitt (2002) found that some of this confusion comes from the way English (pounds) and SI units (grams) are used in identical ways.
It has been found that children view the change of a bulk solid to a powdered solid as likely to result in a decrease in mass (Driver et al. 1994).
Students will have difficulty conserving matter if they cannot distinguish between weight and density (AAAS 1993).
A study by Holding (1987) investigated students’ conceptions of mass. Some students confuse the word mass with the word massive and hence their conception of change in mass was dominated by the bulk appearance of the material..
Research shows that some students have a difficult time conserving matter in a closed container when a gas is involved and the volume of the container changes. They confuse volume with quantity (Sere 1985).