CTS Guide: States of Matter, pp 166-167- Section IV Research Summaries
Phase Changes from Liquid to Gas (boiling, evaporation)
Some middle and high school students view phase changes as chemical changes. As a result, some students think that when a liquid changes to a gas, it separates the molecules into the individual atoms (Aydeniz and Kotowski 2012).
It has been well documented that secondary students think the gas produced from boiling water is a mixture of hydrogen and oxygen gas (Mayer 2011).
Students’ understanding of boiling precedes their understanding of evaporation from surfaces such as dishes and roads. In a sample of students ages 6–8, 70% understood that when water boils, vapor comes from it and that the vapor is made of water. However, the same students did not recognize that when a wet surface dries, the water turns to water vapor (Driver et al. 1994).
An analysis of middle school students’ test results showed that explaining changes of state in molecular terms was among the most difficult tasks for many students. Before instruction, almost none of the students in the study could give molecular explanations of changes of state. After instruction, significantly more students demonstrated understanding, although many students still had difficulties understanding changes of state in molecular terms. For example, one student thought that when molecules are in the water, they move farther apart, they move faster, and then they turn into air (evaporate) (Lee et al. 1993).
Osborne and Cosgrove (1983) studied New Zealand students ages 8–17. An electric kettle was boiled in front of the students so they could see the bubbles in the boiling water. They were asked what the bubbles were made of. Heat, air, oxygen, hydrogen, and steam were common replies, with the percentage of students answering steam increasing between ages 12 and 17. However, most 17-year-old students thought that water can be split into its component elements by heating, or that heat is a substance in its own right, or that air is contained in water. The researchers attribute the idea that water molecules break up to knowing the formula of water is H2O, so naturally it comes apart.
Phase Changes from Solid to Liquid (melting)
Studies show young children have difficulty distinguishing between melting and dissolving. “Although two materials are required for the dissolving process, children tend to focus only on the solid and they regard the process as melting” (Driver et al. 1994, p. 80). When things dissolve they often describe them as “melting away.” When things melt, children have described them as being similar to “ice going runny.”
Cosgrove and Osborne (1980) sampled 8- to 17-year old children and found that they regarded melting and dissolving as similar processes since they were both gradual. They also found that melting was unconnected to the concept of a particular melting point. They also interviewed students about their ideas related to changes in state and noticed that students generally do not regard a change in state as being related to a specific temperature.
Temperature and Phase of Matter
Driver and Russell (1982) carried out a study in which children were shown a beaker of ice with a thermometer reading of 0°C. The children were asked what would happen if more ice was added. Most of the 8-year- old children thought that the temperature would go up when more ice was added. Older children, up to age 14, thought that the temperature would decrease when more ice was added.
Presence of Two Phases
In a study of high school students’ understanding of basic chemical concepts, students were asked to explain a change from ice to water when two phases were present. Some students thought the reason for the constant temperature was because it takes a period of time for the thermometer to change, and others thought the heat was not hot enough (Abraham and Williamson 1994).
A standard laboratory exercise is to plot a time-temperature graph of water as it changes from melting ice to boiling water. Although students can readily see the steady temperature as they make their observations of the boiling water, the counterintuitiveness of the phenomenon often results in disbelief statements such as, “This thermometer is not working properly” (Erickson and Tiberghien 1985, p. 64).
Solids
Students tend to recognize materials like metals and wood as being solids. However, students have difficulty categorizing materials that are not hard or rigid as solids. Fifty percent of 12- to 13-year-olds classified nonrigid solids like dough, sponge, sand, and sugar differently from coins, glass, or chalk. They suggest that “the easier it is to change the shape or state of the solid, the less likely it is to be included in the group of solids” (Kind 2004, p. 6).
Some children regard powders as liquids, and any nonrigid materials, such as a sponge or a cloth, as being somewhere in between a solid and liquid. Their explanation of powders as liquids is often “because they can be poured.” Reasons for nonrigid objects as being neither solid nor liquid is because they “are soft,” “crumble,” or “can be torn.” Thus children characterize the state of matter of a material according to its macroscopic appearance and behavior with the result that solids are associated with hardness, strength, and an inability to bend. By age 11, students tend to regard a powder as being an intermediate state, rather than a liquid (Driver et al. 1994).
Children frequently consider atoms of a solid to have all or most of the macroproperties they associate with the solid (Driver et al. 1994).
A study of children’s ideas about solids conducted with Israeli children ages 5–13 showed that younger children tend to associate solids with rigid materials (Stavy and Stachel 1984).
Liquids
Children can classify liquids more easily than they can solids, perhaps because liquids are less varied in their physical characteristics (Kind 2004).
Some students tend to regard liquids as continuous (nonparticulate) and static (Driver et al. 1994).
Gases
Because students cannot observe gases, they often do not think of a gas as being the same type of matter that makes up solids and liquids (Mayer and Krajcik 2017).
Students may believe that matter does not include gases or that gases are weightless materials (AAAS 2009).
Gases pose special difficulties for children since the ones they commonly experience, like air and helium, are invisible. It is suggested that this invisibility prevents students from developing a scientific conception of a gas. Explicit instruction is needed for children to understand the properties of a gas, including properties like mass and weight. This is in contrast to solids and liquids where students tend to learn about them intuitively (Kind 2004).
Researchers have suggested that one of the reasons students fail to recognize gases as having weight or mass is because one of their most common experiences with gases is with those that tend to rise or float (such as the helium). This view is supported by studies that show that children ages 9–13 tend to predict that gases have negative weight, such that when a gas like helium is added to a balloon, it will weigh less. Students believe that the more gas that is added to a container, the lighter the container becomes (Driver et al. 1994).
Some students believe a warmed gas weighs less than the same gas that is cooler (Driver et al. 1994).
The idea that air or gas has mass is not obvious to children. Yet, when it is taught, it is a concept children can acquire easily and remember (Sere 1985).