Students’ ideas about changes of state

Poor understanding of the four basic aspects of particle theory affects students’ thinking about changes of state. Students’ ideas about this have been studied extensively.

As many students aged up to 18 years do not appreciate that particles are moving, unsurprisingly they find it difficult to explain scientifically what happens when a gas is heated or cooled.

What happens when a gas is heated?

Novick and Nussbaum (1981) report that about 40% of 16-year-olds think increased particle motion is the main effect of heating a gas. Over 40% of students aged 16 suggest that “particles are forced apart”, while another 20% used the notion of repulsive forces. The CLIS study (Brook et al, 1984) reports similar response levels to a question about air pressure in a car tyre.

About 12% of 15-year olds use ideas suggesting that increasing forces between particles cause a change in car tyre pressure during a journey. Séré (1982) studied 11 - 13 year olds’ ideas about air pressure, noting that children use mechanistic terms like “force” to describe visual effects. Brook et al also found replies using ideas like particles “swelling”, or simply occupying more space.

What happens when a gas is cooled?

Decreasing in particle motion on cooling seems to be harder to understand than that particle motion increases on heating. Recall that about 40% of 16 year olds thought that increased particle motion was the main effect heat has on gas particles. The converse question yielded correct responses from less than 30% of 16 - 18 year old students and only 20% of university students (Novick and Nussbaum, 1981).

This difference could be because fewer practical examples of cooling gases are available to assist understanding. Approximately 50% of students of any age offered descriptive responses to the question on cooling of gases, including ideas about particles being able to ‘shrink’, ‘condense’, ‘sink’ or ‘settle’.

Taken to an extreme, the cooling of a gas leads to liquefaction. Novick and Nussbaum found that students may represent this pictorially by drawing particles of air accumulating around the sides or at the bottom of the vessel. Approximately 70% of students from age 13 to university level drew this sort of picture, suggesting that misconceptions about liquefaction are widespread. Novick and Nussbaum (1981) state that

“…many high school students attribute the decrease in volume of a gas on cooling not to decreased particle motion but to increased attractive forces.” (p 192)

Evaporation

…among young children

Young children gain experience of evaporation. Russell et al (1989) report that infants notice evaporation has occurred, but focus on the remaining water, saying some has “disappeared”. About one-fifth of 7 - 9 year olds acknowledge that water has gone, but think an outside agent, like another person or the Sun is responsible. Children may also think water soaks into the pan when it is boiled in front of them (Beveridge, 1985), or “went into the plate” if just left to evaporate (Cosgrove and Osborne, 1981). Closer to particle ideas,Russell and Watt (1990) note that other children in the primary age range think water transforms into mist, steam or spray (28%) while a further group describe water as changing to an imperceptible form (17%), such as water vapour or a `gas’.

“I think the water has split up into millions of tiny micro bits and floated up..” (Russell and Watt, 1990 p 33).

Older children produce the same explanations, but in different proportions, for example, about 57% of the 9 - 11 age group use the idea of an outside agent.

These ideas indicate that thinking about evaporation is linked to understanding conservation of matter. In suggesting that an outside agent has removed the water, children seem to conserve the amount of material, but offer a faulty explanation about why the water disappears. They use sensory-based reasoning, applying what are to them satisfying explanations for an invisible change.

…and among secondary school students

Stavy (1990b) studied the link between evaporation and conservation of matter in detail among 9-15 year olds who had been taught particle theory. She examined their responses to two tasks (also reported in Stavy 1990a). Her results suggest that 50% of 15 year olds do not conserve the amount of matter in evaporation. Stavy suggests that confusion arises because of teaching about density and weight. Students say “gas weighs less than liquid”, so there is less gas present, thus explaining evaporation in terms of weight change (incorrect) rather than density change (correct).

Osborne and Cosgrove (1983; also reported in Cosgrove and Osborne, 1981) studied New Zealand students aged 8 - 17 years. An electric kettle was boiled in front of respondents so that bubbles could be seen in the boiling water. They were asked, “What are the bubbles made of?” The replies included that the bubbles were made of heat, air, oxygen or hydrogen and steam. Over 700 students answered the question, giving the same responses. Proportionately, these varied from age 12 - 17 as follows:-

Heat	30% - 10%
Air	30% - 20%
Oxygen/Hydrogen	25% - 40%
Steam	15% - 30%

These data show that the number offering a correct response, steam, increases between the ages of 12 and 17. However, most 17 year olds think either 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. Osborne and Cosgrove attribute these to the influence of teaching; by this age students know the formula of water is H2O, so imagine that water molecules break up on heating.

Johnson (1998b) carried out a longitudinal study of 11–14-year-olds using Cosgrove and Osborne’s questions to explore their thinking about changes of state. He considers that encouraging students to understand boiling water as a state change is important in developing their idea of “gas” as a substance. He argues that teaching particle ideas plays a key role in helping 11–14-year-olds accept that bubbles in boiling water are water changed to the gas state. In his later paper (1998c), Johnson suggests that the key point is:

“…that pupils needed to develop and understanding of the gas state that could see water both by itself and as a mixture with the air.” (p 708)

Kruger and Summers (1989) used questions similar to those of Cosgrove and Osborne in their work with primary school teachers. They found that these adults did not use particle ideas often, explaining the phenomenon of evaporation in macroscopic terms. This adds to the evidence presented earlier indicating that people do not readily change their naïve ideas about particles and matter, retaining child-like perceptions into adulthood.

Condensation

Osborne and Cosgrove (1983) report children’s ideas about condensation. They held a saucer in the steam leaving a boiling kettle and asked, “What is this on the saucer?”. Many 10 - 13-year-olds said the plate had become “sweaty” or simply “wet”. Others of the same age and older said, “The steam turns back into water”, or “The oxygen and hydrogen recombine to from water.” About one quarter of the 13 - 17-year-olds interviewed gave a correct response.

Osborne and Cosgrove collected four major explanations about the origin of water condensing on the outside surface of a sealed glass jar containing ice. These are: “water comes through the glass” (age 8 - 15); “coldness comes through the glass” (age 12 - 17); “the cold surface and dry air (oxygen and hydrogen) react to form water” (age 12 - 17); and “water in the air sticks to the glass” (age 14 - 17). The proportion of 16 - 17-year-olds thinking that coldness or water came through the glass was very small, although around 30% of this age group used the idea that gases recombine on the surface to give water.

The authors note that correct responses using particle ideas were exceptions, and that

“…more ideas to do with particles moving and colliding appeared to be understood by older pupils, but sustained probing of these ideas did not produce sound scientific explanations in terms of intermolecular forces or of loss of kinetic energy.” ( p 830)

The tenacity of misconceptions suggests that even 16-year-old students may find it difficult to apply basic particle ideas to practical situations.

Melting

Cosgrove and Osborne (1983) report three major ideas expressed by 8 - 17-year-olds shown ice melting on a teaspoon. The response that the ice “just melts and changes into water” was common. 12 - 13-year-olds suggested frequently that the ice is “above its melting temperature” while 14 - 17-year-olds thought commonly that “The heat makes the particles move further apart”. A small number of 14- to 17-year-olds used particle ideas.

Brook et al (1984) asked 15-year-olds to explain what happens to ice when it is removed from a freezer at -10oC and left to warm to -1oC. About half of the replies used particle ideas but showed misconceptions in their application. Examples of these answers include:-

“The block of ice cools and the particles are beginning to break away from each (other) to form gases.” (p 53)

“The particles start to break away from each other because of the rise in temperature. When they have broken away from each other, they turn from a crystal form to a solution form.” (p 53)

The first reply confuses melting with evaporation whilst the second introduces the idea of dissolving.

Other respondents applied macroscopic ideas such as particles expanding and contracting, for example,

“As the temperature rises, the particles take in the heat and begin to expand.” (p 56)

“When a block of ice is taken out of a freezer the sudden change of temperature reacts on the particles making them decrease in size.” (p 57)

Other suggestions included that the particles melted or died. However, the question asked was not testing ideas about change of state explicitly, since the temperatures used in the question were both below zero centigrade. So, some of the ideas expressed by students may have resulted from confusion about what they were actually being asked, or interpreted the question as though the ice would melt.

Freezing

Children’s ideas about freezing have not been widely investigated. Stavy (1990b) found that some 6 - 14 year olds realise that melting is reversible, but notes that:-

“It is possible that pupils of these ages do not have a general conception of the reversibility of the melting process but judge each case specifically.” (p 509)

So, students may think that although water can be frozen and will melt back to water, this will not necessarily apply to other substances. Stavy (1990b) cites how the words “melting” and “freezing” were applied to candle wax and water. Reversibility of the ice - water state change was accepted by almost all respondents, but the notion of the candle wax melting and freezing was understood by 50% of 10-year-olds, rising to 100% only at age 16.

Summary of key difficulties

1. Students are inconsistent in their use of particle ideas

Students do not use particle ideas consistently to explain changes, and if these are expressed, they are frequently incorrect. Examples include thinking that particles can expand, contract, break up and/or are static.

2. State changes are seen as separate events

Students find it hard to appreciate the reversibility of the state changes, thinking of each process as a separate event. Thus, melting and freezing may not necessarily involve the same substance – we do not help by giving solid water the name “ice”, calling liquid water “water” and gaseous water “steam”!

3. Information about one substance cannot be transferred to others

Water is often used as an example for discussion of state changes. Although students may be able to give scientifically correct ideas about the behaviour of water, they cannot apply the same reasoning to other substances. This suggests that rather than having learned and understood state changes in general, they have learned only about the state changes of water. Their learning has not been fundamental in nature, but depends on one example.

4. Ideas about condensation

Students may develop a state change model that involves molecules breaking up on boiling and reforming on condensing. 12-15 year olds may not know where condensed substances come from, saying for example that they “come through the glass” or have “stuck to the glass”.

5. Ideas about melting and freezing

Ice has been used commonly as the substance for investigations about students’ thinking. About half of 15-year-olds think ice particles can shrink, expand, dissolve or melt when changing to liquid water. Melting and dissolving are used frequently as synonyms. Ideas about freezing are less thoroughly investigated – 16-year-olds seem to develop the idea that freezing and melting are “opposites”. The idea that freezing must occur at “cold” temperatures seems to be firmly fixed in many students.

Suggested activities^[1]

1. Provide a wide range of substances

Students need to experience state changes for more than one substance. Encourage investigation of state changes of everyday substances, for example, butter, margarine, chocolate, tomato soup. These examples may help students learn that freezing points are not necessarily “cold” and that boiling points are not always “hot”. Students may experiment with finding the transition temperatures between states for a range of substances and plot these on the same graph to show the variation in values for the same state change.

2. Challenge the “molecules break up” model

Use molecular models while discussing changes of state. Boil water in front of students. Give each person a piece of paper. Tell them to write down what they think is in the bubbles when water boils. Collect the responses. Sort the responses. There is likely to be a range. Ask a few students to explain their thinking. A few will say “steam”. Ask what steam is made from – water molecules. A proportion will suggest hydrogen and oxygen. Use a molecular model of ice showing the hydrogen bonds between molecules to illustrate that it is these bonds which break during state changes, not those within water molecules. Students who use the “molecules break up” model will usually change their thinking as a result.

3. Reinforce particle ideas

Use visual images to explain what happens when state changes occur. Discuss what happens to the particles – do not discuss the bulk substance, but refer to “butter particles”, “chocolate particles” or “tomato soup particles”. Discuss why the temperatures for transition between states differ, relating this to different types of particles and therefore different intermolecular bond strengths. For consistency and to prevent difficulties learning chemical bonding, the term “intermolecular bonding” is best, rather than “attractions” or “attractive forces”.

4. Consider how to present state changes as reversible

Students need to see heating and cooling cycles for themselves, so they can realise that nothing has been added or taken away to the substance. They may think that changes have occurred because appearances change. Re-solidified butter, for example, never looks the same as it did before melting! Using particle ideas will help students realise that the particles have rearranged in a different way so will not give a solid with the same appearance.