Researchers analyse student conceptions of solid-liquid and liquid-vapour phase transitions using data from 117 undergraduate chemistry students, using data from open-ended, short-answer questions. The collected responses offer insights into students' understanding of these phase changes.

Suggestions

• Teachers should explicitly point out the differences between scientific versus colloquial meanings of terms.
• A greater proportion of time should be devoted to the hands-on lab component of science classes.
• Showing various organic substances boiling at "low" temperatures would be powerful real-world examples and would be important in illustrating phase changes for substances other than water.
• Classroom demonstrations lead to a significant increase in understanding. These were most effective when targeting concepts not included in the curriculum.

Learners’ ideas

• Many students conflated physical changes with chemical changes.
• Many students believed that the sizes of atoms changed during a phase transition.
• Some students could not identify the composition of bubbles in a boiling liquid, with some younger students believing bubbles were composed of heat.
• Several students only associated evaporation with water or water solutions, not with other liquids.
• Some students assumed that a substance such as alcohol had the same freezing point as water.
• Some younger students viewed evaporation and boiling as the same process.
• Some students believed that the maximum temperature for a substance is its boiling point.
• Some students from middle school upwards have difficulty understanding that during a phase transition, the addition of heat does not produce a temperature rise.
• Some students interpreted crystallisation or precipitation as being the same as freezing.
• Many students could not correctly identify the relationship between exothermic, endothermic processes and thermodynamic concepts such as 'warm' or 'cold'.
• Some students had difficulty reconciling the ideas of molecular motion with thermodynamic concepts such as 'warm' or 'cold'.
• Many students had difficulty clearly distinguishing systems and surroundings.

Study Structure

Aims

To present students' conceptions of the solid-liquid and liquid-vapour phase transitions and to investigate to what extent students use experience or contextual understandings in their explanations.

Evidence collection

Evidence was produced via a simple survey of four short-answer questions designed for this study. The survey was completed after the topics in question had been taught (though the time between teaching and evaluation ranged from a few days to weeks). Multiple choice elements of the questions were analysed statistically.

Details of the sample

The sample consists of 117 students from two different cohorts of a first-semester undergraduate general chemistry course at a midsized public university.