The differences between elements, compounds and mixtures

Two definitions of “element” illustrate that particle ideas are implicit in making the distinction:-

“A pure substance which cannot be split up into any other pure substance” (Freemantle, 1987 p 123)

“An element is a substance that consists of only one kind of atom.” (Atkins 1989 p 8)

To understand Freemantle’s phrase “cannot be split up”, students must appreciate that matter comprises tiny particles which combine together. To understand Atkins’ definition, students must know the meaning of “atom”. The topic has received relatively little attention from researchers, although Barker (1995), Briggs and Holding (1986) and Ben-Zvi et al (1986) have studied students’ thinking about these ideas.

Making the distinctions

Briggs and Holding (1986) explored how 15 year olds apply particle ideas in making the distinctions between elements, compounds and mixtures. They used coloured dots to represent different atoms in diagrams of a mixture of two elements, a compound and an element alone. About 30% of respondents selected all three correctly. A number of students could not “…discriminate between particulate representations of compounds and elements” (p 43) and so thought the picture of the compound alone, which showed two different coloured dots joined as molecules, represented an element (7%) or a mixture (39%). Briggs and Holding suggest that

”..about half of the students regarded any diagram that contained different symbols for atoms, whatever their location, as a representation of a mixture.” (p 48)

Interviews showed that students seemed to understand the macroscopic nature of an element, but did not use particle ideas, suggesting, for example, that an element was:-

“… a single substance…?”

“…. a form of chemical…”

“An element is one, just made up of one substance…well if it was copper it would be made up of just copper…” (p 50 - 51).

These responses indicate understanding that all parts are the same and that an element is “pure”. Other responses showed considerable confusion about the particles present in an element, for example,

“An element is a particular kind of chemical…and all molecules er atoms er molecules of the same substance..” (p 50)

“…[an element] it is part of an atom, something that makes up an atom…um they can be joined by many of them an element is just one part of an atom.” (p 50)

Ben-Zvi et al (1986) found that nearly half of 15 year olds attributed the bulk physical properties of copper to single atoms of the element itself, thus making each atom a microscopic version of the element. Briggs and Holding (1986) state

“…the overall reluctance of students to use particulate ideas in talking about elements, compounds and mixtures may [arise from or result in] gaps in students’ thinking. If bridges are not continuously made between the macroscopic and particulate levels then students do not readily cross freely from one to another unless strong cues are present.” (p 57)

Barker (1995) carried out a longitudinal study of the understanding of a range of basic chemical ideas among 250 16-18 year old students taking the UK post-16 chemistry course called Advanced (A) level. She found that almost all students starting A level courses in chemistry could distinguish correctly between the Briggs and Holding diagrams.

Briggs and Holding (1986) explored the distinctions 15 year olds make between elements, compounds and mixtures by asking them to identify an element from a list of four substances, each described using basic chemical terminology. Only 21% used particle ideas explicitly when making their choice. Other responses included:-

“I think it is a because elements can not be split into anything except by chromatography…” (p 19)

“…an element can be split into two more substances…” (p 20).

These students seem to recall Atkins’ definition in a confused form. Some respondents suggested that an element burns to give off a gas, or “…most elements need oxygen to stay living” (p 21).

In the same study, students considered if a substance was an element on the basis of specified results of “tests”. Some responses incorporated physical characteristics into a definition of “element”, for example, 

“…no element can have a melting point above 200 ^oC and dissolve in water to give a colourless solution.” (p 31)

Other students confused “element” with chemical characteristics or chemical reactions. Barker’s study (1995) revealed that around 3% of 16 year olds beginning post-16 chemistry courses could give general tests to determine if a substance is an “element” or “compound”, a figure which increased to 17% at the end of the course. She reports that about 43% could define “element” and “compound” correctly at the start of a post-16 course and that this figure remained unchanged at the end. Gabel and Samuel (1987) note with concern that:-

“Even after the study of chemistry students cannot distinguish between some of the fundamental concepts on which all of chemistry is based such as solids, liquids and gases or elements, mixtures and compounds in terms of the particle model.” (p 697)

Implications for teaching

Students who choose to study chemistry post-16 appear to have little difficulty making distinctions between elements, compounds and mixtures when presented with diagrammatic representations of particles. This indicates that the converse may also be true - that “non-chemist” students may find making these distinctions problematic, so this fundamental aspect of chemistry remains a mystery.

These data have significant implications for teaching. Students’ understanding of the differences between elements, compounds and mixtures in particle terms is poor. It is therefore unsurprising that students find chemistry “hard”, as they do not understand a basic principle providing a foundation for more detailed study.

Activities to help students are suggested at the end of the next section.