Students’ thinking about acids, bases and neutralisation

Misconceptions about acids, bases and neutralisation

Workers including Hand and Treagust (1988), Nakhleh (1992), Ross and Munby (1991) and Cros et al (1986, 1988) have studied students’ ideas about the nature of acids, bases and neutralisation. The studies reveal some consistency with earlier discussion about students’ models for chemical reactions.

Hand and Treagust (1988) identified five key misconceptions about acids and bases among sixty 16 year olds. These were :-

“(1) An acid is something which eats material away or which can burn you;

(2) Testing for acids can only be done by trying to eat something away;

(3) Neutralisation is the breakdown of an acid or something changing from an acid;

(4) The difference between a strong and a weak acid is that strong acids eat material away faster than a weak acid; and

(5) A base is something which makes up an acid.” (p 55)

No particle ideas are used here: the students give descriptive statements emphasising a continuous, non-particulate model for acids and bases, some including active, anthropomorphic ideas such as “eating away”. This non-particulate view persists for a minority of students, as Nakhleh (1992) found. 20% of 17 year old chemists in her study drew images consistent with a non-particulate model of an acid when asked how an acid or base would “appear under a very powerful magnifying glass” (p 192). This implies that although students may measure pH and know about the corrosive qualities of acids and bases, some find it hard to associate properties with the particles present.

In Barker’s longitudinal study (1995), participants were asked a two-part question involving hydrochloric acid. In the first part, students were invited to draw a diagram showing how hydrochloric acid forms from hydrogen chloride gas and water. About half of the respondents gave particle-based answers, with about 12% of 16 year olds drawing hydrogen or oxonium ions and 40% hydrogen chloride molecules. At the end of the study, almost 80% used particle ideas, divided between 37% drawing hydrogen/oxonium ions and 40% hydrogen chloride molecules. This supports Ross and Munby’s (1991) interviews with 17 year olds showing that the notion of an “acid containing hydrogen ions” was reasonably well-known.

Even if students “know” that acids “contain hydrogen ions”, the chemical behaviour of acids proves difficult to explain. In the second part of her question, Barker invited the same respondents to explain how hydrogen gas forms when a piece of magnesium is added to the acid. About 6% at the start and 17% at the end of the study answered the first part with “hydrogen/oxonium ions” then used the term “displacement reaction” in the second, suggesting that they understood a chemically correct meaning for this. Students who gave incorrect responses to the first part also used the term “displacement reaction”. For example, around 8% initially drew hydrogen chloride molecules and used this phrase, a figure increasing to about 12% by the end. Around 12% of 18 year olds gave the correct ions, but thought that chlorine was displaced. Students seemed to view the acid / metal reaction as a means for hydrogen to “swap partners” with magnesium, perceiving a reaction between the magnesium and “chlorine”/chloride part of hydrogen chloride, rather than between the magnesium atoms and hydrogen/oxonium ions. These findings have implications for teaching about electrode potentials as well as further detailed work on acid/base equilibria.

Some evidence supports the view that definitions of “acid” and “base” together with changing these also causes difficulties for students. Hand (1989) followed up twenty-four of the students reported in Hand and Treagust (1988). At this later stage, some students had been taught much more sophisticated ideas in a pure chemistry course, while others had studied a broader based science course or biology. A test based on the five original misconceptions was administered to the group. The results indicated that only students studying chemistry could answer basic recall questions correctly, while those studying biology did best overall. The author concluded that the biologists did better because “they were not having any interference from new definitions” (p 142). Carr (1984) agrees with this, stating that students’ difficulties with acids and bases are:-

“more usefully perceived in terms of confusion about the models used in teaching the concept rather than as a conflict between preconceptions and the scientific view” (p 97).

In advanced chemistry courses, acids and bases are redefined under the Brönsted-Lowry theory as “donors” and “acceptors”, moving away from the Arrhenius definitions of an acid being a “substance which yields hydrogen ions” and a base producing hydroxide ions in solution. Hand suggests that presenting students with this new theory confuses them. Hawkes (1992) supports this, stating:-

“It is inherent in human nature that we accept what we are told first and relinquish or change it with difficulty.” (p 543)

Students studying chemistry post-16 may continue to use ideas learned much earlier and see no reason to change them.

Cros et al (1986, 1988) investigated French university science students’ ideas about acids and bases, finding that the concept of bases was far less developed than that of acids. Many students gave the Arrhenius definition of bases being OH- donors. Students could not name bases as easily as acids, giving only ammonia and sodium or potassium hydroxide as responses. Second year students showed no improvement on the first years in these respects.

Summary of key difficulties

1. Acids can burn and eat material away

Students think of acids as active agents that damage skin and other materials. The idea develops in young children, who learn to think of acids as “dangerous”. Cartoons showing scientists making holes in benches with acids also contribute to this image. Acids are not perceived as being particulate, but rather continuous matter with special properties.

2. Neutralisation means an acid breaking down

Rather than considering neutralisation as a reaction between an acid and an alkali, students perceive this as removing acid properties. The alkali may stop the action of an acid, or alternatively the acid may break down. 

3. A base/alkali inhibits the burning properties of an acid

Students tend to meet acids in formal education well before alkalis, so ideas about these chemicals are relatively under-developed. Although dilute alkalis are in fact more corrosive than dilute acids, students’ perceptions are that they have no corrosive properties, instead acting to or inhibit acids “eating away” other material.

4. Hydrogen ions are present in acids, but acids remain molecular in solution

That hydrogen ions are responsible for acidic behaviour is relatively well-known, at least among many 16 year old chemists. However, a common model for acid behaviour seems to be that hydrogen ions remain in a molecule and “swap partners” or are “displaced” from this molecule by reaction with an alkali or metal.

Acid / base reactions feature in most pre-16 chemistry courses. Teachers must therefore be aware of students’ difficulties with these reactions. Students’ problems may arise because acids and alkalis both look like water. Reacting them together needs precision and some way of knowing that neutralisation is complete, so an indicator is required. Addition of this extra chemical adds and extra layer of “mystery”. A common experiment too at this level is to investigate the acid/ base nature of everyday substances using universal indicator. Thus, students find out that toothpaste, baking powder, soap, bleach, vinegar, tomato sauce and other well-known household items have a specific chemical property we “label” as acid or base.

Suggested activities^[1]

1. Introduce acids and bases alongside each other

Rather than allow students to focus mainly on acids, encourage development of knowledge of alkalis too. One common approach is to ask students to test the pH of a range of household substances. Domestic cleaners containing ammonia, toothpaste, “bicarbonate of soda” (used in baking) have alkaline pH values, while fruit juice, vinegar, tomato sauce and shampoos tend to have acidic pH values. This approach needs to be balanced, otherwise children will detect more acids than alkalis. The next step is to test the behaviour of laboratory acids and alkalis to demonstrate that in fact alkalis are more corrosive than acids. This may be done by dropping samples of dilute acid and alkali on to a range of substances such as paper, nylon, aluminium foil and cotton. The samples can be inspected over time. The alkali will cause more corrosion than the acid.

2. Show the difference between “strong” and “weak” and dilute and concentrated

Sequential 1:10 dilutions of a strong acid, strong base, weak acid and weak base will show that pH changes by 1 whole value for every dilution. A strong acid or base will require more dilutions to reach the same value as a weak base. This suggests that there are more particles responsible for acidity/ alkalinity present in strong acids and bases than in weak ones. This can be contrasted with concentrated solutions in which a large amount of any substance may be dissolved in water. This is independent of whether the acid/ base is “strong” or “weak”, but applies equally to non-acidic substances such as sodium chloride or sugar.

3. Introduce “neutralisation” as a reaction involving an acid and a base reacting together

Students can carry out a titration between a strong acid and a strong base, measuring the pH and temperature after the addition of each aliquot of acid. Plotting the results reveals that the highest temperature and the neutral point coincide. Molecular models can be used to reinforce the reaction between the acid and base, showing that water and a salt are formed. In discussion, students can be led towards the notion that formation of bonds in water is a source of the energy. Varying the acid to dibasic and tribasic reveals that more energy is released – this can be directly related to the equations for the reactions.