CHAPTER 5 RESULTS SHOWING SCOPE AND QUALITY OF RESEARCH ON
7.2 DIFFICULTIES WITH PHYSICAL PROPERTIES OF ACIDS AND
Difficulty
number Difficulty Descriptions (In Bold) linked to Propositional Statements (coded)
Difficulty Classification P24 The Arrhenius model is for strong acids and the Brønsted model is for weak acids.
Strong Arrhenius acids or bases are fully dissociated in solution (8.2.2.1) Weak Arrhenius acids are partially dissociated in solution (8.2.2.2)
Stronger Brønsted acids are better proton donors than weaker Brønsted acids. (8.3.1)
2
P25.1 P25.2
All salts have neutral aqueous solutions.
Sodium chloride does not have a neutral aqueous solution.
Salts may have neutral or non-neutral solutions (7.1.3)
Salts where ions are weaker Brønsted acids or bases than water will have neutral solutions.
(7.3.3.3.2.1) such as NaCl (5.1.2)
Salts where ions are stronger Brønsted acids than water will have acidic solutions. (7.3.3.3.2.2) Salts where ions are stronger Brønsted bases than water will have basic solutions (7.3.3.3.2.3) such as sodium ethanoate (5.1.3.1)
2 2
P26 There is no acid-base reaction between water and the ions from a salt.
Ionic compounds dissociate into cations and anions when they dissolve in water (8.2.5.1) If ions are stronger Brønsted acids or bases than water, they will react with water molecules.
(8.3.5)
Hydrolysis of anions or cations changes the [H3O+] and [OH-] (8.3.5.2) Brønsted acid-base reactions include hydrolysis. (7.3.3.3)
Hydrolysis is a chemical reaction between an ion or molecule and water (7.3.3.3.1)
3+
7.2 DIFFICULTIES WITH PHYSICAL PROPERTIES OF ACIDS AND BASES
“burn”. With a stable description across multiple research studies in different contexts, the difficulty can be classified at Level 4. From the research in the reports given above, students seem to over generalise, as not all acids are corrosive. Moreover, students appear to make no distinction between pure acids and solutions of these. Neither do they distinguish between concentrated and dilute solutions. Consequently the difficulty maps to the following propositional knowledge:
• Some acids can be corrosive and appear to burn the skin and eyes. (2.1.1.4) (Young, 2003b),
• Citric acid is irritating to eyes and skin. (2.1.1.4.3.1) (Young, 2003b).
• Properties in concentrated solutions may differ from those in dilute solutions. (1.2.0.1) This difficulty may be related to Difficulty S1 concerning the lack of distinction which students make between acids and their solutions (see Section 6.2.1.1).
7.2.1.2 Difficulty P2: Acids are poisonous or toxic
Coupled with the previous difficulty, students at all levels, from junior secondary to tertiary and even teachers, are fearful about ingesting acidic substances (Cros et al., 1986; Ross & Munby, 1991; Toplis, 1998; Demircioğlu et al., 2004; Ogunniyi & Mikalsen, 2004; Drechsler &
Schmidt, 2005b; Chiu, 2007). The common essence of their descriptions is: All acids are poisonous or toxic. Being found in multiple contexts through independent triangulated studies, I classified this difficulty at Level 4. It appears that students are unaware that many foodstuffs are acidic, which relates to Difficulty S7 concerning everyday examples of acidic and basic substances (see Section 6.4.1). In particular, students may be interested to find phosphoric acid in the list of ingredients in Coca Cola or that they would need to ingest half a kilogram of citric acid (used in sour sweets) for it to be fatal (Young, 2003b). Moreover, if students believe acids are corrosive (Difficulty P1 in the previous section) it is not surprising that they will not ingest them. The corresponding propositional knowledge which has already been given in the previous chapter is:
• Foods often contain acidic substances. (2.1.2.1)
7.2.1.3 Are difficulties P1 and P2 distinct difficulties?
Is it worth separating the difficulties P1 and P2 – could they be one difficulty: ‘acids are dangerous’? If the implications of the student reasoning are different then according to the method outlined in Section 4.5 they should be treated separately. I first consider the implication of acids being ‘poisonous’. The word ‘acid’, coined in the 17th century, came from the Latin word acidus meaning sour or tart. Consequently, the characteristic acidic taste is inherent in the
operational model. Furthermore, an important application of acid chemistry is in food science.
What is more, this conception misleads students into thinking that only acids are poisonous.
Moreover, while many acidic substances are poisonous, this is not necessarily due to their acidic nature. Secondly, what are the implications of a student belief that acids are corrosive? Perhaps students might think that only acids are corrosive, hence they might misclassify some caustic bases as acids. In summary, acids are indeed dangerous, but student difficulties with their corrosive and poisonous properties each have different educational consequences, and so they should be treated as separate difficulties.
7.2.1.4 Difficulty P3: Bases are dangerous
Some junior secondary and elementary students see bases as dangerous, along with acids. From the limited data (Toplis, 1998; Chiu, 2007), the description is not yet entirely clear. Further research is needed to verify whether students distinguish the two aspects as they did with acids.
Consequently, the classification of the difficulty is only Level 3+.This difficulty maps to the following two propositional knowledge statements – both already appeared in the previous Chapter (see Difficulties S5 and S7):
• Alkali is an alternative term for Arrhenius bases. (3.2.1.1)
• Basic substances are found in cleaning materials such as oven cleaner, household ammonia, household bleach; washing soda Na2CO3, and soap. (3.1.2.1)
7.2.2 The acid-base dichotomy
7.2.2.1 Difficulty P4: Acids and bases have dichotomous properties
In addition to a conception that bases are harmless, Nakhleh and Krajcik (1994) identified acid- base dichotomy as a major theme of student conceptions which they substantiate with student quotations such as: “if acids are coloured, bases are clear” and: “if acids taste bitter, bases taste sweet”. Without further analysis I can describe the difficulty as: Acids and bases have dichotomous properties. From one comprehensive study the difficulty is classified at Level 3.
The difficulty description is based at present on macroscopic observations; it may also pertain to student sub-microscopic understanding of acid-base systems, however no research in this regard arose in the search of publications (see results in 5.2). In the interim, the difficulty maps to a complementary, as opposed to inverse, relationship between acidic and basic properties as shown by the following propositional knowledge:
• Acids and bases have complementary properties. (1.2.0.2)
• As solutions become more acidic the pH decreases. (9.3.1)
• As solutions become more basic the pH increases. (9.3.2)
Difficulty P4 is not trivial, as it could inhibit further conceptual development. In this regard, in Section 6.5.1.2 it was argued that students resisted the idea of a substance or species being able to act as both acid and base; that is, being amphoteric (Difficulty S9). The conception may be explained by a dichotomous view of acid-base substances, which would preclude a substance falling into both classifications. Further open-ended research on Difficulty S9 may show that it is a sub-difficulty of P4. Other implications of the Difficulty P4 are given as sub-difficulties below.
7.2.2.2 Sub-difficulty P4.1: All substances are either acid or base.
Arising from the dichotomous view above, students classify every substance as acid or base.
The description of the difficulty, given above, is based on my own interpretation of the data from Nakhleh and Krajcik (1994) and consequently it cannot be classified as more than Level 1, or Suspected.
• Neutral substances and solutions are neither acidic nor basic. (5.1)
7.2.2.3 Sub-difficulty P4.2: Bases are not dangerous.
Published research shows students believe bases to be harmless (Ross & Munby, 1991; Nakhleh
& Krajcik, 1994; Toplis, 1998; Ogunniyi & Mikalsen, 2004). This contrasts so clearly with the common perception of acids as corrosive or poisonous that I believe it arises from the dichotomous conception. The descriptions in the published research map to the following scientific knowledge related to everyday examples and safety aspects.
• Oven cleaner and drain cleaner contain basic substances such as NaOH. (3.1.2.1.1)
• Bases, such as NaOH and KOH and ammonia (Young, 2003a) (3.1.1.4.1), can be corrosive (or caustic) and appear to ‘burn’ skin and eyes. (3.1.1.4)
• Sodium and potassium have common names caustic soda and caustic potash. (3.1.1.4.1.1) The propositional statements given above, in turn suggests the sub-difficulty description: Bases are not dangerous. This conception has been found in triangulated studies covering four different cohorts around the world, so I can classify it at Level 4, or Established.
7.2.3 Difficulties concerning sensory properties of acidic or basic solutions
Three student difficulties (one with sub-difficulties) concerning physical properties of acids and bases can be described directly from the original research. The research, from which the descriptions were derived, concerning taste (P5) and smell (P6) of acids and feel (P7) of bases, is summarised below (Table 7.2). For each difficulty, the propositional knowledge statements to which it could be mapped are given below the evidence.
Table 7.2 Summary of research on acid-base sensory properties
Difficulty number
Difficulty description
Classification level
Reason for classification
Educational level of
students Author(s) P5.1 Only acidic substances
have taste.
1 Unanticipated data Senior secondary Pre-service teachers
Nakhleh & Krajcik (1994)
Dhindsa (2002) P5.2 Acidic taste is called
bitter.
2 Little research reported, could be a language difficulty (Clerk & Rutherford, 2000)
Senior secondary Senior secondary Pre-service teachers
Ross & Munby (1991) Nakhleh & Krajcik (1994)
Bradley & Mosimege (1998)
P5.3 Acid solutions taste sweet.
1 Unvalidated MCQ distractor Pre-service teachers Bradley & Mosimege (1998)
• Weakly acidic solutions taste sour. (2.1.1.3), as do lemons. (2.1.1.3.1)
• Weakly basic substances taste bitter (3.1.1.3), as does soap (3.1.1.3.1) P6 Acids are recognized
by strong smells
3 Two studies combined to give the same picture
Senior secondary Senior secondary
Ross & Munby (1991) Nakhleh & Krajcik (1994)
• Acidic substances may smell ‘sharp’ (2.1.1.5.1) and may make you feel like choking (2.1.1.5.1.1)
• Ammonia has a strong pungent smell (3.1.1.5.1), as does urine. (3.1.1.5.1.1) P7 Bases do not have a
characteristic feel.
1 Unvalidated MCQ distractor Pre-service teachers Bradley & Mosimege (1998)
• Weakly basic solutions feel soapy (3.1.1.4.2)
• Corresponding propositional statements shown bulletted
Difficulty P6 has the highest classification because data from two reports gave a coherent
‘picture’, while the other difficulties all have low classifications due to the lack of reported research data to substantiate coherent difficulty descriptions. In particular, Bradley and Mosimege (1998) gave no indication of any research on which they based distractors used in two multiple-choice items, resulting in little insight being gained into the nature of the relevant difficulties (P5.3 and P7).
7.3 DIFFICULTIES WITH ACID-BASE CHEMICAL CHARACTERISTICS