Theydo not incorporate the corresponding aggregation states. (iii)
They omit the double arrow. (iv) They confuse the chemical
equation with the particular experimental situation; for example,
they write AgCl(s) →←
3Cl(aq) + 3Ag+(aq) according to the
number of ions in solution drawn in the figure.
From the particle representations in question 4, one can
evaluate whether the students have assimilated the three main
characteristics of a system in equilibrium:
1. Reversible aspect: The student must take into consideration
the coexistence of all the species in the new
equilibrium situation. For example, some of them
dissolve all the salt or completely precipitate it.
2. Constancy of the concentrations: the students must keep
the number of particles in solution constant.
3. Dynamic aspect: They must maintain the same particles
in each phase and change the numbers of the particles.
Alternative conceptions regarding these three characteristics
of chemical equilibrium have been investigated in various
studies with students in their final year of secondary school
and first year of university (8–11). Nevertheless, studies have
not been conducted on how students visualize and interpret
solubility equilibrium, although secondary students’ conceptions
of solubility were identified by Ebenezer and Erickson (12).
The particulate nature of matter is a fundamental concept
in chemistry, and an improper understanding of it can lead
to difficulties with other concepts that are built upon it (13).
In questions 3–5 that the students must
1. Keep the number of ions in the closed container
constant (conservation of matter);
2. Correctly represent the ionic solid (Taber applied the
term “molecular framework” to the tendency to perceive
some ions within an ionic lattice to be bonded
to one another as in a molecular solid [14]);
3. Properly represent dissociated reactants and products
(some students depict all dissolved species as molecules
of AgCl or HCl–AgOH; this difficulty was also
mentioned by Smith and Metz [15]);
4. Consider the kinetic aspect of the model, the translation
movements (students systematically draw the ions in
the same place as in the original situation); and
5. Take into account the distribution of particles according
to the corresponding aggregation state of matter.
In applying Le Châtelier’s principle or the common ion
effect in question 5, students fail to maintain the solution’s
neutrality, even though most assert that the solubility of AgCl
decreases with the addition of AgNO3. Some students formulate
it in quantitative terms; they calculate Ksp from the
number of ions per unit of volume and obtain as a result
that Ksp = [Ag+][Cl] = 3 × 3 = 9. And in order to keep Ksp
constant, they draw a possible final situation with 9 Ag+ ions,
1 Cl ion, and 8 NO3
ions, supposing that 8 Ag+ and 8 NO3
ions had been added and 2 Ag+ and 2 Cl ions precipitated. This
Ksp calculation, made on the basis of nonconventional units
of concentration, may be considered valid in this simulation.