Comparison of the effects of conceptual change texts implemented after and before instruction on secondary school students’ understanding of acid-base concepts

The descriptive measures of the tests for the experimental groups and the control group are given in Table 3. It is seen that students’ mean scores of pre-CAT were similar in the experimental and control groups. A one-way analysis of variance (ANOVA) was used to determine whether statistically significant differences existed among the mean scores of EG₁, EG₂ and CG for the results of the pre-CAT at the 0.05 level. The findings showed that there were no differences among the groups with respect to the students’ initial understandings of the concepts of acids and bases (F_(2;73)=0,042; p>0.05) before the treatment. These results indicated that students in all groups were similar in respect to the initial understandings of the concepts of acids and bases before the intervention.

Table 3. Comparison of performance of experimental and control groups on pre-tests and post-tests

		Pre-CAT		Post-CAT		Pre-Post mean difference (CAT)
Group	N	Mean	SD	Mean	SD	Pre-Post mean difference (CAT)
EG₁	26	34.62	17.72	77.88	12.50	43.26
EG₂	25	35.40	17.55	70.60	13.17	35.20
CG	25	36.00	16.14	60.00	15.74	24

Note: SD: Standard Deviation; EG: Experimental group; CG: Control group

After the treatment, the analysis of covariance (ANCOVA) was run to compare the effects of intervention on students’ understanding of acid-base concepts in post-CAT. The adjusted mean scores of EG₁, EG₂, and CG were 79.1, 70.1 and 58.2, respectively. Although the students in each group increased their achievement scores from pre-test to post-test, these gains were not too high. While the mean score of students in EG₁ had risen from 34.62 to 77.88 with a gain score of 43.3, in EG₂ it had risen from 35.40 to 70.6 with a gain score of 35.2 and in the control group it had risen from 36 to 60.2 with a gain score of 24.2 after the treatment. As seen in Table 4, the results of ANCOVA showed that the treatments resulted in significant effects on the mean scores (F_(2;71)= 24,84, p<0.01). When the marginal mean scores of the groups were compared by using the Tukey test, statistically significant differences were found between the experimental groups and the control group (p<0. 01), and a remarkably significant difference was also found between the experimental groups in favor of EG₁ (P<0.05). The alternative conceptions concerning the acid and base concepts were determined by the students’ responses to the multiple choice items in the CAT. Ratios of the alternative conceptions held by more than 20% of students in each group were presented in Table 5. The alternative conceptions, as reflected by the distracters in multiple-choice items, are the most common in a certain conceptual area.

Table 4. Summary of the analysis of covariance comparing the mean post-CAT scores of students in the experimental and control groups

Source of Variance	df	Mean Square	F	p	Eta-Square η²	Differences (Tukey HSD)*
Pre-CAT (cov)	1	3880.89	24.84	0.000	0.256	EG1-CG; EG2-CG; EG1-EG2
Treatment	2	1562.69	10.00	0.000	0.217
Error	72	156.388

*significant differences among the groups according to Tukey test (at the 0.05 level)

Twenty alternative conceptions were identified and were grouped under the headings of the application of properties of acids and bases (A), pH scale (B), salts solutions (C), neutralization and titration (D), indicators (E), strength of acids and bases (F) and hydrolysis (G). While the percentages of students’ alternative conceptions in pre-test ranged from 26.29% to 80.77% in EG₁, the students in EG₂ ranged from 24% to 84% and those in the control group ranged from 24% to 80% (Table 5). The students in each group held almost the same alternative conceptions in the pre-test at about the same percentages. Each group showed progress in changing their alternative conceptions to the scientifically acceptable ones, but the experimental groups performed better overall. Although the experimental groups performed better, this does not mean that all alternative conceptions were remedied. The reasons for this are discussed in detail below.