Saša Zeljković

Bulletin of the Chemists and Technologists of Bosnia and Herzegovina

The Impact of Selected School Experiments on Pupils' Attitudes Toward Chemistry

B. Ilić, S. Subotić, M. Balaban, S. Zeljković

https://doi.org/10.35666/2232-7266.2025.65.02

School experiments, as a teaching method, represent one of the visual ways of explaining relevant natural phenomena to pupils. The application of experiments in chemistry teaching is often interpreted as an excellent way to achieve good learning outcomes. The impact of conducted school experiments on pupils was examined using psychological instruments, administered before and after performing the selected school experiments in front of the pupils. Feedback from 143 respondents, pupils from primary and secondary schools in Bosnia and Herzegovina, was collected. Adequate statistical analysis was performed. Two key measures were identified: Affinity towards Chemistry and positive experiment experience. A slight but significant increase in the affinity scores was noted after demonstrating school experiments, showing a mildly positive impact on students' attitudes toward chemistry. Positive Experiment Experience scores were high, indicating a favorable perception of the demonstrations. The data suggest that students' initial affinity towards chemistry is the best predictor of their attitudes after the experiment, suggesting that pre-existing attitudes are key. No evidence was found for a "magic" intervention that significantly changes students' affinity. These findings highlight the importance of continuous engagement over isolated demonstrations, which alone cannot replace the impact of ongoing work with students.

Chemistry, Teaching, Experiments, Pupils, Affinities

Original Scientific Article

Reaction Kinetics, Mechanisms, and Catalysis

Kinetics and mechanism of cerium nitrate thermal decomposition in different atmospheres

S. Zeljković, D. Jelić

https://doi.org/10.1007/s11144-025-02884-2

The thermal decomposition of cerium(III) nitrate hexahydrate into cerium oxide was studied under argon and air atmospheres via simultaneous thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), scanning electron microscopy (SEM), and X-ray diffraction (XRD). XRD and SEM analyses revealed that the synthesized cerium oxide had an average crystallite size of 20 nm, although agglomeration resulted in secondary particles up to 1 μm in size. The decomposition was monitored under nonisothermal conditions, and the thermal analysis data were evaluated from a kinetics point of view by isoconversional and model-fitting methods. The isoconversional expanded Friedman demonstrated a clear dependence of the activation energy (Eₐ) on the degree of conversion (α), which supports a complex mechanism. Both atmospheres yielded CeO₂ formation, but through rather different mechanisms. Namely, the Ar atmosphere forms CeO₂ through multiple stages, keeping a high concentration of Ce(III) ions, and preventing the oxidation process from Ce(III) to Ce(IV) ions. Decomposition in the air was more energetically favorable compared to the argon atmosphere. The activation energy values were obtained as follows: 156 and 232 kJ mol−1 (expanded Friedman), 190 and 285 kJ mol−1 (Discrete model), and 166 and 280 kJ mol−1 (N-th order) for air and Ar atmosphere. Based on these results, a reaction mechanism was proposed for the thermal decomposition of cerium(III) nitrate hexahydrate in both atmospheres.

Cerium nitrate, Thermal decomposition, Kinetics, Mechanism, Solid-state reactions

Original Scientific Article

Food Chemistry

Advancing food fortification with kinetics: Stabilizing vitamin D3 through calcium carbonate-based vehicles

D. Jelić, M. Vraneš, S. Zeljković, S. Veličković, F. Veljković, A. Cvetkovski, B. Pejova, S. Papović

https://doi.org/10.1016/j.foodchem.2025.144811

The addition of vitamin D3 in food products is an important health strategy. The vitamin D is heat, moisture, oxidation and pH sensitive. Given the complexity and diversity of food matrices, the use of various encapsulation strategies can more effectively address the need for vehicle systems that ensure the long-term chemical and physical stability of vitamin D. Calcium carbonate-based vehicle, modified with polyvinyl alcohol (PVA) and D-α-tocopherol succinate, were synthesized and loaded with vitamin D3 (D3/X/CaCO3). The vehicle has a form of beadlet designed to protect vitamin D3. The obtained nanostructured D3/X/CaCO3 demonstrated the desirable characteristics for powdered food matrix-vitamin D based systems, such as: enhanced thermal stability of vitamin D3, efficient loading, and an extended shelf-life. A detailed analysis of the physicochemical behavior of vitamin D, both structurally and kinetically, was provided presenting the fully functionalized pre-food model systems. The proposed kinetic perspective could serve as a roadmap for vitamin D₃ stability.

Vitamin D, Calcium carbonate, Vitamin E derivative, Food vehicles, Kinetics

Original Scientific Article

Chemical Papers

Moving towards sustainable tribology: waste silica fumes as a novel lubricant base

S. Zeljković, V. D’Urso, T. Ivas, D. Bogdanić, A. Senatore

https://doi.org/10.1007/s11696-025-04062-z

We conducted ball-on-disk tests to explore the tribological properties of a carbon sodium silicate lubricant (CSS). This lubricant consists of a water-based sodium silicate (SS) suspension and amorphous carbon particles. Its solidification occurs as water gradually evaporates from the mixture. The CSS samples were derived from waste silica fume (SF), which is a mixture of SiO2 and carbon. We analyzed the SF by using X-ray fluorescence and optical microscopy. In order to investigate the impact of lubrication on hot metalworking we used various proportions of SF and NaOH, creating samples S2, S3, and S4 with modulus 1, 1.5, and 2. Additionally, a comparative SS lubricant sample (Sample S5 with modulus 1.5) was made from pure SiO2. We conducted two types of tests: a steady-state test at a constant sliding speed of 50 mm s−1 and a fretting test involving oscillations at a 75° angle and a frequency of 2 Hz, both at 100 °C. Distilled water served as the benchmark tribological fluid (Sample S1) for comparing coefficients of friction (CoF) values. The average CoF (fretting test) values for S1, S2, S3, S4 and S5 are 0.43 ± 0.026, 0.29 ± 0.048, 0.22 ± 0.042, 0.34 ± 0.041, and 0.25 ± 0.041, respectively. In steady-state CoF test, average values for S1, S2, S3, S4, and S5 are 0.27 ± 0.076, 0.28 ± 0.034, 0.40 ± 0.027, 0.42 ± 0.077, and 0.35 ± 0.051, respectively. Statistical analysis, including one-way ANOVA and post-hoc comparisons, revealed significant differences in friction means between sample types in both test types (p-values < 10−15). These results highlight the potential of repurposing SF waste material as a novel lubricant base.

Sustainability, Solid lubricants, Recycling, Carbon sodium silicate, Green tribology

Original Scientific Article