Scientific literacy responds to what is important to know, value and be able to do in situations involving science and technology and as an educational goal, requires rethinking science teaching and the corresponding assessment methods. This work presents the systematic analysis of 106 items from tests to assess scientific literacy run from 2000-2015 in PISA (Programme for International Student Assessment). In line with recommendations from the specialised research, results show that questions are mainly situated in real life contexts and scenarios that bring purpose and meaning to the knowledge to be used, demanding not only conceptual knowledge but also an understanding of epistemic aspects of science, though procedural knowledge is underrepresented. Nevertheless, the bi-dimensional analysis of the type of knowledge and the cognitive demand shows that over 80% of questions focus on the application of low order skills (remember and understanding). These results suggest the need to increase activities that require the evaluation and creation of knowledge to enhance high order cognitive skills and to better prepare individuals for a critical and active participation in a world shaped by science and technology. La alfabetización científica responde a qué es importante saber, valorar y saber hacer en situaciones en las que intervienen la ciencia y la tecnología y como objetivo educativo, requiere un cuestionamiento de las formas de enseñar y evaluar el aprendizaje de las ciencias. Este trabajo presenta el análisis sistemático de 106 ítems de las pruebas realizadas entre los años 2000 y 2015 dentro del programa PISA (Programme for International Student Assessment). La finalidad es analizar el tipo de conocimiento evaluado y el papel que los contextos y la demanda cognitiva juegan en la evaluación de la alfabetización científica en dicho programa. Los resultados muestran que las actividades se ubican mayoritariamente en contextos cotidianos o utilitarios y requieren no solo el dominio de conceptos científicos, sino también la comprensión de aspectos epistémicos de la ciencia con una infrarrepresentación de aspectos procedimentales. El análisis bidimensional del tipo de conocimiento evaluado y la demanda cognitiva exigida muestra que más del 80% de las actividades se asocian con una demanda cognitiva baja (recordar y comprender). Los resultados sugieren la necesidad de reforzar el número de actividades que requieren la aplicación, evaluación y creación de conocimiento con objeto de enfatizar habilidades cognitivas de orden superior, especialmente relevantes en un mundo dominado por la ciencia y la tecnología. Palabras clave: Alfabetización científica; Aprendizaje basado en contextos; Demanda cognitiva; Conocimiento científico; PISA. Knowledge, Cognitive demand and contexts in the assessment of scientific literacy in PISA Scientific literacy responds to what is important to know, value and be able to do in situations involving science and technology and as an educational goal, requires rethinking science teaching and the corresponding assessment methods. This work presents the systematic analysis of 106 items from tests to assess scientific literacy run from 2000-2015 in PISA (Programme for International Student Assessment). In line with recommendations from the specialised research, results show that questions are mainly situated in real life contexts and scenarios that bring purpose and meaning to the knowledge to be used, demanding not only conceptual knowledge but also an understanding of epistemic aspects of science, though procedural knowledge is underrepresented. Nevertheless, the bi-dimensional analysis of the type of knowledge and the cognitive demand shows that over 80% of questions focus on the application of low order skills (remember and understanding). These results suggest the need to increase activities that require the evaluation and creation of knowledge to enhance high order cognitive skills and to better prepare individuals for a critical and active participation in a world shaped by science and technology. Keywords: Scientific literacy; Context-based learning; Cognitive demand; Scientific knowledge; PISA.

Activation barrier heights for the dehydration reaction of geminal carbinols and silanediols R′R″X(OH)₂ (X = C, Si) were estimated at the B3LYP and MP2 levels of theory employing Dunning's correlation-consistent triple-zeta basis sets. It was shown that the barrier height for carbon derivatives steadily decreases upon substitution by R groups, usually termed as electron-donating, such as alkyl and amino groups. Substitution by electron-withdrawing groups leads, however, only to small changes in barrier heights compared to that of methanediol. A similar tendency was also found for silicon derivatives, but high activation barriers of this reaction remain even for amino group substituted silanediols. Introduction of additional water molecules into the reactive space of carbinol dehydration drastically reduces barrier heights and brings the transition state energy for methanediol close to the experimental value. The difference between dehydration barrier heights for both methanediol and carbinols with electron-rich substituents becomes well-defined for dimeric species. The higher acidity of the hydroxyl group protons in molecules containing halogens and CO groups brings about a noticeable growth in the dehydration barrier heights of these compounds. This difference in barrier heights for oligomeric species may be the reason for the stability of carbinols with electron-rich substituents.