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Cet article présente les résultats d’une recherche concernant les conceptions d’élèves de cycle 1 sur les concepts de masse et de volume et sur le comportement d’objets après leur immersion dans l’eau. Les résultats obtenus montrent que les élèves de maternelle ont des difficultés à dissocier la masse du volume et associent essentiellement la flottabilité des objets à leur taille. Nous avons étudié l’impact d’activités autour de la flottabilité en mettant en œuvre deux ingénieries comparatives en classe de moyenne section de maternelle. Les résultats montrent que le niveau de conceptualisation des élèves est supérieur en ce qui concerne la dissociation de la masse et du volume chez les élèves qui ont travaillé la flottabilité en utilisant pour chaque matériau testé des objets de tailles variées. In this paper, we present the result of our research into the way preschool pupils perceive the concepts of mass and volume and the way objects act after being submerged in water. The results show that preschool pupils have difficulty separating the notion of mass from volume and tend to associate the buoyancy of objects with the size of these objects. We studied the impact of activities involving buoyancy by means of the implementation of two comparative didactic engineering in preschool (pupils aged 4-5). Our results show an increased level of pupil conceptualisation on the dissociation of mass and volume in pupils who had worked on buoyancy by using objects of various sizes for each tested materials.

Using time-resolved dielectric relaxation spectroscopy, we have studied the kinetics of the first-order phase transformation of caffeine from its high temperature rotational solid form to the room temperature phase. The results indicate that the kinetics can be fitted by a highly stretched exponential law. The kinetics, thus, differ from that of a ‘typical’ nucleation and growth process. This unusual non-sigmoidal evolution can be understood as being dominated by slow nucleation in grains with a dispersion of their size ranging below the characteristic dimension of the nucleation and growth process. The relaxation times of polarization are characterized by an activation energy closely correlated with that of the nucleation process.

International audience; Using dielectric relaxation data we have characterized the molecular mobilities of caffeine both in phase I (stable and metastable) and in phase II. In phase I effects of sublimation and phase transformation kinetics were carefully considered. In plane rotational motions were followed on a wide temperature range. A noticeable antiferroelectric short range order developing at the approach of the glass-like transition is characterized. Condition for occurrence of a critical-like behaviour is discussed. At high temperature the emergence of an additional ultra slow relaxation process is highlighted. Possible molecular mechanisms are proposed for both processes. In phase II the existence of a less intense relaxation process is confirmed. Close similarity with the main process developing in phase I hints at a common origin of the dipolar motions. Careful consideration of recent structure determinations leads to suggest that this process is associated to similar molecular in plane rotations but developing at the surface of crystalline samples. Lower cooperativity at the surface is reflected in the smaller activation entropy of the relaxation.

International audience; Raman investigations are carried out both in crystalline forms of caffeine and during the isothermal transformation of the orientationally disordered form I into the stable form II at 363 K. The time dependence of the Raman spectrum exhibits no significant change in the intramolecular regime (above 100 cm–1), resembling the spectrum of the liquid state. By contrast, significant changes are observed below 100 cm–1, and the low-frequency spectra of forms I and II are observed to be different from that of the liquid. The temperature dependence of the 5–600 cm–1 spectrum gives information on the static disorder through the analysis of collective motions, while information on dynamic disorder are obtained from the study of the 555 cm–1 band corresponding to internal vibrations in the pyrimidine ring. This analysis indubitably reveals that form II is also orientationally disordered with a local molecular arrangement that mimics that in form I and the liquid state. The comparison of the low-frequency spectra recorded in theophylline and form II of caffeine allows one to describe the stable form of caffeine from the packing arrangement of anhydrous theophylline with the consideration of reorientational molecular disorder.