Experiences

Mixtures and compounds

Elena Margheri

Italy

Vocational School

Chemistry

Teaching in class, Laboratory, Online Learning

Direct

It is a difficult task to make students understand the real difference between a mixture and a compound, beyond the simple enunciation of Lavoisier and Proust Laws.
Students hardly understand that the variability of the composition is casual in mixtures (only related to the solubility, for the homogeneous ones), while, on the contrary, in compounds it is ruled by the ratio of weight ratio combination.
The sequence of the issues I proposed to the class, was:
- difference between homo and heterogeneous mixtures and related comments during lab activity;
- strong emphasis on the variability of the weight ratio among components in mixtures;
- variability of the weight of the elements involved in the synthesis of a compound, always related to a characteristic weight ratio, making possible to define its chemical formula.
Concerning this last activity it is difficult to find a link with lab practice; in fact also the experiences proposed by the textbooks have no sure results, to confirm that weight laws are more intuitive than deductive.
This is why, in our experience, it revealed useful a description of mixtures based on molecular spheres, pictures and also slides found on-line.
The results obtained by the laboratorial approach and by the use of spheres models were positive, because they increased the interest and the comprehension, due to the manual activity and the direct observation of samples.
Using multimedia material was also of impact, because this kind of language is near to students’ world.
At the end of the activity, most of the students recognized different kind of mixtures and differentiated them from compounds; however, the real comprehension of the nature of a compound was not completely clear to many of them.