A methodology for teaching the relationship between grain size and magma cooling rates, using sodium chloride as a model, aligned with the Curriculum for Wales.

Pupils frequently struggle to understand how magma cooling rates influence crystal size, as illustrated by differences between basalt and granite. To address this, a classroom appropriate methodology using household materials was developed to investigate the effects of temperature and solution volume on sodium chloride crystal formation. Temperature and volume significantly affected evaporation time, but neither variable produced significant differences in crystal size, and no correlation was found between evaporation time and crystal sizes. However, residue morphology varied, as higher volumes of unheated solutions were more likely to produce cubic crystals rather than amorphous layers. These findings suggest the treatments may not have altered nucleation rates, supersaturation dynamics, or growth kinetics sufficiently to influence crystal size. The methodology, nevertheless, provides a practical way to demonstrate the principles underlying magma cooling and grain size, and aligns with the Curriculum for Wales.