Breaking down a compound into its elements is considered a chemical change because it involves the breaking of chemical bonds and the formation of new substances with different properties. This process alters the composition and structure of the original compound, resulting in a fundamental change at the molecular level.

Yes, the sample of sand could still be a heterogeneous mixture even if all the grains were the same size, because heterogeneity refers to the composition and not just the size of particles. If the sand grains are made up of different minerals or substances, the sample would still be considered heterogeneous due to the varied composition of the grains.