Final answer:
The huge difference in melting points between 60-70% and 35% solutions of hexafluorosilicic acid is due to freezing point depression, a colligative property that lowers the melting point of a solvent when a solute is added. A higher concentration of the acid will result in greater disruption of the ice structure, leading to a higher melting point compared to a lower concentration.
Step-by-step explanation:
The significant difference in the melting point of hexafluorosilicic acid solutions with differing concentrations can be attributed to a phenomenon known as freezing point depression. When a solute is added to a solvent, in this case, hexafluorosilicic acid to water, the solute particles disrupt the formation of the solid lattice structure, making it more difficult for the solvent to freeze, thus lowering the freezing/melting point.
In the case of hexafluorosilicic acid, a higher concentration of solute (acid) will further disrupt the ice lattice formation compared to a lower concentration, resulting in a greater depression of the melting point. Therefore, a 60-70% solution has a higher melting point compared to the 35% solution, because the lower concentration of acid in the 35% solution can't disrupt the ice structure as effectively as the higher concentration does.
Colligative properties, like freezing point depression, depend on the concentration of the solute particles rather than their identity. This is why varying concentrations of the same substance can have vastly different melting points.