Here is a potential broad question about solubility that could help determine if cocoa mix should be added to water before or after heating to make hot chocolate:
How does temperature affect the solubility of solids in liquids?
The solubility of solids in liquids can vary significantly with temperature changes. Understanding how temperature impacts solubility in general could provide insight into the best approach for dissolving cocoa mix into hot water to make hot chocolate.
Specifically, investigating whether solubility typically increases or decreases with higher temperatures would shed light on whether it is better to add the cocoa mix before heating the water or after. If solubility tends to improve at higher temperatures, then adding the cocoa mix after heating the water could help it dissolve more readily. However, if higher temperatures impede solubility, then adding the cocoa beforehand and heating could be problematic.
Broadly exploring the relationship between temperature and solubility of solids in liquids would provide a framework to reason through the best sequencing for preparing hot chocolate. The findings could then be applied to determine the ideal order of steps for maximum efficiency and optimal dissolving of the cocoa powder.