Why do substances dissolve more easily at higher temp?

Firstly, what do you mean by 《easily》? more repidly or higher solubility ?

At higher temperature, a solid dissolves more rapidly in solvent (e.g. water). This is because the particles move more rapidly at higher temperature, and this makes the particles of solute and those of solvent mix together more rapidly.

At higher temperature, some solids dissolve more but some dissolve less. If the process of dissolving is exothermic (放熱的), solubility of the solid decreases. If the process of dissolving is endothermic (吸熱的), solubility of the solid increases. This can be explained by considering the process in terms of equilibrium, and applying Le Chatelier's principle.

既然你識得entropy,你應該都識得 gibbs free energy.
當然溫度高d, difference of g會係負數,個reaction就會行(因為已經考慮晒energetic 同kinetic factor)
但係如果係gas就會相反,gas溫度越高,越溶唔到多野,因為個vapour pressure大,vapour就已經好濃,就溶唔到幾多野入去啦.

因為冷縮熱脹，分子間的空隙加大。冷縮熱脹好像只有冰是例外，因此這現象沒有例外。