Why does sugar dissolve better in hot water than in cold?

When you pour a sugar cube into your coffee, it dissolves quickly. But when the same sugar cube is poured into a cold drink, it takes a long stirring before it finally breaks down. This is because the dissolution of sugar in a solvent is an endothermic process, which requires heat. Water molecules are more agitated in a hot liquid than in cold water. As a result, they move faster, which leads to more frequent shocks with the sugar molecules. These will quickly disintegrate and disperse in the water, forming a homogeneous solution.

More sugar can be dissolved in hot water than in cold water

This is also true for most soluble solids, such as salt, which melts faster in a pot of boiling water (although this is much less obvious than with sugar). By the same process, hot water also helps the tea molecules to diffuse, making it faster to prepare than cold water. Note that the amount of sugar that we can dilute in water increases with the temperature. At 20°C, it is possible to dissolve 2 kg of sugar per litre of water, but it is possible to incorporate up to 4 kg of sugar per litre of water at 90°C.

Gases less soluble in hot water

Conversely, gases are less soluble in hot water than in cold water. The dissolution of gas in water is exothermic: the gas produces heat when it dissolves, increasing water molecules' agitation. As a result, the water molecules will collide with the gas molecules, then disintegrate and escape into the air. Because of this phenomenon, soft drinks should be kept in the fridge so that the maximum amount of CO2 remains inside. But this is also a problem for fish, which have less oxygen in the water when the ocean warms up.

These are simplified explanations because, in reality, many other phenomena come into play, such as the entropy of the liquid. There are also exceptions, such as calcium carbonate and calcium sulfate, whose solubility decreases with temperature.