I was stirring some sugar in some water the other day to feed my bouquet of flowers and it occurred to me that sugar would dissolve faster in hot water.
Why do powders dissolve faster in heated liquids? Another example i can think of is jello powder, which wouldn't dissolve at all if your water isn’t hot. Silk Linings
James Tytko posed this to Philip Broadwith from Chemistry World...
Philip - Okay, so the thing that you've got to work out is, sugar is a relatively small molecule, and inside a granule of sugar, there are lots of those molecules packed together, and they have bonds or interactions between them that are holding them together to dissolve it into the solution. What you need to do is separate all of those molecules out from each other, so break those bonds holding the molecules together, and replace those with interactions with the solvent molecules. Water is quite a good solvent for sugar. It will dissolve relatively easily, but the more energy you put into the water, so the hotter you make it, the easier it is to break up those bonds that are holding the sugar molecules together, and the faster the molecules are moving so the more times they'll collide with each other, the more opportunities there are to transfer that energy and make that dissolving happen.
James - Christie suggests correctly that sugar will eventually dissolve in cold water. It'll be slower, but it will eventually dissolve. Whereas with a different solid, jello or jelly powder, it will only dissolve into the solution if there is enough energy in the water, if there's enough heat. What's the difference?
Philip - Okay, so sugar is quite a small molecule and it's quite soluble in water. Jelly's a much bigger molecule. It's actually a protein, so it's chemistry is quite different - the way it interacts with the solvent. The water is a little bit different, but mostly it's because it's a much bigger molecule and those big molecules have a lot more bonds between the molecules, so it's much harder for the water to break them apart. It takes much more to get in between the chain, the gelatin protein, to break them apart and dissolve them in the water. In fact, they never really truly dissolve in a sense. They make what's called a gel or a colloid. That's how jelly is happening. When it's warm, it behaves kind of like a liquid, it's sort of like a solution. But, as it cools, the chains, the proteins start to stick back to each other and try to make a solid. But, in doing so, they trap water in between them. So instead of making a kind of dry, powdery, solid, they make a gel or a jelly.
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