Why does fine powder (like flour or powder milk) mixes evenly when stirred in cold water, but not in hot water (where the mixture becomes lumpy)?

Question

Why does fine powder (like flour or powder milk) mixes evenly when stirred in cold water, but not in hot water (where the mixture becomes lumpy)?

In: Chemistry

Answer 1

Most powders dissolve better in hot water because the molecules of water have more energy and are “bumping” into the powder molecules. Hot water molecules have more energy and move more than cold molecules.

In some powders, mostly those that contain starches, the rapid mixing of the powder and hot water becomes a problem. The hot water immediately bonds with the outer layer of the bits of powder, but they form a shell which prevents water from reaching the subsequent layers of powder.