Asked by: Michelle du Bois, Portland, Oregon, USA
In hot water, molecules are moving around more, so there are more collisions between the water molecules and a solid.
A solvent is a substance that can dissolve other substances – water is a good solvent. Dissolving a solid requires energy input to disrupt the forces holding the molecules together, both in the solid and the water.
Most solids, including sugar and salt, become more soluble with increasing temperature. This is because heat increases molecular movement, causing more collisions between the water molecules and the solid. But the opposite is true for gases, which tend to become less soluble as a solvent’s temperature increases. As the gas molecules become more active, they can break free from the liquid, which explains why drinks soon lose their carbon dioxide fizz on a hot day.
Dr Emma Davies is a science writer and editor with a PhD in food chemistry from the University of Leeds. She writes about all aspects of chemistry, from food and the environment to toxicology and regulatory science.