Intuitively you may think that hot water will always melt ice faster than a colder counterpart, but this isn’t always the case. Throw a few ice cubes into warm water, and then place a few more under some cold tap water, and see what happens. Spoiler alert: the cold tap will melt the ice faster. For those who just tried it, or for those just taking our word for it–why does cold water melt ice better than hot water?
Why Does Cold Water Melt Ice Better than Hot Water?
If you did some critical thinking, you probably noticed some odd phrasing on our part. We implied that the warm water was standing, while the cold water was moving. The caveat here is how heat gets transferred around.
Melting only occurs as a result of heat transfer. This should make sense on a very primal level, when you touch something hot with something cold, the cold things gets warmer and the hot thing gets a little colder. Eventually the cold thing can get hot enough to melt. Whether you’ve thought about it enough or not, it probably makes sense to you that heat transfer can only occur on points of contact. Contact with air molecules counts.
Since heat can only transfer on points of contact, if you want to melt your ice even faster, you may want to crush it so more surface area is touching the ice’s environment.
Shuffling Things Around
So you’ve figured out that the linchpin to melting ice better with cold water is motion, but why does that work?
Well that’s because water doesn’t equally distribute its temperature instantaneously. When you throw your ice cube into that standing warm water, you end up with a patch of cold water. So there’s the ice cube, some radius of cooler water around it, and then the warm water. The cooler water is going to be a relatively similar temperature to the ice cube, at least compared to the warm water around the cold patch.
Since heat only transfers by point of contact, the warm water has now lost its direct line to the ice cube. It has to transfer heat to the colder water, which then transfers heat to the ice cube. In short, the ice cube ends up insulating itself.
This isn’t the case for your cold tap water. Since the water is moving, there’s never a point where the water’s temperature becomes relatively equal to the ice cube. You have a system where the relative maximum amount of heat transfers to the ice cube at all times. Therefore, you can make cold water melt ice better than warm water.
Want to melt things that aren’t just ice? Try your hand at seeing what you shouldn’t melt here.