Why do a few big ice cubes seem to melt more slowly than many smaller cubes? asks a reader.
If the ice in your drive-through drink disappears on the short trip home, don’t blame the kid who filled your cup. Some of that crushed ice had vanished before your drink even settled into the cup holder.
But when you arrive home and plop in some big cubes, they seem to last forever. How come?
Large cubes melt more slowly because of their surface area-to- volume ratio. Imagine a big cube of ice, with six sides. Heat flows from the air (or your drink) into the ice, through the surface. As the temperature of the ice rises, it begins to melt.
Now imagine chopping that cube into pieces. Cut the cube in half down the middle, and the two remaining cubes will have a total of 12 sides. Cut those two, and the four blocks sport 24 sides. The result: Same volume of ice, but much more surface area. With more “faces” exposed to air or liquid, it’s easier for heat to diffuse into the ice. And so smaller cubes melt faster than the same volume of bigger cubes. Crushed ice, unfortunately, melts fastest of all.
(Shape matters, too. A sphere is the most compact shape for physical matter, the smallest surface area for a given volume. So ice spheres will melt more slowly than traditional cubes.)
Knowing the principle behind ice-cube melting can help you plan your icy-drink strategy. For quick cooling, use crushed ice, which absorbs heat fastest. To keep ice floating in your drink longer, use big, old-fashioned ice-tray cubes.
Meanwhile, if you bang your knee or suffer a sports-related injury, try using a bag of crushed ice (or frozen peas) rather than big cubes. The bag will mold itself to the curve of your leg, and the small, icy bits will make for a quick blast of cold.
The surface-to-volume effect also applies to the sugar added to your iced tea. A spoonful of finely ground sugar will dissolve faster than an equal amount of coarsely-ground crystals. Likewise, a big bar of soap will last longer than the same bar divided into hotel- sized pieces.
Surface vs. volume also affects how animals fare in cold and hot weather. On a frigid day, a mouse loses body heat faster than the cat in chilly pursuit. On the other hand, a large, bulky animal or human has more trouble getting rid of built-up internal heat on a hot summer day.
The simple ratio can even have deadly implications. A silo full of intact corn kernels or a warehouse full of wooden boards isn’t in much danger of suddenly exploding. But grind the corn into a fine flour or the wood into sawdust, and the greater surface-to-volume ratio present dangers. If a large amount of such dust hangs suspended in oxygen-rich air, and there is an ignition source–hot machinery, static electricity, or simply friction between particles–there can be a fiery blast. In fact, coal dust is one cause of deadly mine explosions.