Get ready to head back to your high school chemistry class, because it’s time for a lesson on water molecules. And this time, this lesson will actually apply to your daily life.
Imagine it’s a hot summer day and you want some ice to add to that homemade lemonade, but you don’t have any on hand. Is it quicker to freeze your ice cube tray using hot or cold water? Common sense tells us to opt for cold water, since it’s closer in temperature to what you need for water to be frozen.
Hate to break it to you, but that’s actually false. You’re going to want to fill those ice cube trays with hot water instead. The real reason has to do with the properties of the molecules. That’s right, I wasn’t kidding, it’s time for a chemistry lesson.
It’s been a while…can you give me a chemistry refresh?
There’s a reason we call water H2O, and it’s not because the phrase sounds trendy or hip. H2O is actually the molecular term for the chemical substance of water. Water molecules consist of two hydrogen atoms and one oxygen atom. Hence the two after the H, and only one O, in H2O.
Atoms within a molecule are actually attached by a covalent bond. Covalent bonds occur between atoms due to the electron pairs within those atoms. They stick together—kind of like how gooey melted cheese binds together gobs of macaroni in these comfort food dishes.
Why do I need to know the science behind it?
These facts are vitally important for understanding why hot water freezes faster than cold water, and we can blame it all on a recent study done at the School of Electrical and Electronic Engineering at Nanyang Technological University in Singapore. They wanted to find a reason for the Mpemba effect, which is the fancy title for the chemical reaction we are trying to solve (hot water freezing faster).
According to their study, the change in covalent bonds between hot and cold H2O molecules is what makes all the difference when trying to freeze water. Unlike some covalent bonds that soften when applying heat, H2O molecules actually tighten up. The energy from heat squeezes and strengthens the bond between the hydrogen atoms and the oxygen atom, while simultaneously spreading out from the other molecules in the water. AKA, it’s getting ready to become water vapor.
Why hot water freezes faster.
When water is frozen, a similar reaction happens. The molecules initially tighten up and bind together to create a solid form. Since hot water’s molecules are already tightened up, it has a head start on looking it should in its frozen state. Cool water would take time to reach this point. Ergo, hot water freezes in less time than cold water does.
So next time you’re in a pinch and need ice cubes ASAP, just boil up some water to freeze. You’ll have ice cubes in no time!