How Does Antifreeze Work, Anyway?

It’s a question we’ve all asked, or been a little embarrassed to ask, for years: How come a 60/40 antifreeze-water mix freezes at a lower temperature than either 100 percent water OR 100 percent antifreeze? The answer may surprise, because it has nothing to do with any reaction between the water and the ethylene or propylene glycol that makes up the majority of automotive antifreeze products. In fact, it’s all about apples and grapefruit; it’s very much like packing fruit in a crate. Water molecules (the familiar H2O) “freeze” by surrendering some heat to the surroundings and become less active. Instead of moving around freely in the liquid state, they settle down into neat, ordered patterns (which chemists call “crystal lattices”) like grapefruit packed in a shipping crate. Adding antifreeze to water is like adding apples to the grapefruit; the apples get in the way and prevent the grapefruit from easily settling into their neat rows in the crate. The same thing happens when big antifreeze molecules get in the way of the water molecules trying to solidify into ice, delaying freezing until the temperature drops below 0C. Why doesn’t 100 percent antifreeze work even better? Because pure antifreeze acts like a crate of apples: they settle into their own neat pattern unless something else, like water (or in the apple box, grapefruit) interrupts the neat packing. Again, freezing is delayed by the interference, but this time it’s water which interferes with the antifreeze. In the end, each needs each other to be effective, although they don’t react with each other in the solution. And why is freezing coolant so deadly to rads and engine blocks in the first place? Because water is one of the few liquids on earth that expands when it freezes. It’s a good thing, because if it didn’t, lakes and oceans would freeze from the bottom up, and life on earth would never have evolved!