Back in middle school physics class, I had a question that bugged me: why is the temperature of an ice-water mixture always zero degrees?
For instance, if I take a big block of ice out of the freezer at minus 18 degrees and pour some tap water over it, does the temperature instantly become zero? Or if I press my hand against ice and a tiny bit melts at that spot, is the temperature right there zero degrees? What does “the temperature is zero degrees” even mean? What exactly is going on with an ice-water mixture?
I asked my teacher at the time. He went through the textbook explanation again, but I was still confused afterward – something just didn’t click.
Today, while out with my kid, I bought a bottle of ice-cold mineral water, and the question popped into my head again. Looking at it now with fresh eyes, it finally makes sense: “the temperature of an ice-water mixture is zero degrees” actually means that in a container at equilibrium, where both ice and water coexist, the water portion is at zero degrees. The ice itself can actually be colder than zero. To put it more precisely, in an ice-water mixture that has reached thermal equilibrium, the water part is at zero degrees – and that’s what the physics textbook is really saying.
I often think about what it means to truly understand something. For me, it’s when I can explain a concept in plain language, from multiple different angles. Plain language, because it’s the most fundamental and solid way I think about the world. And examining something from multiple angles to arrive at a consistent answer is a form of cross-validation. After all, in the real world, we may never know what absolute truth is. The best we can do is verify from different directions, using logic and common sense to minimize the chance of being wrong.