If you listen with a linguist’s ear, you’ll find that the ice in your drink isn’t actually popping, which implies a certain hollowness. But it certainly does snap and, on occasion, crackle.
First, the snap.
When you plunge a cold ice cube into a warmer liquid, the water warms up parts of the ice cube, which tends to make those parts expand slightly. This places a stress on the ice crystal, because ice has a very rigid structure and it can’t just expand here and there at random. The only way the crystal can relieve these stresses is to crack. That’s the snap you hear.
Next, the crackling, which sounds like a rapid series of tiny explosions. And that’s exactly what it is. Unless you’ve made your ice cubes out of boiled water, there was some dissolved air in the water that went into your ice tray, or, if you’re one of those most fortunate of human beings, into your automatic ice maker.
As the water froze, there was no room for the air in the rigid, solid structure that is ice, so the air had to settle out into tiny, isolated bubbles. These bubbles are what makes the ice cloudy instead of crystal clear.
Now put that bubble-filled ice cube into a drink. The water works away at melting the surface of the cube, eating; its way in deeper and deeper. As it goes, it encounters an air bubble. When that bubble was formed, it was freezer-cold air. But now it is being warmed up by the advancing water, and it wants to expand.
It can’t expand, however, until its imprisoning ice wall has been thinned enough to allow it to break through. When it does, Crack! It explodes its way out Thousands of those tiny breakouts, happening all over the ice surfaces, make a faint crackling or sizzling noise.
The crackles of icebergs and glaciers as they move south into warmer water can be heard loud and clear by the inhabitants of Arctic-prowling submarines.
Boil some water for several minutes to get most of the dissolved air out of it. Let it cool, pour it into an ice-cube tray, and freeze it. You’ll find that there won’t be many bubbles in the ice cubes. Compare one with an ordinary ice cube by holding them up to a strong light.
When you put the boiled cubes into a drink they may snap, but they won’t crackle much. You’ll enjoy a relatively quiet drink.
