|Naengmyun, SuperH-mart, Houston, TX|
It goes back to our friend, colligative properties. Remember, in earlier posts, that the freezing point of water is decreased the more things dissolved in it - and we can take advantage of this as a way of concentrating solutions without having to heat it up to boil off the extra water. Well, there's another thing: the freezing point of water is also decreased under increasing pressure. That's how ice skates work: by putting the full weight of a person under thin blades, the space under the blades are under high pressure, and the ice temporarily melts - allowing skater to move on a thin film of water as lubrication. As soon as the pressure is off, the water then refreezes.
And this is what's going on in the bag of ice. A piece of ice sitting atop another will exert enough pressure to allow a little bit of ice to melt, until the point when the pressure is low enough to permit refreezing. Eventually, the two pieces will merge.
So, the best way to keep the ice as separate pieces? Keep the place really cold. So cold that even pressure induced melting will not happen.
Or just freeze water in ice cube trays.
I have to thank Dr. John Coupland on Twitter for referring me to this review article on the physics of phase transitions of ice. Basically, what I described above on nature of water ice is a bit of an oversimplification. In fact, the slippery nature of ice has long been postulated to be due to a layer of liquid water on ice - well below the freezing point. Experiments exploring this extend as far back to the 1800s. But to the point of merging ice chunks - looks like nothing short of absolute zero is going to keep the ice from merging with each other given water's tendency to form this liquid skin.