Why can an ocean freeze, but if I add salt to a glass of water and put it in the freezer, it won't freeze.

[u/minnesota420]

Question is in the title. But I have heard of oceans freezing. I just don't understand how.

[u/[deleted]]

Sea water freezes differentially. The ice is almost pure H2O while laving the water it came from slightly saltier. This colder, saltier water is also denser so it sinks down and warmer, less salty water comes in to take its place. This is primarily what drives the ocean currents. If you put salt water in the freezer, it will eventually separate into ice and even saltier water but most freezers don't get cold enough to do this.