If you’re freezing a cup of ocean water (about 3.5% salt), it’s not going to freeze at the same 32°F (0°C) as your tap water. Salt lowers the freezing point, a phenomenon called freezing-point depression—it’s why we throw salt on icy roads in winter. For seawater, that magic number drops to around 28.4°F (-2°C).
So, if your freezer is set to a standard 0°F (-18°C), seawater will freeze eventually, but it’ll lag behind a plain water ice cube by a solid chunk of time. In a typical home freezer, a shallow tray of salt water might take two to three hours to fully freeze, versus 90 minutes for fresh water.
But here’s where it gets ironic: the first ice that forms isn’t pure salt water. It’s mostly fresh. Salt gets pushed out of the crystal lattice as ice forms, leaving behind a briny slush. So your frozen blob is actually a mix of clean ice and trapped salty pockets.
What about a giant lake or the Arctic Ocean?
Scale changes everything. A small puddle of salt water in your backyard might freeze in a few hours on a 20°F night. But the Arctic Ocean? That bad boy takes months to develop a thick, multi-year ice pack. The ocean surface starts freezing when air temps hit around 28°F, but wind and waves mix things up.
How long does it take salt water to freeze? - YouTube
In the ocean, ice forms in a process called frazil ice—tiny needles of pure ice that clump together. It’s a slow dance because the salt acts like an invisible bouncer, holding back the freezing party. Scientists say it can take roughly 10 to 14 days of sustained cold below -4°F (-20°C) to build ice thick enough for a polar bear to stroll on.
And even then, the ice is never 100% salt-free. As it ages, brine drains out through little channels, making it “multiyear ice” that’s mostly fresh but still slightly salty. So, your driveway puddle freezes faster than a polar ice cap—go figure.
