Salt water freezes at a lower temperature than fresh water, so polar seas stay liquid longer.

The longer version

Dissolved salt disrupts the neat lattice that water molecules must adopt in order to freeze, so a lower temperature is needed before ice can form at all. Typical seawater begins freezing roughly two degrees below the freshwater point.

Ice itself rejects most of the salt, leaving concentrated brine pockets that drain downward and make the water beneath denser. The resulting cover then insulates against further freezing, so thickening slows quickly. Because ice formation removes fresh water, salinity nearby rises temporarily, driving local convection. This is why polar seas stay liquid well below zero. That same rejection of salt also helps drive deep circulation in polar regions.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.