In the winter time you may melt the ice on your front sidewalk simply by scattering salt on it. You can do this because salt, like all materials that dissolve in water lowers the freezing point of water. You have created a mixture, salt water, which no longer freezes at the freezing temperature of pure water but at a lower temperature. Because the ocean contains salt, it does not freeze during the winter in temperate climates. But if the temperature falls low enough, as it commonly does in polar regions, even salt water will freeze.