In winter, the weather is cold and the river is easy to freeze. Why is it difficult for sea water to freeze?

The freezing conditions of seawater are different from those of fresh water. Anyone who has lived by the sea knows that when the cold air strikes in early winter, the pool on the shallow sea will soon freeze into a thin layer of ice. In late winter, the rivers are frozen, but the sea is still rough. Only when frequent cold snap strikes and the air stays cold for a long time will the sea water freeze. The freezing point of seawater is -2℃, which is lower than that of fresh water. In addition, there are many salts in seawater, most of which are in the form of ions and constantly move in the solvent, so even at the temperature of zero degrees Celsius, it is difficult for water molecules to condense into crystals, which is the main reason why seawater cannot become ice.

The density of water surface is different from that of deep layer, which causes strong convection inside the water and cannot freeze. In addition, sea ice will only form when the air is cold for a long time, and cold currents often appear, which makes the water temperature in the deep and surface layers close to freezing point. There are three conditions for seawater to freeze. The first condition is that the temperature is lower than the water temperature, resulting in a large loss of heat in the water.

The second condition is that it is a little too cold relative to the temperature (freezing point) at which water begins to freeze. The third condition is that there are impurities such as suspended particles and snowflakes in the condensation nucleus of water. The high salinity of seawater requires a lower temperature than fresh water to freeze. The freezing point of seawater decreases with the increase of salinity. When the surface of seawater tends to freeze, the density increases and the seawater sinks, causing vertical convection and mixing of water. The surface water began to freeze, and the released salt increased the salinity of the adjacent layer, which made the freezing point of the adjacent water drop again. So it is difficult to freeze on the sea.

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