Why does seawater become lighter when it freezes? Why is it freshwater after the sea freezes

The conditions for seawater to freeze are not the same as for freshwater to freeze. Anyone who lives by the sea has experienced that when the cold air comes in early winter, the shallow ponds on land quickly freeze into a thin layer of ice; In the depths of winter, the rivers freeze, but the sea is still choppy, with undulating waves. Only when cold waves break out frequently and the air is at a low temperature for a long time will the sea water freeze.

There are three conditions required for seawater to freeze: the air temperature is lower than the water temperature, and a large amount of heat is lost in the water; There is already a small amount of supercooling relative to the temperature at which the water begins to freeze (freezing point); There are condensation nuclei of impurities such as suspended particles and snowflakes in the water. Fresh water is most dense around 4, and the water temperature can freeze when it drops below 0.

Seawater contains a lot of salt, and due to its high salinity, the temperature required for freezing is lower than that of fresh water, and the water temperature is lower than 4 at its highest density. As the salinity increases, the freezing point of the seawater and the temperature at its highest density gradually decreases.

When sea ice is first born, it is a needle-like or flake-like ice crystal; This is followed by the formation of a paste or sponge; After further freezing, it becomes an ice skin or ice cake floating on the surface of the sea, also called lotus leaf ice; When the sea is covered with this ice, it extends in the direction of thickness, forming gray and white ice that covers the sea.

Sea ice is a mixture of freshwater ice crystals, "brine" and bubbles containing salt. According to the development stage, it can be divided into six categories: primary ice, Nile ice, cake ice, early ice, one-year ice and old ice. According to the state of movement, it can be divided into two categories: fixed ice and drift ice. Fixed ice freezes together with the coast, seabed or island, and can rise and fall with the surface of the sea, extending several meters or hundreds of kilometers from the sea.

Drift ice floats on the surface of the sea and moves in all directions with the surface wind direction and currents.

The salinity of the seawater is very high, about around, and the freezing point of the seawater at this salinity is about -2. Even when -2 is reached, the density of the surface water and the density of the underlying sea water are different, resulting in strong convection and greatly hindering the formation of sea ice. In addition, the ocean is heavily influenced by ocean currents, waves, storms, and tides, and ice crystals are difficult to form when the temperature is not too low.

The difficulty of freezing the ocean is very beneficial to us human beings, and the fact that the ocean does not freeze in winter makes the world's climate relatively humid and easy for the development of organisms. In addition, the fact that the seas do not freeze allows our ships to continue sailing in winter.

The places where sea ice is now present all year round are the Antarctic and the Arctic. In the Arctic, the area of sea ice is the size of Oceania, even in summer.

Because when the seawater freezes, the pure water in the seawater will be automatically separated, and the salts contained in the seawater will be excluded, so the ice will become light, and the water when the ice melts will become fresh water, and the phenomenon that the seawater freezing can eliminate other impurities can inspire people to use the condensation method for seawater desalination.

Seawater is water in or from the sea. Seawater is fluid, and there is no limit to the amount of water available to humans who are in dire straits. Seawater is a veritable liquid mineral, with an average of 35.7 million tonnes of minerals per cubic kilometre of seawater, and 80% of the more than 100 elements known in the world can be found in seawater.

Seawater is also the first freshwater on land and the regulator of climate, the ocean evaporates 4.5 million cubic kilometers of fresh water every year, of which 90% is returned to the ocean through rainfall, 10% becomes rain and snow falling on the earth, and then returns to the ocean down the river. Seawater desalination technology is developing into an industry. Some people predict that with the deterioration of the ecological environment, the last way for humans to solve the water shortage is likely to be desalination.

There are many reasons why the sea does not freeze, the ocean is a whole, so we should all protect the ocean, pollution has no "national borders" here, here is a little bit of my understanding:

1) The existence of monsoons and ocean currents enables the seawater to exchange heat and energy, from the equator to the poles or from the poles to the equator, the ocean is generally connected, the four oceans they are a whole, and there is a certain balance of heat transfer.

2) The salinity of the seawater is very high, which lowers the freezing point of the seawater. Freshwater freezing is at 0 degrees Celsius, with 10 degrees of salinity, while 35 degrees of salinity is the average salinity of the oceans on Earth, so the freezing point of seawater is around the limit.

3) The maximum density of seawater is not at 4, but decreases with increasing salinity. It decreases faster than the freezing point decreases as salinity increases. As can be seen from the figure below, the two descending lines intersect at the coordinates of salinity.

Because the average salinity of seawater is much exceeded, when the seawater reaches the freezing point, it has not yet reached the maximum density of seawater, so the convective mixing of seawater does not stop, which greatly hinders the freezing of seawater.

4) The energy brought by submarine volcanoes or crustal movements is also very large.

Wait a minute. The following ** is a story about water circulation and exercise that I hope will help you!

1. There are monsoons and ocean currents in reserve, so that the seawater can exchange heat energy, from the equator to the poles or from the poles to the equator, the ocean is generally connected, the four oceans they are a whole, and there is a certain balance of heat transfer.

2) The salinity of the seawater is very high, which lowers the freezing point of the seawater. Freshwater freezes at 0 degrees Celsius, with 10 degrees of salt destruction and 35 degrees of salinity. The average salinity of the oceans on Earth is , so the freezing point of the sea is around .

3) The maximum density of seawater is not at 4, but decreases with increasing salinity. It decreases faster than the freezing point decreases with increasing salinity. As you can see from the image below, the two descending lines intersect on the coordinates of salinity and .

Because the average salinity of seawater is much exceeded, when the seawater reaches the freezing point, it has not yet reached the maximum density of seawater, so the convective mixing of seawater does not stop, which greatly hinders the freezing of seawater.

4. The energy brought by submarine volcanoes or crustal jujube transportation operations is also very large.