Ocean water freezes at a lower temperature than freshwater

Ocean water freezes just like freshwater, but at lower temperatures. Fresh water freezes at 0 degrees Celsius (32 degrees Fahrenheit), but seawater freezes at about -1.9 degrees Celsius (28.4 degrees Fahrenheit) because of the salt in it. When seawater freezes, however, the ice contains very little salt because only the water part freezes. It can be melted down to use as drinking water.

At least 15 percent of the ocean is covered by sea ice some part of the year. On average, sea ice covers almost about 25 million square kilometers (10 million square miles) of the Earth.

Sea water becomes more and more dense as it becomes colder, right down to its freezing point. Fresh water, on the hand, is most dense while still at 4 degrees Celsius (39.2 degrees Fahrenheit), well above the freezing point. The average temperature of all ocean water is about 3.5 degrees Celsius (38.3 degrees Fahrenheit).

At what temperature does ocean water freeze?

The freezing temperature of salt water depends on how much salt you put in. Oceanographers define the “salinity” of water as the number of grams of salt per 1000 grams of water. (Since 1000 grams of water is almost exactly 1 liter, it’s also grams of salt per liter).

Here’s a table which gives the freezing point of water at a number of different salinities:

S(g/kg) 0 10 20 24.7 30 35 T(freezing) (C) 0 -0.5 -1.08 -1.33 -1.63 -1.91

Away from rivers and glaciers, the ocean has a salinity of about 35. The Atlantic is about 1 salinity units saltier than the Pacific. So ocean water freezes at about -1.91 degrees C.

The results of your experiment are strange. Usually, fresh water freezes at 0 C. Did you make sure that your thermometer was completely surrounded by the ice? Don’t freeze the thermometer into the ice: ice expands when it freezes, and you could easily break the thermometer. Instead, take the ice out of the freezer, put it it a styrofoam cup, and let it melt, and put the temperature in the melt water. (the meltwater will be the same temperature as the ice until the ice has melted completely). If your salt water didn’t freeze at -18 C, you must have put an awful lot of salt in! Be sure to measure the amount of salt you put in. As long as the amount of salt isn’t too large, you should expect the freezing temperature to drop by .054 degrees for each gram of salt. Try using 35 or 100 grams per liter.

Another thing to watch out for: when salt water freezes, the salt can’t join with the ice crystals, and tends to stay in the liquid water. As a result, ice made from salt water has less salt in it than the water that’s left behind. The unfrozen water gets saltier and saltier, and eventually becomes so salty that your freezer isn’t cold enough to freeze it. So there will be a little bit of very salty brine left in the bottom of the cup. This is called “brine rejection”, and is very important for the study of sea ice in the Arctic and Antarctic oceans.

You can do an experiment to demonstrate this. Take a 100 milliliters of water and put about 10 grams of salt in it (if you do the math, this means the salinity is 100). Put it in the freezer until it’s half ice, half water. Take it out, and pour the liquid into a cup labeled “brine”, and put the ice into another cup labeled “ice”. Let the ice melt, and then taste the water in both cups. (only a sip! if you drink a lot, it’ll upset your stomach!) The “ice” cup should be a lot less salty than the “brine” cup.

I confess I haven’t actually done this experiment, but it should work. You might have to play with the amount of salt to get it to freeze right.

Temperature of Ocean Water

Most of the solar radiation (light and heat) that hits the ocean is absorbed in the first few tens of meters of water. Waves and turbulence mix this heat downward quickly. The surface layer of the ocean is well mixed from the top to the bottom of that layer. The temperature of the surface waters (the mixed layer) varies mainly with latitude. The polar seas (high latitude) can be as cold as -2 degrees Celsius (28.4 degrees Fahrenheit) while the Persian Gulf (low latitude) can be as warm as 36 degrees Celsius (96.8 degrees Fahrenheit). The average temperature of the ocean surface waters is about 17 degrees Celsius (62.6 degrees Fahrenheit).
There is a boundary between surface waters of the ocean and deeper layers that are not mixed. The boundary usually begins around 100-400 meters and extends several hundred of meters downward from there. This boundary region, where there is a rapid decrease of temperature, is called the thermocline. 90 % of the total volume of ocean is found below the thermocline in the deep ocean. Here, temperatures approach 0 degrees Celsius. So even though surface waters can be a comfortable 20 degrees Celsius (good for swimming in!), the majority of our ocean water has a temperature between 0-3 degrees Celsius (32-37.5 degrees Fahrenheit).

The density of ocean water continuously increases with decreasing temperature until the water freezes. Ocean water, with an average salinity of 35 psu, freezes at -1.94 degrees Celsius (28.5 degrees Fahrenheit). At very high latitudes, ocean water can reach these low temperatures and freeze. Dissolved salts in the water tend to be rejected by the forming ice so that sea ice is only about 1 % salt. An interesting tidbit for those of you interested in survival tactics - because of the lessened amount of salt, melted sea ice would be fit to drink even if sea water is not! Sea ice formation at high latitudes ultimately drives circulation of the deep waters of the ocean.

At the surface of the ocean, temperature measurements are often taken by thermometers placed on buoys. Recently, the Argo program has taken on the task of monitoring the state of ocean surface waters across the globe. The Argo program is deploying floats that measure salinity and temperature throughout the surface layer of the ocean. 3,000 floats will be deployed all over the ocean by 2003. Each float is programmed to sink 2,000 meters down, drifting at that depth for about 10 days. The float then makes its way to the surface measuring temperature and salinity the whole time. Data is transmitted to a satellite once the float reaches the surface, so that scientists have access to the state of the ocean within hours of the data collection. Each float will last 4-5 years. At a greater depth in the water, in situ measurements are often made with a CTD instrument, where the instrument is placed in the ocean water from a ship or a platform.