|There was a pond with about 10 to 12 feet of water in it; divers in a search and rescue mission were working all night in wet suits and very cold temperatures. The ambient air temperature was 29 to 31 degrees during the search. One official said the water was 24 degrees when the first dive team went in. Could a pond be 24 degrees and not be frozen?|
|Question Date: 2019-11-22|
The phenomena described here could be either "freezing point depression" or "supercooling" of the water. Let's take a look at both.
Freezing point depression is a reduction in the melting temperature (or freezing temperature) of a substance due to an impurity dissolved in that substance. When we dissolve something in water, we change the boiling and melting points of the water. One of the most common things dissolved in water is salt (after all, the ocean is pretty salty!). The dissolved salt prevents the water from freezing as easily, so it reduces the freezing point of water. So, it is likely that this pond has some impurities in it that are causing the freezing point of water to decrease. We use this phenomenon in de-icers on the road. When it's cold and snowy out, the road can sometimes ice over. CalTrans will then pour salt on the road to make the ice form at a lower temperature--in effect, melting the ice at water's normal freezing point. This makes the road less icy.
Another phenomena that could be happening in the lake is called "supercooling". Supercooling happens when water goes below its normal freezing point and doesn't form ice. This is because there are very few impurities in the water to cause the ice to nucleate and form a crystal, around which more ice forms. You may have seen this in videos online where someone freezes a bottle of water and pulls it out of the freezer still in the liquid state. Then, when they jostle the water, ice instantly forms, because it nucleates really fast and the rest of the water wants to be in the frozen state. The water was supercooled because it was still liquid below its freezing temperature (until it was jostled, wherein it became regular ice).
Both of these explanations could be true; however, what do you think would happen if the water was supercooled and the diver jumped in the water? Would the water stay liquid? If the water was experiencing freezing point depression, what would happen if the diver jumped in?
Good question! You might guess that the pond should be frozen at 24 degrees Fahrenheit since pure water freezes at 32°F, but there are multiple reasons the pond can still be liquid.
If water has things mixed into it, like salt then freezing temperature actually changes and generally becomes lower. This is called "freezing point depression" and you can think of it as the water has a harder time packing together into ice when there is other stuff mixed in. Ponds usually have all sorts of things mixed into the water, so they will freeze at some temperature lower than 32°F.
Another reason the pond can still be liquid below 32°F is because of a phenomenon called "super-cooling". Pure water can be cooled to a little below 32°F and not freeze. This is because although the water wants to freeze, there is a barrier to going from the liquid into the ice phase. Once a little bit of ice forms, it's easy for the rest of the water to attach onto the ice crystal, but forming that first little bit of ice is hard, so a super-cooled liquid can be stable for a long time and not freeze.
Between the two effects of "freezing point depression" and "supercooling" a pond could easily reach 24°F and still not freeze.
Before we get to the answer, let's clarify a few things. Some of us have seen frozen lakes or ponds and found that the water below has not frozen. Why is that? When the temperature drops below 32 degrees Fahrenheit, the first thing that will freeze in a pond is the surface water because the surface is exposed directly to the cold air. After a thick enough layer of ice forms on the top, the layer essential insulates the rest of the water from the cold air, given that the air does not continue to get much colder AND that the pond is deep enough. The water below the ice will then stay above freezing temperature, so even if the air is at 24 degrees, the water in the pond can still remain unfrozen.
There is also the possibility that the pond is salty enough not to freezing at a temperature below 32 degrees F, because salts that are dissolved in water will lower the temperature at which the water freezes. However, it is not likely that a pond is salty enough not to freeze at 24 degrees (it would have to contain about 5% salt whereas seawater only has about 3.5% salt), so the most likely explanation is that the pond surface was frozen over while the water underneath was not.
Hope these help! Thank you!
Solutes, such as salts, lower the freezing point of water, so if the pond was not pure freshwater, it could be below 32 degrees Fahrenheit and not freeze.
Salty water freezes at a lower temperature, because the salt ions disorganize the water molecules and prevent them from forming ice crystals at their normal freezing temperature.
The freezing point of salty ocean water is 28.4 degrees F.* Do you know whether the 24-degree temperature was in Fahrenheit or Centigrade? Water freezes at zero degrees Centigrade. Lengths in 'feet' suggest the temperatures would be in Fahrenheit, but the British often use lengths in feet and temperatures in Centigrade.
*28.4 degrees Fahrenheit
Ocean water freezes just like freshwater, but at lower temperatures. Fresh water freezes at 32 degrees Fahrenheit but seawater freezes at about 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.
Well, you didn't tell me what the temperature scale they were using was. If it's Celsius, then it's easy, but from the fact that you mention that it's cold, I'm guessing you mean Fahrenheit.
The presence of salts will lower the freezing temperature of water. Brine can have freezing points as low as 21 degrees Fahrenheit (or -5 Celsius), so yes, it's possible.
Yes. The reason for this is that pure water at equilibrium freezes at 32 degrees at atmospheric pressure. The key words here are pure and equilibrium. The general idea behind equilibrium systems is that macroscopically, the system is not changing. A pond outside with animals swimming around in it and currents going in and out is generally speaking not at equilibrium. For example, if there are convective currents in the water, it may be hard for ice crystals to form. This can keep the water from freezing. And you can imagine a pond might have a lot of other stuff in it besides water that could affect the freezing point. Think of how salt water freezes at a lower temperature than pure water. It’s the same idea. So it can be liquid at 24 degrees.
Yes! This topic can lead into a very interesting field called Thermodynamics, but the main phenomena occurring hear is called “Freezing Point Depression.” Essentially, 32 F is the freezing point of water under “standard conditions.” This mean normal pressure levels and perfectly pure water. Increasing/decreasing the pressure can change the freezing point. The main factor in the situation you described, however, is that pond water is not pure water. When water has impurities mixed into it, the freezing point will be lowered below 32 F. This is actually true regardless of what the impurity is. This phenomena is why cities with harsh winters salt their roads- adding salt to the water will prevent it from freezing at normal temperatures, thus preventing the roads from becoming icy even if it is well below 32 F outside.
So all of the particulate matter, minerals, etc., mixed in the pond water, significantly lowered its freezing point so that it was still liquid at 24 F.
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