Thermophysical properties and freezing point of aqueous solutions of NaCl and CaCl2. Experiments with ice

At what temperature does water freeze? It would seem - the simplest question that even a child can answer: the freezing point of water at normal atmospheric pressure of 760 mm Hg is zero degrees Celsius.

However, water (despite its extremely wide distribution on our planet) is the most mysterious and not fully understood substance, so the answer to this question requires a detailed and reasoned discussion.

In Russia and Europe, the temperature is measured on the Celsius scale, the highest value of which is 100 degrees.
The American scientist Fahrenheit developed his own scale with 180 divisions.
There is another unit of temperature measurement - kelvin, named after the English physicist Thomson, who received the title of Lord Kelvin.

States and types of water

Water on planet Earth can take three main states of aggregation: liquid, solid and gaseous, which can transform into different forms that simultaneously coexist with each other (icebergs in sea water, water vapor and ice crystals in clouds in the sky, glaciers and free-flowing rivers ).

Depending on the characteristics of the origin, purpose and composition, water can be:

fresh;
mineral;
nautical;
drinking (here we include tap water);
rain;
thawed;
brackish;
structured;
distilled;
deionized.

The presence of hydrogen isotopes makes water:

light;
heavy (deuterium);
superheavy (tritium).

We all know that water can be soft and hard: this indicator is determined by the content of magnesium and calcium cations.

Each of the types and aggregate states of water we have listed has its own freezing and melting point.

Freezing point of water

Why does water freeze? Ordinary water always contains some amount of suspended particles of mineral or organic origin. It can be the smallest particles of clay, sand or house dust.

When the ambient temperature drops to certain values, these particles take on the role of centers around which ice crystals begin to form.

Air bubbles, as well as cracks and damage on the walls of the vessel in which water is located, can also become crystallization nuclei. The rate of water crystallization is largely determined by the number of these centers: the more of them, the faster the liquid freezes.

Under normal conditions (at normal atmospheric pressure), the temperature of the phase transition of water from a liquid to a solid state is 0 degrees Celsius. It is at this temperature that water freezes on the street.

Why does hot water freeze faster than cold water?

Hot water freezes faster than cold water - this phenomenon was noticed by Erasto Mpemba, a schoolboy from Tanganyika. His experiments with mass for making ice cream showed that the rate of freezing of the heated mass is much higher than that of the cold one.

One of the reasons for this interesting phenomenon, called the "Mpemba paradox", is the higher heat transfer of a hot liquid, as well as the presence in it of a larger number of crystallization nuclei compared to cold water.

Are the freezing point of water and altitude related?

With a change in pressure, often associated with being at different heights, the freezing point of water begins to radically differ from the standard, characteristic of normal conditions.
Crystallization of water at a height occurs at the following temperature values:

Paradoxically, at an altitude of 1000 m, water freezes at 2 degrees Celsius;
at an altitude of 2000 meters, this happens already at 4 degrees Celsius.

The highest freezing temperature of water in the mountains is observed at an altitude of over 5,000 thousand meters (for example, in the Fann Mountains or the Pamirs).

How does pressure affect the process of water crystallization?

Let's try to link the dynamics of changes in the freezing point of water with changes in pressure.

At a pressure of 2 atm, water will freeze at a temperature of -2 degrees.
At a pressure of 3 atm, the temperature of -4 degrees Celsius will begin to freeze water.

With increased pressure, the temperature of the beginning of the water crystallization process decreases, and the boiling point increases. At low pressure, a diametrically opposite picture is obtained.

That is why in conditions of high mountains and a rarefied atmosphere it is very difficult to cook even eggs, since the water in the pot boils already at 80 degrees. It is clear that at this temperature it is simply impossible to cook food.

At high pressure, the process of ice melting under the blades of the skates occurs even at very low temperatures, but it is thanks to him that the skates glide on the ice surface.

The freezing of skids of heavily loaded sleds in the stories of Jack London is explained in a similar way. Heavy sleds that put pressure on the snow cause it to melt. The resulting water facilitates their sliding. But as soon as the sleds stop and linger for a long time in one place, the displaced water, freezing, freezes the skids to the road.

Crystallization temperature of aqueous solutions

Being an excellent solvent, water easily reacts with various organic and inorganic substances, forming a mass of sometimes unexpected chemical compounds. Of course, each of them will freeze at different temperatures. Let's put this in a visual list.

The freezing point of a mixture of alcohol and water depends on the percentage of both components in it. The more water added to the solution, the closer to zero its freezing point. If there is more alcohol in the solution, the crystallization process will begin at values close to -114 degrees.
It is important to know that water-alcohol solutions do not have a fixed freezing point. Usually they talk about the temperature of the beginning of the crystallization process and the temperature of the final transition to the solid state.
Between the beginning of the formation of the first crystals and the complete solidification of the alcohol solution lies a temperature interval of 7 degrees. So, the freezing point of water with alcohol of 40% concentration at the initial stage is -22.5 degrees, and the final transition of the solution to the solid phase will occur at -29.5 degrees.

The freezing point of water with salt is closely related to the degree of its salinity: the more salt in the solution, the lower the position of the mercury column it will freeze.

To measure the salinity of water, a special unit is used - "ppm". So, we have found that the freezing point of water decreases with increasing salt concentration. Let's explain this with an example:

The salinity level of ocean water is 35 ppm, while the average value of its freezing is 1.9 degrees. The degree of salinity of the Black Sea waters is 18-20 ppm, so they freeze at a higher temperature in the range from -0.9 to -1.1 degrees Celsius.

The freezing point of water with sugar (for a solution whose molality is 0.8) is -1.6 degrees.
The freezing point of water with impurities largely depends on their amount and the nature of the impurities that make up the aqueous solution.
The freezing point of water with glycerin depends on the concentration of the solution. A solution containing 80 ml of glycerin will freeze at -20 degrees, when the glycerol content is reduced to 60 ml, the crystallization process will begin at -34 degrees, and the beginning of freezing of a 20% solution will be minus five degrees. As you can see, there is no linear relationship in this case. To freeze a 10% solution of glycerin, a temperature of -2 degrees will suffice.
The freezing point of water with soda (meaning caustic alkali or caustic soda) presents an even more mysterious picture: a 44% caustic solution freezes at +7 degrees Celsius, and 80% at + 130.

Freezing of fresh water

The process of ice formation in freshwater reservoirs occurs in a slightly different temperature regime.

The freezing point of water in a lake, just like the freezing point of water in a river, is zero degrees Celsius. The freezing of the cleanest rivers and streams does not start from the surface, but from the bottom, on which there are crystallization nuclei in the form of bottom silt particles. At first, snags and aquatic plants are covered with a crust of ice. As soon as the bottom ice rises to the surface, the river instantly freezes through.
Frozen water on Lake Baikal can sometimes cool down to negative temperatures. This happens only in shallow water; the water temperature in this case can be thousandths, and sometimes hundredths of one degree below zero.
The temperature of the Baikal water under the very crust of the ice cover, as a rule, does not exceed +0.2 degrees. In the lower layers, it gradually rises to +3.2 at the bottom of the deepest basin.

Freezing point of distilled water

Does distilled water freeze? Recall that for water to freeze, it is necessary to have some crystallization centers in it, which can be air bubbles, suspended particles, as well as damage to the walls of the container in which it is located.

Distilled water, completely devoid of any impurities, does not have crystallization nuclei, and therefore its freezing begins at very low temperatures. The initial freezing point of distilled water is -42 degrees. Scientists managed to achieve supercooling of distilled water to -70 degrees.

Water that has been exposed to very low temperatures but has not crystallized is called "supercooled". You can place a bottle of distilled water in the freezer, get it supercooled, and then demonstrate a very effective trick - see the video:

By gently tapping on a bottle taken from the refrigerator, or by throwing a small piece of ice into it, you can show how it instantly turns into ice, which looks like elongated crystals.

Distilled water: does this purified substance freeze or not under pressure? Such a process is possible only in specially created laboratory conditions.

Freezing point of salt water

If you notice, then in the sea water freezes at temperatures well below zero degrees. Why is this happening? It all depends on the concentration of salt in it. The higher it is, the lower the freezing point. On average, an increase in the salinity of water by two ppm lowers its freezing point by one tenth of a degree. So judge for yourself what the ambient temperature should be so that a thin layer of ice forms on the surface of the sea, with a salinity of 35 ppm. It should be at least two degrees below zero.

The same Sea of \u200b\u200bAzov, with a salinity of 12 ppm, freezes at a temperature of minus 0.6 degrees. At the same time, the Sivash adjacent to it remains unfrozen. The thing is that the salinity of its water is 100 ppm, which means that for the formation of ice here, at least six degrees of frost is needed. In order for the surface of the White Sea, where the salinity level of the water reaches 25 ppm, to be covered with ice, the temperature must drop to minus 1.4 degrees.

The most surprising thing is that in sea water chilled to minus one degree, snow does not melt. He just continues to swim in it until he turns into a piece of ice. But getting into the chilled fresh water, he immediately conceals.

The process of freezing sea water has its own characteristics. Initially, primary ice crystals begin to form, which are incredibly similar to thin transparent needles. There is no salt in them. It is squeezed out of the crystals and remains in the water. If we collect such needles and melt them in some kind of dish, then we will get fresh water.

Porridge of ice needles, outwardly similar to a huge greasy spot, floats on the surface of the sea. Hence its original name - salo. With a further decrease in temperature, the fat freezes, forming a smooth and transparent ice crust, which is called nilas. Unlike lard, nilas contains salt. She appears in it in the process of freezing fat and capturing with needles, droplets of sea water. It's a pretty chaotic process. That is why salt in sea ice is distributed unevenly, as a rule, in the form of individual inclusions.

Scientists have found that the amount of salt in sea ice depends on the temperature of the surrounding air, which took place at the time of its formation. With a slight frost, the rate of formation of nilas is low, the needles capture little sea water, hence the salinity of the ice is low. In cold weather, the situation is exactly the opposite.

When sea ice melts, the first thing that comes out of it is salt. As a result, it gradually becomes insipid.

Young naturalists are always haunted by seemingly simple questions. At what temperature does sea water usually freeze? Everyone knows that zero degrees is not enough to turn the sea surface into a good ice rink. But at what temperature does this happen?

What is sea water made of?

How is the content of the seas different from fresh water? The difference is not so great, but still:

Much more salt.
Magnesium and sodium salts predominate.
The density differs slightly, within a few percent.
Hydrogen sulfide can form at depth.

The main component of sea water, no matter how predictable it may sound, is water. But unlike the water of rivers and lakes, it contains large amounts of sodium and magnesium chlorides.

Salinity is estimated at 3.5 ppm, but to be more clear - at 3.5 thousandths of a percent of the total composition.

And even this, not the most impressive figure, provides water not only with a specific taste, but also makes it undrinkable. There are no absolute contraindications, sea water is not a poison or a toxic substance, and nothing bad will happen from a couple of sips. It will be possible to talk about the consequences if a person is at least throughout the day. Also, the composition of sea water includes:

Fluorine.
Bromine.
Calcium.
Potassium.
Chlorine.
sulfates.
Gold.

True, in percentage terms, all these elements are much less than salts.

Why can't you drink sea water?

We have already briefly touched on this topic, let's look at it in a little more detail. Together with sea water, two ions enter the body - magnesium and sodium.

Sodium	Magnesium
Participates in maintaining the water-salt balance, one of the main ions along with potassium.	The main effect is on the central nervous system.
With an increase in the number Na in the blood, fluid is released from the cells.	Very slowly excreted from the body.
All biological and biochemical processes are disturbed.	An excess in the body leads to diarrhea, which aggravates dehydration.
Human kidneys are not able to cope with so much salt in the body.	Perhaps the development of nervous disorders, inadequate condition.

It cannot be said that a person does not need all these substances, but needs always fit within certain limits. After drinking a few liters of such water, you will go too far beyond their limits.

However, today the urgent need for the use of sea water may arise only among the victims of shipwrecks.

What determines the salinity of sea water?

Seeing a little higher figure 3.5 ppm , you might think that this is a constant for any sea water on our planet. But everything is not so simple, salinity depends on the region. It just so happened that the further north the region is located, the greater this value.

The south, on the contrary, boasts not so salty seas and oceans. Of course, all rules have their exceptions. Salt levels in the seas are usually slightly lower than in the oceans.

What is the geographic division in general? It is not known, researchers take it for granted, there is everything. Perhaps the answer should be sought in the earlier periods of the development of our planet. Not at the time when life was born - much earlier.

We already know that the salinity of water depends on the presence of:

magnesium chloride.
sodium chloride.
other salts.

Perhaps, in some parts of the earth's crust, the deposits of these substances were somewhat larger than in neighboring regions. On the other hand, no one canceled the sea currents, sooner or later the general level had to level off.

So, most likely, a small difference is associated with the climatic features of our planet. Not the most unfounded opinion, if you remember the frosts and consider what exactly water with a high salt content freezes more slowly.

Desalination of sea water.

Regarding desalination, everyone has heard at least a little, some now even remember the film "Water World". How realistic is it to put one such portable distiller in every house and forever forget about the problem of drinking water for humanity? Still fiction, not reality.

It's all about the energy expended, because for efficient operation huge capacities are needed, no less than a nuclear reactor. A desalination plant in Kazakhstan operates on this principle. The idea was also submitted in the Crimea, but the power of the Sevastopol reactor was not enough for such volumes.

Half a century ago, before numerous nuclear disasters, one could still assume that a peaceful atom would enter every home. There was even a slogan. But it is already clear that no use of nuclear micro-reactors:

In household appliances.
At industrial enterprises.
In the construction of cars and aircraft.
And yes, within the city limits.

Not expected in the next century. Science may take another leap and surprise us, but so far these are just the fantasies and hopes of careless romantics.

At what temperature can sea water freeze?

But the main question has not yet been answered. We have already learned that salt slows down the freezing of water, the sea will be covered with a crust of ice not at zero, but at sub-zero temperatures. But how far should the thermometer readings go to minus so that the inhabitants of the coastal regions do not hear the usual sound of the surf when they leave their homes?

To determine this value, there is a special formula, complex and understandable only for specialists. It depends on the main indicator - salinity level. But since we have an average value for this indicator, can we also find the average freezing point? Oh sure.

If you do not need to calculate everything up to a hundredth, for a particular region, remember the temperature at -1.91 degrees.

It may seem that the difference is not so great, only two degrees. But during seasonal temperature fluctuations, this can play a huge role where the thermometer falls at least 0. It would be only 2 degrees cooler, the inhabitants of the same Africa or South America could see ice near the coast, but alas. However, we do not think that they are very upset by such a loss.

A few words about the oceans.

And what about the oceans, fresh water reserves, pollution levels? Let's try to find out:

The oceans are still standing still, nothing has happened to them. In recent decades, the water level has been rising. Perhaps this is a cyclical phenomenon, or maybe the glaciers are actually melting.
Fresh water is also more than enough, it's too early to panic about this. If another global conflict happens, this time with the use of nuclear weapons, we can and will, like in Mad Max, pray for saving moisture.
The last point is very fond of conservationists. And sponsorship is not so difficult to achieve, competitors will always pay for black PR, especially when it comes to oil companies. But it is they who cause the main damage to the waters of the seas and oceans. It is not always possible to control oil production and emergency situations, and the consequences are catastrophic every time.

But the oceans have one advantage over humanity. It is constantly updated, and its real self-cleaning capabilities are very difficult to assess. Most likely, he will be able to survive human civilization and see its decline in a completely acceptable state. Well, then the water will have billions of years to clear itself of all the "gifts".

It is even difficult to imagine who needs to know at what temperature sea water freezes. A general educational fact, but to whom it is really useful in practice is a question.

Video experiment: freezing sea water

If you cool a solution of any salt in water, you will find that the freezing point has dropped. Zero degrees is passed, and solidification does not occur. Only at a temperature a few degrees below zero will crystals appear in the liquid. These are pure ice crystals; salt does not dissolve in solid ice.

The freezing point depends on the concentration of the solution. By increasing the concentration of the solution, we will decrease the crystallization temperature. The saturated solution has the lowest freezing point. The decrease in the freezing point of the solution is not at all small: for example, a saturated solution of sodium chloride in water will freeze at -21 ° C. With the help of other salts, an even greater decrease in temperature can be achieved; calcium chloride, for example, allows you to bring the solidification temperature of the solution to -55°C.

Let us now consider how the freezing process proceeds. After the first ice crystals fall out of the solution, the strength of the solution will increase. Now the relative number of foreign molecules will increase, the interference with the water crystallization process will also increase, and the freezing point will drop. If the temperature is not lowered further, crystallization will stop.

With a further decrease in temperature, water (solvent) crystals continue to separate. Finally, the solution becomes saturated. Further enrichment of the solution with the dissolved substance becomes impossible, and the solution solidifies immediately, and if we examine the frozen mixture through a microscope, we can see that it consists of ice crystals and salt crystals.

Thus, the solution freezes differently than a simple liquid. The freezing process extends over a large temperature interval.

What happens if you sprinkle some icy surface with salt? The answer to this question is well known to the janitors: as soon as the salt comes into contact with the ice, the ice will begin to melt. For the phenomenon to take place, it is, of course, necessary that the freezing point of a saturated salt solution be below the air temperature. If this condition is met, then the ice-salt mixture is in a foreign region of the state, namely, in the region of stable existence of the solution. Therefore, a mixture of ice and salt will turn into a solution, i.e., the ice will melt, and the salt will dissolve in the resulting water. In the end, either all the ice will melt, or a solution of such a concentration is formed, the freezing point of which is equal to the temperature of the environment.

The courtyard area of 100 m 2 is covered with an ice crust of 1 cm - this is not a little ice, about 1 ton. Let's calculate how much salt is needed to clean the yard if the temperature is -3 ° C. This crystallization (melting) temperature has a salt solution with a concentration of 45 g / l. Approximately 1 liter of water corresponds to 1 kg of ice. This means that 45 kg of salt are needed to melt 1 ton of ice at -3°C. In practice, they use much smaller quantities, since they do not achieve complete melting of all the ice.

When ice is mixed with salt, the ice melts and the salt dissolves in water. But melting requires heat, and ice takes it from its surroundings. Thus, adding salt to ice causes the temperature to drop.

We are used to buying factory-made ice cream now. Previously, ice cream was prepared at home, and at the same time, a mixture of ice and salt played the role of a refrigerator.

The table shows the thermophysical properties of a solution of calcium chloride CaCl 2 depending on temperature and salt concentration: specific heat of the solution, thermal conductivity, viscosity of aqueous solutions, their thermal diffusivity and the Prandtl number. The concentration of salt CaCl 2 in solution is from 9.4 to 29.9%. The temperature at which the properties are given is determined by the salt content of the solution and ranges from -55 to 20°C.

calcium chloride CaCl 2 may not freeze up to minus 55°С. To achieve this effect, the concentration of salt in the solution should be 29.9%, and its density will be 1286 kg/m 3 .

With an increase in the salt concentration in a solution, not only its density increases, but also such thermophysical properties as the dynamic and kinematic viscosity of aqueous solutions, as well as the Prandtl number. For example, dynamic viscosity of CaCl 2 solution with a salt concentration of 9.4% at a temperature of 20°C is 0.001236 Pa s, and with an increase in the concentration of calcium chloride in the solution to 30%, its dynamic viscosity increases to a value of 0.003511 Pa s.

It should be noted that the temperature has the strongest influence on the viscosity of aqueous solutions of this salt. When a solution of calcium chloride is cooled from 20 to -55°C, its dynamic viscosity can increase by 18 times, and kinematic by 25 times.

Given the following thermophysical properties of CaCl 2 solution:

, kg / m 3;
freezing point °С;
dynamic viscosity of aqueous solutions, Pa s;
Prandtl number.

The density of a solution of calcium chloride CaCl 2 depending on temperature

The table shows the values of the density of a solution of calcium chloride CaCl 2 of various concentrations depending on the temperature.
The concentration of calcium chloride CaCl 2 in solution is from 15 to 30% at a temperature of -30 to 15°C. The density of an aqueous solution of calcium chloride increases with a decrease in the temperature of the solution and an increase in the salt concentration in it.

Thermal conductivity of CaCl 2 solution depending on temperature

The table shows the thermal conductivity of a solution of calcium chloride CaCl 2 of various concentrations at low temperatures.
The concentration of CaCl 2 salt in solution is from 0.1 to 37.3% at a temperature of -20 to 0°C. As the concentration of salt in the solution increases, its thermal conductivity decreases.

Heat capacity of CaCl 2 solution at 0°С

The table shows the values of the mass heat capacity of a solution of calcium chloride CaCl 2 of various concentrations at 0°C. The concentration of salt CaCl 2 in solution is from 0.1 to 37.3%. It should be noted that with an increase in the concentration of salt in a solution, its heat capacity decreases.

Freezing point of solutions of NaCl and CaCl 2 salts

The table shows the freezing point of solutions of salts of sodium chloride NaCl and calcium CaCl 2 depending on the salt concentration. The salt concentration in the solution is from 0.1 to 37.3%. The freezing point of a saline solution is determined by the salt concentration in solution and for sodium chloride NaCl can reach a value of minus 21.2°C for a eutectic solution.

It should be noted that sodium chloride solution may not freeze to a temperature of minus 21.2 ° C, and a solution of calcium chloride does not freeze at temperatures up to minus 55°С.

Density of NaCl solution as a function of temperature

The table shows the values of the density of a solution of sodium chloride NaCl of various concentrations depending on temperature.
The concentration of the NaCl salt in the solution is from 10 to 25%. The density values of the solution are indicated at temperatures from -15 to 15°C.

Thermal conductivity of NaCl solution as a function of temperature

The table shows the thermal conductivity values of a solution of sodium chloride NaCl of various concentrations at negative temperatures.
The concentration of the NaCl salt in the solution is from 0.1 to 26.3% at a temperature of -15 to 0°C. According to the table, it can be seen that the thermal conductivity of an aqueous solution of sodium chloride decreases as the concentration of salt in the solution increases.

Specific heat capacity of NaCl solution at 0°С

The table shows the values of the mass specific heat of an aqueous solution of sodium chloride NaCl of various concentrations at 0°C. The concentration of the NaCl salt in the solution is from 0.1 to 26.3%. According to the table, it can be seen that with an increase in the concentration of salt in a solution, its heat capacity decreases.

Thermophysical properties of NaCl solution

The table shows the thermophysical properties of a solution of sodium chloride NaCl depending on temperature and salt concentration. The concentration of sodium chloride NaCl in solution is from 7 to 23.1%. It should be noted that when an aqueous solution of sodium chloride is cooled, its specific heat capacity changes slightly, the thermal conductivity decreases, and the viscosity of the solution increases.

Given the following thermophysical properties of NaCl solution:

solution density, kg/m 3 ;
freezing point °С;
specific (mass) heat capacity, kJ/(kg deg);
thermal conductivity coefficient, W/(m deg);
dynamic viscosity of the solution, Pa s;
kinematic viscosity of the solution, m 2 /s;
thermal diffusivity, m 2 /s;
Prandtl number.

The density of solutions of sodium chloride NaCl and calcium CaCl 2 depending on the concentration at 15 ° C

The table shows the density values of solutions of sodium chloride NaCl and calcium CaCl 2 depending on the concentration. The concentration of the NaCl salt in the solution is from 0.1 to 26.3% at a solution temperature of 15°C. The concentration of calcium chloride CaCl 2 in the solution is in the range from 0.1 to 37.3% at its temperature of 15°C. The density of solutions of sodium chloride and calcium increases with increasing salt content.

Volumetric expansion coefficient of solutions of sodium chloride NaCl and calcium CaCl 2

The table shows the values of the average coefficient of volumetric expansion of aqueous solutions of sodium chloride NaCl and calcium CaCl 2 depending on the concentration and temperature.
The volume expansion coefficient of the NaCl salt solution is indicated at a temperature of -20 to 20°C.
The volumetric expansion coefficient of the CaCl 2 chloride solution is given at -30 to 20°C.

Sources:

Danilova G. N. et al. Collection of tasks on heat transfer processes in the food and refrigeration industry. M.: Food industry, 1976.- 240 p.