FREEZING POINT: HOW TO CALCULATE IT AND EXAMPLES - CHEMISTRY - 2024

The freezing point is the temperature at which a substance experiences a liquid-solid transition equilibrium. When it comes to substance, it can be a compound, a pure element or a mixture. Theoretically, all matter freezes as the temperature drops to absolute zero (0K).

However, extreme temperatures are not necessary to observe the freezing of liquids. Icebergs are one of the most obvious examples of frozen bodies of water. Likewise, the phenomenon can be tracked in real time using liquid nitrogen baths, or using a simple freezer.

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What is the difference between freezing and solidification? That the first process is highly dependent on temperature, on the purity of the liquid, and is a thermodynamic equilibrium; while the second is more related to changes in the chemical composition of the substance that solidifies, even without being completely liquid (a paste).

Therefore, a freeze is a solidification; but the reverse is not always true. Furthermore, to rule out the term solidification, there must be a liquid phase in equilibrium with the solid of the same substance; icebergs do this: they float on liquid water.

Thus, one is facing the freezing of a liquid when a solid phase is formed as a consequence of a decrease in temperature. Pressure also influences this physical property, although its effects are less in liquids with low vapor pressures.

What is the freezing point?

As the temperature drops, the average kinetic energy of the molecules decreases, and therefore they slow down a bit. As you go slower in the liquid, there comes a point where they interact enough to form an ordered arrangement of molecules; this is the first solid, from which larger crystals will grow.

If this first solid "wobbles" too much, then it will be necessary to lower the temperature even further until its molecules remain still enough. The temperature at which this is achieved corresponds to the freezing point; from there, the liquid-solid equilibrium is established.

The previous scenario occurs for pure substances; but what if they are not?

In that case, the molecules of the first solid must manage to incorporate the foreign molecules. As a result, an impure solid (or solid solution) is formed, which requires a temperature lower than the freezing point for its formation.

We speak then of the lowering of the freezing point. As there are more foreign molecules, or more correctly speaking, impurities, the liquid will freeze at lower and lower temperatures.

Freezing vs solubility

Given a mixture of two compounds, A and B, as the temperature drops, A freezes, while B remains liquid.

The scenario is similar to what has just been explained. A part of A has not yet frozen, and is therefore dissolved in B. Is it then the solubility equilibrium that we speak of rather than a liquid-solid transition?

Both descriptions are valid: A precipitates or freezes, separating from B as the temperature drops. All A will have precipitated when there is no longer any of it dissolved in B; which is the same as saying that A will have frozen completely.

However, it is more convenient to treat the phenomenon from the point of view of freezing. Thus, A freezes first because it has a lower freezing point, while B will need cooler temperatures.

However, the "ice of A" actually consists of a solid that has a richer composition of A than B; but B is there too. This is because A + B is a homogeneous mixture, and therefore some of that homogeneity is transferred to the frozen solid.

How to calculate it?

How can you predict or calculate the freezing point of a substance? There are physicochemical calculations that allow obtaining an approximate value of this point under other pressures (other than 1atm, the ambient pressure).

However, these lead to the enthalpy of fusion (Δ _Fus); since, the fusion is the process in opposite sense of the freezing.

Furthermore, experimentally it is easier to determine the melting point of a substance or mixture than its freezing point; Although they may seem the same, they show certain differences.

As mentioned in the previous section: the higher the concentration of impurities, the greater the drop in the freezing point. This can also be said as follows: the lower the mole fraction X of the solid in the mixture, the lower the temperature will freeze.

Temperature decrease equation

The following equation expresses and summarizes everything that has been said:

LnX = - (Δ _Fus / R) (1 / T - 1 / Tº) (1)

Where R is the ideal gas constant, which has almost universal use. Tº is the normal freezing point (at ambient pressure), and T is the temperature at which the solid will freeze to a mole fraction X.

From this equation, and after a series of simplifications, the following, better known, is obtained:

ΔTc = K _F m (2)

Where m is the molality of the solute or impurity, and K _F is the cryoscopic constant of the solvent or liquid component.

Examples

A brief description of the freezing of some substances will be given below.

Water

The water freezes around 0ºC. However, this value may decrease if it contains a solute dissolved in it; to say, salt or sugar.

Depending on the amount of dissolved solute, there are different molalities m; and as m increases, X decreases, whose value can be substituted in equation (1) and thus solve for T.

For example, if a glass of water is placed in a freezer, and another with sweetened water (or any water-based drink), the glass of water will freeze first. This is because its crystals form faster without the disturbance of glucose molecules, ions, or other species.

The same would happen if you put a glass of sea water in the freezer. Now, the glass of sea water may or may not be frozen first than the glass of sweetened water; the difference will depend on the amount of solute and not on its chemical nature.

It is for this reason that the decrease in Tc (freezing temperature) is a colligative property.

Alcohol

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Alcohols freeze at colder temperatures than liquid water. For example, ethanol freezes around -114 ° C. If mixed with water and other ingredients, there will be an increase in the freezing point on the contrary.

Why? Because water, a liquid substance miscible with alcohol, freezes at a much higher temperature (0ºC).

Returning to the fridge with the glasses with water, if this time one with an alcoholic drink is introduced, this will be the last to freeze. The higher the ethyl grade, the freezer will need to cool it further to freeze the drink. It is for this reason that drinks like tequila are more difficult to freeze.

Milk

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Milk is a water-based substance, in which fat is dispersed along with lactose and calcium phosphates, in addition to other lipoproteins.

Those components that are more soluble in water determine how much its freezing point will vary with the composition.

On average, milk freezes at a temperature around -0.54ºC, but it ranges between -0.50 and -0.56 depending on the percentage of water. Thus, it is possible to know if the milk has been adulterated. And as you can see, a glass of milk will freeze almost on par with a glass of water.

Not all milk freezes at the same temperature, as its composition also depends on its animal source.

Mercury

Mercury is the only metal that is in liquid form at room temperature. To freeze it, it is necessary to lower the temperature to -38.83ºC; And this time the idea of ​​pouring it into a glass and putting it in a freezer will be avoided, as it could lead to terrible accidents.

Note that mercury freezes before alcohol. This may be due to the fact that the mercury crystal vibrates less as it consists of atoms linked by metallic bonds; whereas in ethanol, they are relatively light CH ₃ CH ₂ OH molecules that must be accommodated slowly.

Gasoline

Of all the freezing point examples, gasoline is the most complex. Like milk, it is a mixture; but its base is not water, but a group of various hydrocarbons, each with its own structural characteristics. Some of small molecules, and some large ones.

Those hydrocarbons with lower vapor pressures will freeze first; while the others will remain in a liquid state, even if a glass of gasoline is surrounded by liquid nitrogen. It will not properly form a “gasoline ice”, but a gel with yellow-green tones.

To freeze gasoline completely, it may be necessary to cool the temperature down to -200ºC. At this temperature it is likely that gasoline ice will form, as all components of the mixture will have frozen; that is, there will no longer be a liquid phase in equilibrium with a solid.

References

1. Department of Physics, University of Illinois at Urbana-Champaign. (2018). Q&A: Gasoline freezing. Recovered from: van.physics.illinois.edu
2. Ira N. Levine. (2014). Principles of physicochemistry. (Sixth edition). Mc Graw Hill.
3. Glasstone. (1970). Physicochemical Treaty. Aguilar SA de Ediciones, Juan Bravo, 38, Madrid (Spain).
4. Walter J. Moore. (1962). Physical Chemistry. (Fourth edition). Longmans.
5. Sibagropribor. (2015). Determination of the Freezing Point of Milk. Recovered from: sibagropribor.ru
6. Helmenstine, Anne Marie, Ph.D. (June 22, 2018). Freezing Point of Alcohol. Recovered from: thoughtco.com