SOLIDIFICATION: SOLIDIFICATION POINT AND EXAMPLES - CHEMISTRY - 2025

The solidification is a liquid which undergoes change when it passes to the solid phase. The liquid can be a pure substance or a mixture. Likewise, the change may be due to a drop in temperature or as a result of a chemical reaction.

How can this phenomenon be explained? Visually, the liquid begins to turn petrified or hardened, to the point that it stops flowing freely. However, solidification actually consists of a series of steps that occur on microscopic scales.

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An example of solidification is a liquid bubble that freezes. In the image above you can see how a bubble freezes on contact with snow. What is the part of the bubble that begins to solidify? The one that is in direct contact with the snow. The snow works as a support on which the molecules of the bubble can settle.

Solidification is quickly triggered from the bottom of the bubble. This can be seen in the "glazed pines" that extend to cover the entire surface. These pines reflect the growth of crystals, which are nothing more than ordered and symmetrical arrangements of molecules.

For solidification to occur it is necessary that the particles of the liquid can be arranged, in such a way that they interact with each other. These interactions become stronger as the temperature decreases, which affects the molecular kinetics; that is, they slow down and become part of the crystal.

This process is known as crystallization, and the presence of a nucleus (small aggregates of particles) and a support accelerates this process. Once the liquid has crystallized, it is then said to have solidified or frozen.

Solidification enthalpy

Not all substances solidify at the same temperature (or under the same treatment). Some even "freeze" above room temperature, such as high-melting solids. This depends on the type of particles that make up the solid or liquid.

In the solid, they interact strongly and remain vibrating in fixed positions in space, without freedom of movement and with a defined volume, while in the liquid, they have the ability to move as numerous layers that move over each other, occupying the volume of the container that contains it.

The solid requires thermal energy to pass to the liquid phase; in other words, it needs heat. It gets heat from its surroundings, and the smallest amount it absorbs to generate the first drop of liquid is known as latent heat of fusion (ΔHf).

On the other hand, the liquid must release heat to its surroundings in order to order its molecules and crystallize into the solid phase. The heat released is then the latent heat of solidification or freezing (ΔHc). Both ΔHf and ΔHc are equal in magnitude but with opposite directions; the first has a positive sign, and the second a negative sign.

Why does the temperature remain constant in solidification?

At a certain point the liquid begins to freeze, and the thermometer reads a temperature T. As long as the liquid has not completely solidified, T remains constant. Since ΔHc has a negative sign, it consists of an exothermic process that releases heat.

Therefore, the thermometer will read the heat given off by the liquid during its phase change, counteracting the imposed temperature drop. For example, if the container that contains the liquid is put into an ice bath. Thus, T does not decrease until solidification is completely complete.

What units accompany these heat measurements? Usually kJ / mol or J / g. These are interpreted as follows: kJ or J is the amount of heat required by 1 mole of liquid or 1 g to be able to cool or solidify.

For the case of water, for example, ΔHc is equal to 6.02 kJ / mol. That is, 1 mole of pure water needs to give off 6.02 kJ of heat in order to freeze, and this heat is what keeps the temperature constant in the process. Similarly, 1 mole of ice needs to absorb 6.02 kJ of heat to melt.

Solidification point

The exact temperature where the process occurs is known as the solidification point (Tc). This varies in all substances depending on how strong their intermolecular interactions are in the solid.

Purity is also an important variable, since an impure solid does not solidify at the same temperature as a pure one. This is known as lowering the freezing point. To compare the solidification points of a substance it is necessary to use as a reference the one that is as pure as possible.

However, the same cannot be applied to solutions, as in the case of metal alloys. To compare their solidification points, mixtures with equal mass proportions must be considered; that is, with identical concentrations of its components.

Certainly, the solidification point is of great scientific and technological interest with regard to alloys and other varieties of materials. This is because, by controlling the time and how they are cooled, some desirable physical properties can be obtained or those inappropriate for a given application can be avoided.

For this reason the understanding and study of this concept is of great importance in metallurgy and mineralogy, as well as in any other science that deserves to manufacture and characterize a material.

Solidification and melting point

Theoretically Tc should be equal to the temperature or melting point (Tf). However, this is not always true for all substances. The main reason is because, at first glance, it is easier to mess up the solid molecules than to order the liquid ones.

Hence, it is preferred in practice to use Tf to qualitatively measure the purity of a compound. For example, if a compound X has many impurities, then its Tf will be more distant from that of pure X compared to one with higher purity.

Molecular ordering

As has been said so far, solidification proceeds to crystallization. Some substances, given the nature of their molecules and their interactions, require very low temperatures and high pressures to be able to solidify.

For example, liquid nitrogen is obtained at temperatures below -196ºC. To solidify it, it would be necessary to cool it further, or increase the pressure on it, thus forcing the N ₂ molecules to group together to create nuclei of crystallization.

The same can be considered for other gases: oxygen, argon, fluorine, neon, helium; and for the most extreme of all, hydrogen, whose solid phase has attracted much interest for its possible unprecedented properties.

On the other hand, the best known case is dry ice, which is nothing more than CO _2, whose white vapors are due to its sublimation at atmospheric pressure. These have been used to recreate haze on stage.

For a compound to solidify it does not depend only on Tc, but also on pressure and other variables. The smaller the molecules (H ₂) and the weaker their interactions, the more difficult it will be to get them to the solid state.

Supercooling

The liquid, whether it is a substance or a mixture, will begin to freeze at the temperature at the solidification point. However, under certain conditions (such as high purity, slow cooling time, or a very energetic environment), the liquid can tolerate lower temperatures without freezing. This is called supercooling.

There is still no absolute explanation of the phenomenon, but the theory supports that all those variables that prevent the growth of the crystallization nuclei promote supercooling.

Why? Because from the nuclei large crystals are formed after adding molecules from the surroundings to them. If this process is limited, even if the temperature is below Tc, the liquid will remain unchanged, as happens with the tiny drops that make up and make clouds visible in the sky.

All supercooled liquids are metastable, that is, they are susceptible to the slightest external disturbance. For example, if you add a small piece of ice to them, or shake them a bit, they will instantly freeze, which is a fun and easy experiment to do.

Examples of solidification

-Although it is not a solid itself, gelatin is an example of a solidification process by cooling.

-Fused glass is used to create and design many objects, which after cooling, retain their final defined shapes.

-Just as the bubble froze on contact with snow, a soda bottle can undergo the same process; and if it is supercooled, its freezing will be instantaneous.

-When the lava emerges from the volcanoes covering their edges or the earth's surface, it solidifies when it loses temperature, until it becomes igneous rocks.

-Eggs and cakes solidify with an increase in temperature. Likewise, the nasal mucosa does it but because of dehydration. Another example can also be found in paint or glues.

However, it should be noted that solidification does not occur in the latter cases as a product of cooling. Therefore, the fact that a liquid solidifies does not necessarily mean that it freezes (it does not reduce its temperature appreciably); but when a liquid freezes, it ends up solidifying.

Others:

- The conversion of water to ice: this occurs at 0 ° C producing ice, snow or glacial cubes.

- The candle wax that melts with the flame and solidifies again.

- The freezing of food for its preservation: in this case the water molecules are frozen within the cells of meat or vegetables.

- Glass blowing: this melts to give it shape and then solidifies.

- The manufacture of ice cream: they are generally dairy that solidify.

- In obtaining caramel, which is melted and solidified sugar.

- Butter and margarine are fatty acids in solid state.

- Metallurgy: in the manufacture of ingots or beams or structures of certain metals.

- Cement is a mixture of limestone and clay that, when mixed with water, has the property of hardening.

- In the manufacture of chocolate, cocoa powder is mixed with water and milk, which when dried, solidifies.

References

1. Whitten, Davis, Peck & Stanley. Chemistry. (8th ed.). CENGAGE Learning, p 448, 467.
2. Wikipedia. (2018). Freezing. Taken from: en.wikipedia.org
3. Loren A. Jacobson. (May 16, 2008). Solidification.. Taken from: infohost.nmt.edu/
4. Fusion and solidification. Taken from: juntadeandalucia.es
5. Dr. Carter. Solidification of a melt. Taken from: itc.gsw.edu/
6. Experimental explanation of supercooling: why water does not freeze in the clouds. Taken from: esrf.eu
7. Helmenstine, Anne Marie, Ph.D. (June 22, 2018). Solidification Definition and Examples. Taken from: thoughtco.com