BOILING: CONCEPT, TYPES AND EXAMPLES - CHEMISTRY - 2024

The boiling is the change of state or a liquid phase to the gaseous state. It occurs when a liquid is heated to its boiling point: the temperature at which the vapor pressure of the liquid equals the atmospheric pressure that is exerted on the surface of the liquid.

In general, in boiling the heat is supplied in the lower part of the container that contains the liquid. There begins the formation of bubbles that increase in volume as they rise towards the surface of the liquid, because the pressure on them decreases as they rise.

When a liquid boils, it is said to have reached its boiling point. Source: Pixabay.

When the liquid that is heated is water, the bubbles contain water vapor; that is, they already contain water in a gaseous state. In addition, the bubbles are distributed throughout the volume of the liquid. The boiling temperature of water is around 100ºC, at a pressure of 1 atm (760 mmHg).

To produce the change of state from the liquid phase to the gaseous one, it is necessary to supply energy (enthalpy of vaporization). While boiling occurs, the temperature of the water remains constant at 100 ºC, as thermal energy is lost as liquid water molecules come out as water vapor.

Types of boiling

There are two main types of boiling: nucleated, and critical heat flux.

In nucleate boiling, small-volume bubbles form at different places in the volume of the liquid.

The formation of bubbles is observed after the rise in temperature

Meanwhile, in critical heat flow boiling occurs when the surface where heat is supplied to cause boiling, heats up above a critical temperature value, forming a layer of vapor on the surface.

Boiling point

Factors that determine the boiling point

Atmospheric pressure

An increase in atmospheric pressure results in a rise in the boiling point, since it is necessary to increase the water vapor pressure to equalize the atmospheric pressure. To achieve this, the water temperature must be increased, necessitating a higher caloric expenditure.

On the contrary, when the atmospheric pressure decreases, such as occurs in a high altitude mountain above sea level, the boiling point decreases, since a lower vapor pressure is required that equals atmospheric pressure.

Intermolecular forces

Molecules in solution have several types of interactions, including: dispersion or London forces, dipole-dipole forces, and hydrogen bonds. The greater the magnitude of these forces, the higher the boiling point.

Heat energy is required to break the interaction between the molecules so that they can have enough energy for boiling. For example: methyl ether (C ₂ H ₆ O) has a boiling point of 25 ° C, while ethyl ether (C ₄ H ₁₀ O) has a boiling point of 78.5 ° C.

The difference between the boiling points, despite having similar chemical structures, is explained because ethyl ether has a higher molecular mass; They both form hydrogen bonds, but the dispersive forces in C ₄ H ₁₀ O are stronger than in C ₂ H ₆ O.

Difference between boiling and evaporation

The boiling originates in the liquid near the heat source, then spreading throughout the volume of the liquid. It is seen in this illustration:

Meanwhile, evaporation is a phenomenon of the liquid surface.

Evaporation occurs when a molecule of the liquid at the air-liquid interface has enough energy to overcome the surface tension exerted on it; so it escapes from the sinus of the liquid and passes into the gaseous phase.

Evaporation can occur at any temperature, but the possibility of its occurrence increases with temperature. In the following image you can see water evaporating from the ground:

Examples of boiling

Steam sterilization

It is carried out in equipment called an autoclave, which has the ability to generate high pressures exerted by water vapor, which is unable to escape. Likewise, there is an increase in the boiling point of water, which allows it to reach temperatures above 100 ºC.

In the autoclave, materials for tissue culture, surgical materials, materials for use in laboratories, culture media, etc. are sterilized. The conditions used for sterilization in the autoclave are: 15 pounds of pressure, temperature of 121 ºC and a duration of 15 minutes.

Cooking food

Food is heated by placing it in water. During its cooking, temperatures are used that equal the boiling point of water (100 ºC). Food is heated, during the time that experience indicates, to reach the optimal conditions for ingestion.

Chinese food uses minimal boiling and steaming in order to preserve the color, texture, and flavor of the food. The type of cooking known as simmer uses temperatures below the boiling point. Cooking food is also used by using steam.

Pressure cooker

The pressure cooker is used in cooking food. Its operation is based on its ability to limit the escape of the water vapor produced into the atmosphere, which generates an increase in internal pressure.

The increase in pressure exerted on the surface of the liquid in the pot translates into an increase in the boiling point and reaches temperatures above 100 ºC. This reduces the cooking time and therefore saves on fuel use.

Heat dissipation

Water is boiled on hydrophilic surfaces to cool nuclear reactors and high-power electronic devices, thus preventing them from overheating. To reach the boiling point and boil, the water must take heat from its surroundings and this produces a decrease in its temperature.

Determination of the molar mass of solutes

Increasing the boiling point of water is a colligative property; and therefore dependent on the concentration of the dissolved solute. Knowing it, the molar mass of the solute can be estimated. However, there are more precise methods, such as mass spectrometry, which is still a useful method.

Sugar industry

In the refining of cane sugar for the production of crystalline sugar, the cane juice is boiled and the temperature it reaches depends on the concentration of the sugar in it.

The elevation of the boiling point of cane juice is a measure of the concentration of the sugar in the solution. This is important information to achieve the crystallization of sugar.

References

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