STATE CHANGES: TYPES AND THEIR CHARACTERISTICS (WITH EXAMPLES) - CHEMISTRY - 2025

The state changes or phase are where material undergoes physical changes reversible thermodynamic phenomenon. It is said to be thermodynamic because a heat transfer occurs between matter and the surroundings; or what is the same, there are interactions between matter and energy that induce a rearrangement of the particles.

The particles that undergo the change of state remain the same before and after it. Pressure and temperature are important variables in how they are accommodated in one phase or another. When a change of state occurs, a biphasic system is formed, made up of the same matter in two different physical states.

State changes. Source: Gabriel Bolívar

The image above shows the main state changes that matter undergoes under normal conditions.

A solid cube of a bluish substance can turn liquid or gaseous depending on the temperature and pressure of its surroundings. By itself it represents a single phase: the solid. But, at the moment of melting, that is, melting, a solid-liquid equilibrium called fusion is established (red arrow between the bluish cube and the drop).

For fusion to occur, the cube needs to absorb heat from its surroundings to increase its temperature; therefore, it is an endothermic process. Once the cube is completely melted, it returns to a single phase: that of the liquid state.

This bluish drop can continue to absorb heat, which increases its temperature and results in the formation of gaseous bubbles. Again, there are two phases: one liquid and the other gas. When all of the liquid has evaporated through its boiling point, it is then said to have boiled or vaporized.

Now the bluish drops turned into clouds. So far, all processes have been endothermic. The bluish gas can continue to absorb heat until it becomes hot; however, given the terrestrial conditions, it tends to cool down and condense back into the liquid (condensation).

On the other hand, clouds can also deposit directly on a solid phase, again forming the solid cube (deposition). These last two processes are exothermic (blue arrows); that is, they release heat to the environment or surroundings.

In addition to condensation and deposition, a change of state occurs when the bluish drop freezes at low temperatures (solidification).

Types of state changes and their characteristics

The image shows the typical changes for the three (most common) states of matter: solid, liquid and gas. The changes accompanied by the red arrows are endothermic, involving the absorption of heat; while those accompanied by the blue arrows are exothermic, they release heat.

A brief description of each of these changes will be made below, highlighting some of their characteristics from molecular and thermodynamic reasoning.

- Fusion

Fusion is the change of state of a substance from solid to liquid.

In the solid state, the particles (ions, molecules, clusters, etc.) are “prisoners”, located in fixed positions in space without being able to move freely. However, they are capable of vibrating at different frequencies, and if they are very strong, the rigorous order imposed by intermolecular forces will begin to "fall apart".

As a result, two phases are obtained: one where the particles remain confined (solid), and another where they are freer (liquid), enough to increase the distances that separate them from each other. To achieve this, the solid must absorb heat, and thus its particles will vibrate with greater force.

For this reason the fusion is endothermic, and when it begins it is said that an equilibrium occurs between the solid-liquid phases.

The heat required to bring about this change is called heat or molar enthalpy of fusion (ΔH _Fus). This expresses the amount of heat (energy, mainly in units of kJ) that a mole of substance in solid state must absorb to melt, and not simply raise its temperature.

Snowball

Melting snow by hand. Source: Pixabay

With this in mind, it is understood why a snowball melts in the hand (top image). Snow absorbs body heat, which is enough to raise the temperature of the snow above 0 ° C.

The ice crystals in snow absorb just enough heat to melt and for their water molecules to adopt a messier structure. While the snow is melting, the water formed will not increase its temperature, since all the heat from the hand is used by the snow to complete its melting.

- Vaporization

Vaporization is the change of state of a substance from liquid to gaseous state.

Continuing with the example of water, now placing a handful of snow in a pot and lighting the fire, it is observed that the snow quickly melts. As the water heats up, tiny bubbles of carbon dioxide and other possible gaseous impurities begin to form inside it.

Boiling water. Source: Pixabay

Heat molecularly expands the disordered configurations of water, expanding its volume and increasing its vapor pressure; therefore, there are several molecules that escape from the surface as a result of increasing evaporation.

Liquid water increases its temperature slowly, due to its high specific heat (4.184J / ° C ∙ g). There comes a point where the heat it absorbs is no longer used to raise its temperature, but to start the liquid-vapor equilibrium; that is, it begins to boil and all the liquid will go to the gaseous state while absorbing heat and keeping the temperature constant.

This is where you see the intense bubbling on the surface of the boiled water (top image). The heat that liquid water absorbs so that the vapor pressure of its incipient bubbles equals the external pressure is called enthalpy of vaporization (ΔH _Vap).

The role of pressure

Pressure is also a determining factor in changes of state. What is its effect on vaporization? The higher the pressure, the greater the heat that the water must absorb to boil, and therefore, it vaporizes above 100 ° C.

This is because the increase in pressure makes it difficult for the water molecules to escape from the liquid to the gaseous phase.

Pressure cookers use this fact to their advantage to heat food in water to a temperature above its boiling point.

On the other hand, since there is a vacuum or a decrease in pressure, liquid water needs a lower temperature to boil and go into the gas phase. With high or low pressure, when the water boils it needs to absorb its respective heat of vaporization to complete its change of state.

- Condensation

Condensation is the change of state of a substance from the gaseous state to the liquid state.

The water has vaporized. Whats Next? Water vapor can still increase in temperature, becoming a dangerous current capable of causing severe burns.

However, let's assume it cools instead. How? Releasing heat to the environment, and releasing heat is said to be an exothermic process occurring.

By releasing heat, the very energetic gaseous water molecules begin to slow down. Also, their interactions become more effective as the temperature of the steam decreases. First, water droplets will form, condensed from the steam, followed by larger drops that end up being attracted by gravity.

To completely condense a given quantity of vapor, you need to release the same energy, but with opposite sign, at ΔH _Vap; that is, its enthalpy of condensation ΔH _Cond. Thus, the inverse vapor-liquid equilibrium is established.

Damp windows

Water condensation. Source: Pexels

Condensation can be seen on the windows of homes themselves. In a cold climate, the water vapor contained inside the house collides with the window, which due to its material has a lower temperature than other surfaces.

There, it is easier for the vapor molecules to clump together, creating a thin whitish layer easily removable by hand. As these molecules release heat (heating the glass and air), they begin to form more numerous clusters until the first drops can condense (top image).

When the drops become very large, they slide down the window and leave a trail of water.

- Solidification

Solidification is the change of state of a substance from the liquid state to the solid state.

Solidification occurs as a result of cooling; in other words, the water freezes. To freeze, water must release the same amount of heat that ice absorbs to melt. Again, this heat is called the enthalpy of solidification or freezing, ΔH _Cong (-ΔH _Fus).

As water molecules cool down, they lose energy and their intermolecular interactions become stronger and more directional. As a result, they are arranged thanks to their hydrogen bonds and form so-called ice crystals. The mechanism by which ice crystals grow has an impact on their appearance: transparent or white.

Ice sculpture. Source: Pixabay

If ice crystals grow very slowly, they do not occlude impurities, such as gases that dissolve in water at low temperatures. Thus, the bubbles are escaping and cannot interact with the light; and consequently, you have an ice as transparent as that of an extraordinary ice statue (top image).

The same thing that happens with ice, it can happen with any other substance that solidifies by cooling. Perhaps this is the most complex physical change in terrestrial conditions, since several polymorphs can be obtained.

- Sublimation

Sublimation is the change of state of a substance from the solid to the gaseous state.

Can water be sublimated? No, at least not under normal conditions (T = 25 ° C, P = 1 atm). For sublimation to occur, that is, the change of state from solid to gas, the vapor pressure of the solid must be high.

Likewise, it is essential that their intermolecular forces are not very strong, preferably if they consist only of dispersion forces.

The most emblematic example is solid iodine. It is a crystalline solid with grayish-purple hues, which presents a high vapor pressure. Such is the case, that in the act of it a purple vapor is given off, whose volume and expansion become noticeable when subjected to heating.

Sublimation of iodine. Source: Belkina NV, from Wikimedia Commons

The image above shows a typical experiment where solid iodine is evaporated in a glass container. It is interesting and striking to observe how the purple vapors are diffused, and the initiated student can verify the absence of liquid iodine.

This is the main characteristic of sublimation: there is no presence of a liquid phase. Likewise, it is endothermic, since the solid absorbs heat to increase its vapor pressure until it is equal to the external pressure.

- Deposition

Deposition of iodine crystals. Source: Stanislav.nevyhosteny, from Wikimedia Commons

Deposition is the change of state of a substance from the gaseous state to the solid state.

Parallel to the iodine sublimation experiment, there is that of its deposition. Deposition is the opposite change or transition: the substance goes from the gaseous state to the solid without the formation of a liquid phase.

When purple iodine vapors come into contact with a cold surface, they release heat to warm it, losing energy and regrouping their molecules back into the grayish-purple solid (top image). It is then an exothermic process.

Deposition is widely used for the synthesis of materials where they are doped with metal atoms by sophisticated techniques. If the surface is very cold, the heat exchange between it and the vapor particles is abrupt, omitting the passage through the respective liquid phase.

The heat or enthalpy of deposition (and not deposition) is the inverse of that of sublimation (ΔH _Sub = - ΔH _Dep). In theory, many substances can be sublimated, but to achieve this it is necessary to manipulate the pressures and temperatures, in addition to having their P vs T diagram at hand; in which, its distant possible phases can be visualized.

Other status changes

Although no mention is made of them, there are other states of matter. Sometimes they are characterized by having "a little of each", and therefore being a combination of them. To generate them, pressures and temperatures must be manipulated to very positive (large) or negative (small) magnitudes.

Thus, for example, if gases are heated excessively, they will lose their electrons and their positively charged nuclei in that negative tide will constitute what is known as plasma. It is synonymous with "electric gas", since it has a high electrical conductivity.

On the other hand, when temperatures drop too low, matter can behave in unexpected ways; that is, they exhibit unique properties around absolute zero (0 K).

One of these properties is superfluidity and superconductivity; as well as the formation of Bose-Einstein condensates, where all the atoms behave as one.

Some research even points to photonic matter. In them the particles of electromagnetic radiation, photons, group together to form photonic molecules. That is, it would be giving mass to bodies of light, theoretically.

References

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3. Dorling Kindersley. (2007). Changing states. Recovered from: factmonster.com
4. Meyers Ami. (2019). Phase Change: Evaporation, Condensation, Freezing, Melting, Sublimation & Deposition. Study. Recovered from: study.com
5. Bagley M. (April 11, 2016). Matter: Definition & the Five States of Matter. Recovered from: livescience.com
6. Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.