Change of State of Matter – Solid, Liquid and Gas

Have you ever noticed the phase changes of the matter when the ice melts from its solid state to a liquid state or when boiling water changes to vapor form? Have you ever wondered why this happens? The phase changes of matter occur when it absorbs or loses energy. When energy is absorbed by a substance, there is an increase in the movement of atoms and molecules due to an increase in kinetic energy. This kinetic energy forces the particles to move far away from each other and makes them change form. Read on to understand the basics of the science behind the phase changes of matter. 

The states of matter undergo transitions from one form to another. There are three states of matter—solids, liquids, and gases. These three states of matter most commonly undergo the six-phase changes described below.

What are Changes in the State?

A change of state refers to a physical alteration in the matter. These changes are reversible and do not change the chemical composition of matter. The common phase changes in the state of matter include freezing, sublimation states of matter, melting, condensation, deposition, and vapourization. 

Why Do Phase Changes Occur?

The change in the state of phases of matter generally occurs when there is an alteration in its temperature or pressure. The molecules within the matter interact with each other more when there is an increase in temperature or pressure. It is easier for molecules and atoms to settle in the form of a more rigid structure when the pressure increases or temperature decreases. On the other hand, when there is a decrease in pressure, the particles easily move away from each other.

For instance, ice melts as there is an increase in temperature at normal atmospheric pressure. If the temperature is held constant but the pressure is decreased, eventually, a point is reached where direct sublimation states of matter of ice into water vapor occur.

Melting (Solid to Liquid)

Melting is the process by which a substance undergoes a change from solid to liquid. Pure crystalline solids have a fixed temperature at which they melt. This temperature is called their melting point. It varies from one substance to another. On the other hand, an impure solid’s change from solid to liquid occurs over a range of temperatures under the principal constituent’s melting point. Above is an example of an ice cube melting into a liquid state (water). 

Freezing (Liquid to Solid)

Freezing can be defined as a process through which there is a change in a substance from a liquid state to a solid state. Freezing is experienced by all liquids except helium, which occurs when the temperature decreases sufficiently. The temperature at which this freezing of a substance occurs is called its freezing point, which for water is equal to 0°C (32°F). Other substances may have higher or lower freezing points. For instance, iron has a freezing point of 1535°C, and oxygen has a freezing point of -219°C. Above is an example of the process of freezing where sweetened cream is changed into ice cream.

Vaporization (Liquid to Gas)

Vaporization can be defined as the process by which molecules of matter spontaneously change from a state of liquid to a gaseous state. It is called boiling if conditions allow the formation of bubbles within a liquid during the process of vaporization. Heat is the main factor responsible for the vaporization of a substance. If the heat in the surroundings is not enough, the system itself may produce it by the process of temperature reduction. A liquid has constituent atoms or molecules that are held together by strong, cohesive forces that must be separated during its conversion to vapor form. The image given above indicates the vaporization of alcohol into the gaseous phase.

Condensation (Gas to Liquid)

Condensation may be defined as the process by which there is a change in the gaseous state of matter into a liquid state. It is the reverse of the process of vaporization, and here the heat is liberated into the surroundings by the condensing vapour. The amount of heat liberated in the process is characteristic of the substance. The image given above depicts the process of condensation of water vapor into dew drops.

Deposition (Gas → Solid)

Deposition can be defined as the process by which particles settle or sediment onto a surface. The particles may originate from a solution, suspension, vapor, or mixture. During the process of deposition, there is also a change in the state of matter from the gaseous state to the solid state. The above image depicts the deposition of vapor of silver onto a surface in a vacuum to produce a solid layer for a mirror.

Sublimation (Solid to Gas)

Sublimation is defined as the process where matter transitions from a solid state to a gaseous state without crossing the intermediate liquid state. The image above shows an example of the sublimation states of matter of the solid form of carbon dioxide into CO₂ gas. Another example can be seen on a cold and windy winter day when ice directly converts to water vapor.

Ionization (Gas to Plasma)

Ionization may be defined as the process where energy is utilized to remove an electron from an atom or ion in the gaseous phase. You can notice the process of ionization happening inside a plasma ball novelty toy or during the formation of an aurora in the upper atmosphere.

Recombination (Plasma → Gas)

Recombination is defined as the process where ionized particles return to a gaseous state. There is a transfer of electrons or a combination of charges in a gas which causes the ions to neutralize.

Phase Changes of States of Matter

Another way to look at the change in the matter is by:

Solids: Solids either melt to give a liquid state or undergo sublimation, involving direct solid-to-gas phase change. The deposition of gaseous or freezing liquids gives rise to solids.

Liquids: Liquids either undergo vaporization to give gases or undergo freezing to give solids. They are formed when solids melt, or gases condense.

Gases: Gases undergo ionization to give plasma and condensation to give liquids or direct deposit into the solid state. They are formed when liquids evaporate, solids sublime, or plasma recombines.

Plasma: Plasma undergoes recombination into a gas. It is most commonly formed when a gas undergoes ionization, but sufficient space and energy must be available. A liquid or solid can directly ionize into a gas.

While you observe a situation, phase changes are not necessarily clear or distinct. Suppose, if you observe how dry ice sublimes to give carbon dioxide gas, the white vapor seen is primarily the water that is undergoing condensation from water vapor in the atmosphere into droplets of fog. More than one phase occurs at a particular time. Suppose nitrogen in frozen form converts into both a liquid and a gaseous state. Multiple phase changes can occur at once. For example, when exposed to normal temperature and pressure, frozen nitrogen will form both the liquid and the vapor phases.

Conclusion

It is interesting to know that every object that exists undergoes a change in the state of matter. It only depends on how much heat is supplied to it. If enough heat is supplied, almost anything in this universe can be made to undergo a change in state. The thing to note is, though, that not every substance follows the same solid-liquid-gas path. Some substances do not enter the liquid state to change from liquid to gas; rather, they directly change from their solid to the gaseous state through the process of sublimation. Dry ice or solid CO₂, iodine, and the burning and sublimation of high-quality coal into vapor are examples of sublimation.

Frequently Asked Questions

Q1) What do solids change into on reaching the melting point?

A) On reaching their melting point, solids transform into liquids by undergoing liquefaction. This occurs because, on increasing the temperature, the bonds between different constituent molecules weaken. 

Q2) What are boiling points and melting points?

A) A temperature where there is a change from liquid to gas of pure liquids is called boiling point, while a melting point is a temperature at which solids melt. The boiling point and melting point of a substance determine its phase change from one state of matter to another. 

Q3) Can there be a direct solid-to-gas phase change?

A) Yes, there can be a direct solid-to-gas phase change. Solids can directly transform into a gaseous state through a process called sublimation. The change from the liquid to gas phase is skipped in the process.

Q4) Write three facts about the states of matter.

• Out of the three states of matter, solids are the hardest.
• The air around us contains all the constituting elements in the gaseous state.
• The state of matter called plasma is very rare to be found in everyday life.