SOLID STATE: CHARACTERISTICS, PROPERTIES, TYPES, EXAMPLES - SCIENCE - 2025

The solid state is one of the main ways in which matter aggregates to create condensed or solid bodies. The entire earth's crust, leaving out the seas and oceans, is a motley conglomerate of solids. Examples of objects in solid state are a book, a stone, or grains of sand.

We can interact with solids thanks to the repulsion of our electrons with those of their atoms or molecules. Unlike liquids and gases, as long as they are not severely toxic, our hands cannot go through them, but rather crumble or absorb them.

The wooden statue of this horse is made of strongly cohesive natural polymers. Source: Pxhere.

Solids are generally much easier to handle or store than a liquid or gas. Unless its particles are finely divided, a wind current will not carry it in other directions; they are fixed in the space defined by the intermolecular interactions of their atoms, ions or molecules.

Solid concept

The solid is a state of matter in which there is a rigid volume and shape; the particles that form materials or objects in solid state are fixed in one place, they are not easily compressible.

This state of matter is the most varied and rich in terms of chemistry and physics. We have ionic, metallic, atomic, molecular, and covalent solids, each with its own structural unit; that is, with its own crystals. When their mode of aggregation does not allow them to establish orderly internal structures, they become amorphous and intricate.

The study of the solid state converges in the design and synthesis of new materials. For example, wood, a natural solid, has also been used as an ornamental material and for the construction of houses.

Other solid materials allow the manufacture of automobiles, airplanes, ships, spaceships, nuclear reactors, sporting goods, batteries, catalysts and many other objects or products.

General characteristics of solids

Spring and wood, components of a caliper, example of solid

The main characteristics of solids are:

-They have defined mass, volume and shapes. A gas, for example, does not have an end or a beginning, since these depend on the container that stores it.

-They are very dense. Solids tend to be denser than liquids and gases; although there are a few exceptions to the rule, especially when comparing liquids and solids.

-The distances that separate its particles are short. This means that they have become very cohesive or compacted in their respective volume.

-Its intermolecular interactions are very strong, otherwise they would not exist as such and would melt or sublimate in terrestrial conditions.

Differences between the particles of a solid, a liquid and a gas

-The mobility of solids is usually quite limited, not only from a material point of view, but also molecularly. Its particles are confined in a fixed position, where they can only vibrate, but not move or rotate (in theory).

Properties

Melting points

All solids, unless they are decomposed in the process, and regardless of whether or not they are good conductors of heat, can pass into a liquid state at a certain temperature: their melting point. When this temperature is reached, its particles finally manage to flow and escape from their fixed positions.

This melting point will depend on the nature of the solid, its interactions, the molar mass and the crystalline lattice energy. As a general rule, ionic solids and covalent networks (such as diamond and silicon dioxide) tend to have the highest melting points; while the molecular solids, the lowest.

The following image shows how an ice cube (solid state) turns into a liquid state:

Stoichiometry

Much of the solids are molecular, since they are compounds whose intermolecular interactions allow them to coalesce in such a way. However, many others are ionic or partially ionic, so their units are not molecules, but cells: a set of atoms or ions arranged in an orderly way.

It is here where the formulas of such solids must respect the neutrality of the charges, indicating their composition and stoichiometric relationships. For example, the solid whose hypothetical formula is A ₂ B ₄ O ₂ indicates that it has the same number of A atoms as O (2: 2), while it has twice the number of B atoms (2: 4).

Note that the subscripts of the formula A ₂ B ₄ O ₂ are integers, which shows that it is a stoichiometric solid. The composition of many solids is described by these formulas. The charges on A, B and O must add equal to zero, because otherwise there would be a positive or negative charge.

For solids it is especially useful to know how to interpret their formulas since, generally, the compositions of liquids and gases are simpler.

Defects

The structures of solids are not perfect; they present imperfections or defects, however crystalline they may be. This is not the case with liquids, nor with gases. There are no regions of liquid water that can be said in advance to be "dislocated" from their surroundings.

Such defects are responsible for the solids being hard and brittle, showing properties such as pyroelectricity and piezoelectricity, or ceasing to have defined compositions; that is, they are non-stoichiometric solids (eg A _0.4 B _1.3 O _0.5).

Reactivity

Solids are usually less reactive than liquids and gases; but not due to chemical causes, but to the fact that their structures prevent reactants from attacking the particles inside them, reacting first with those on their surface. Therefore, reactions involving solids tend to be slower; unless they are pulverized.

When a solid is in powder form, its smaller particles have more area or surface to react. That is why fine solids are often labeled as potentially dangerous reagents, as they can ignite rapidly, or react vigorously in contact with other substances or compounds.

Often times, the solids are dissolved in a reaction medium to homogenize the system and carry out a synthesis with higher yield.

Physical

With the exception of the melting point and the defects, what has been said so far corresponds more to the chemical properties of the solids than to their physical properties. The physics of materials is deeply focused on how light, sound, electrons and heat interact with solids, whether they are crystalline, amorphous, molecular, etc.

This is where what is known as plastic, elastic, rigid, opaque, transparent, superconducting, photoelectric, microporous, ferromagnetic, insulating or semiconductor solids comes in.

In chemistry, for example, materials that do not absorb ultraviolet radiation or visible light are of interest, since they are used to make measurement cells for UV-Vis spectrophotometers. The same happens with infrared radiation, when you want to characterize a compound by obtaining its IR spectrum, or study the progress of a reaction.

The study and manipulation of all the physical properties of solids requires enormous dedication, as well as their synthesis and design, choosing “pieces” of inorganic, biological, organic or organometallic construction for new materials.

Types and examples

Since there are several types of solids chemically, representative examples will be mentioned separately for each.

Crystalline solids

On the one hand, there are crystalline solids. These elements are characterized because the molecules that make them up are configured in the same way, which is repeated as a pattern throughout the crystal. Each pattern is called a unit cell.

Crystalline solids are also characterized by having a defined melting point; This means that, given the uniformity of the arrangement of its molecules, there is the same distance between each unit cell, which allows the entire structure to transform constantly under the same temperature.

Examples of crystalline solids can be salt and sugar.

Amorphous solids

Amorphous solids are characterized by the fact that the conformation of their molecules does not respond to a pattern, but varies over the entire surface.

Since there is no such pattern, the melting point of amorphous solids is not defined, unlike crystalline ones, which means that it melts gradually and under different temperatures.

Examples of amorphous solids can be glass and most plastics.

Ionics

Ionic solids are characterized by having cations and anions, which interact with each other by electrostatic attraction (ionic bonding). When the ions are small, the resulting structures are usually always crystalline (taking into account their defects). Among some ionic solids we have:

-NaCl (Na ⁺ Cl ^-), sodium chloride

-MgO (Mg ²⁺ O ^2-), magnesium oxide

-CaCO ₃ (Ca ²⁺ CO ₃ ^2-), calcium carbonate

-CuSO ₄ (Cu ²⁺ SO ₄ ^2-), copper sulfate

-KF (K ⁺ F ^-), potassium fluoride

-NH ₄ Cl (NH ₄ ⁺ Cl ^-), ammonium chloride

-ZnS (Zn ²⁺ S ^2-), zinc sulfide

-Fe (C ₆ H ₅ COO) ₃, iron benzoate

Metallic

As their name indicates, they are solids that have metallic atoms interacting through the metallic bond:

-Silver

-Gold

-Lead

-Brass

-Bronze

-White gold

-Pewter

-Steels

-Duralumin

Note that alloys also count as metallic solids, of course.

Atomic

Metallic solids are also atomic, since in theory there are no covalent bonds between metallic atoms (MM). However, noble gases do essentially count as atomic species, as only London dispersive forces predominate among them.

Therefore, although they are not high application solids (and difficult to obtain), crystallized noble gases are examples of atomic solids; i.e.: helium, neon, argon, krypton, etc., solids.

Molecular and polymeric

Molecules can interact through Van der Walls forces, where their molecular masses, dipole moments, hydrogen bonds, structures and geometries play an important role. The stronger such interactions, the more likely they are to be in solid form.

On the other hand, the same reasoning applies to polymers, which due to their high average molecular masses are almost always solids, and several of them are amorphous; since, its polymeric units find it difficult to arrange themselves neatly to create crystals.

Thus, we have among some molecular and polymeric solids the following:

-Dry ice

-Sugar

-Iodine

-Benzoic acid

-Acetamide

-Rhombic sulfur

-Palmitic acid

-Fullerenos

-Match

-Caffeine

-Naphthalene

-Wood and paper

-Silk

-Teflon

-Polyethylene

-Kevlar

-Bakelite

-Polyvinyl chloride

-Polystyrene

-Polypropylene

-Proteins

-Chocolate bar

Covalent networks

Finally, we have the covalent networks between the hardest and highest melting solids. Some examples are:

-Graphite

-Diamond

-Quartz

-Silicium carbide

-Boron nitride

-Aluminum phosphide

-Gallium arsenide

References

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2. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
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5. Dr. Michael Lufaso. (sf). Solid State Chemistry Lecture Notes. Recovered from: unf.edu
6. askIITians. (2019). General Characteristics of Solid State. Recovered from: askiitians.com
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