The 7 states of condensed matter at room temperature

In ‘Chemistry Essentials for Dummies‘, John T. Moore writes:

Matter is anything that has mass and occupies space. It can exist in one of three classic states: solid, liquid, and gas. When a substance goes from one state of matter to another, the process is called a change of state, or phase change.

As he puts it, this a classical and popular view of states of matter where there are only two states of condensed matter, namely, solids and liquids. But this differentiation is not useful from the moment we are interested for technical or scientific reasons in them. Some more precision when it comes to differentiate states of condensed matter is needed. If we should mention those that we may find in our daily life outside a laboratory and, to narrow the search a little bit, are stable at room temperature (some 20 ºC), we need to focus on how atoms or molecules relate to each other. We’ll find that the two classic ones turn into, at least, seven.

Atomic crystals

Atomic structure of a crystal of sodium chloride. Source: Wikimedia Commons

Atoms can bond together the form regular crystals. A crystal is made of small units reproduced many times and built into a regular array.

Molecular crystals

Sucrose crystals. Source: Wikimedia Commons

It is also possible that atoms will bind together to form molecules, and the molecules will stick together via weak Van der Waals bonds to form so-called molecular crystals.

Molecular structure of sucrose crystals. Source: Crystallography365

Liquids

The formation of a spherical droplet of liquid water minimizes the surface area, which is the natural result of surface tension in liquids. Source: José Manuel Suárez / Wikimedia Commons

Here, atoms are attracted to each other, but not so strongly that they form permanent bonds (or the temperature is high enough to make the bonds unstable). Liquids (and gases) are disordered configurations of molecules where the molecules are free to move around into new configurations.

Structure of a classical monatomic liquid. Atoms have many nearest neighbors in contact, yet no long-range order is present. Source: Kaneiderdaniel / Wikimedia Commons

Amorphous solids and glasses

Interior of the rose at Strasbourg Cathedral. Source: Clostridium / Wikimedia Commons

Somewhere midway between the idea of a crystal and the idea of a liquid is the possibility of amorphous solids and glasses. In this case the atoms are bonded into position in a disordered configuration. Unlike a liquid, the atoms cannot flow freely.

The amorphous structure of glassy silica (SiO2) in two dimensions. No long-range order is present, although there is local ordering with respect to the tetrahedral arrangement of oxygen (O) atoms around the silicon (Si) atoms. Source: Jdrewitt / Wikimedia Commons

Liquid crystals

A Casio watch with a liquid crystal display (LCD). Source: Ricce / Wikimedia Commons

The system orders in some ways but remains disordered in other ways. For example, in so-called smectic phases the system is crystalline (ordered) in one direction, but remains disordered within each plane. One can also consider cases where the molecules are always oriented the same way but are at completely random positions (known as a “nematic”). There are a huge variety of possible liquid crystal phases of matter. In every case it is the interactions between the molecules (“bonding” of some type, whether it be weak or strong) that dictates the configurations.

Schematic of alignment in the smectic phases. The smectic A phase (left) has molecules organized into layers. In the smectic C phase (right), the molecules are tilted inside the layers. Source: Kebes / Wikimedia Commons

 

Quasicrystals

The material holmium–magnesium–zinc (Ho–Mg–Zn) forms quasicrystals, which can take on the macroscopic shape of a dodecahedron. (Only a quasicrystal, not a normal crystal, can take this shape.) The edges are 2 mm long. Source: The Ames Laboratory – US Department of Energy

These are ordered but nonperiodic arrangements. In a quasicystal component units are assembled together with a set of regular rules which appears to make a periodic structure, but in fact the pattern is non-repeating.

Atomic model of fivefold icosahedral-Al-Pd-Mn quasicrystal surface. Source: J.W. Evans, The Ames Laboratory – US Department of Energy

Polymers

Appearance of real linear polymer chains as recorded using an atomic force microscope on a surface, under liquid medium. Chain contour length for this polymer is ~204 nm; thickness is ~0.4 nm. Source: Y. Roiter and S. Minko (2005) AFM Single Molecule Experiments at the Solid-Liquid Interface: In Situ Conformation of Adsorbed Flexible Polyelectrolyte Chains, Journal of the American Chemical Society, vol. 127, iss. 45, pp. 15688-15689 .

Polymers are macromolecules composed of many repeating units. Another way of seeing them is as long chains of atoms.

Microstructure of part of a DNA double helix biopolymer. Source: Wikimedia Commons

References:

Dove, M.T. (2002) Structure and Dynamics: An Atomic View of Materials Oxford Master Series in Condensed Matter Physics / Oxford University Press

Moore, J.T. (2010) Essential Chemistry for Dummies John Wiley & Sons

Simon, S.H. (2013) The Oxford Solid State Basics Oxford University Press

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

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