States of Matter
States of matter are one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and plasma. Many intermediate states are known to exist, such as liquid crystal, and some states only exist under extreme conditions, such as Bose-Einstein condensates, neutron-degenerate matter, and quark-gluon plasma, which only occur, respectively, in situations of extreme cold, extreme density, and extremely high energy.
Historically, the distinction is made based on qualitative differences in properties. Matter in the solid state maintains a fixed volume and shape, with component particles (atoms, molecules or ions) close together and fixed into place. Matter in the liquid state maintains a fixed volume, but has a variable shape that adapts to fit its container. Its particles are still close together but move freely. Matter in the gaseous state has both variable volume and shape, adapting both to fit its container. Its particles are neither close together nor fixed in place. Matter in the plasma state has variable volume and shape, and contains neutral atoms as well as a significant number of ions and electrons, both of which can move around freely.
The term “phase” is sometimes used as a synonym for state of matter, but it is possible for a single compound to form different phases that are in the same state of matter. For example, ice is the solid state of water, but there are multiple phases of ice with different crystal structures, which are formed at different pressures and temperatures.
A crystalline solid: atomic resolution image of strontium titanate. Brighter atoms are strontium and darker ones are titanium.
- In a solid, constituent particles (ions, atoms, or molecules) are closely packed together.
- The forces between particles are so strong that the particles cannot move freely but can only vibrate. As a result, a solid has a stable, definite shape, and a definite volume.
- Solids can only change their shape by an outside force, as when broken or cut.
- The particles (atoms, molecules, or ions) are packed in a regularly ordered, repeating pattern in crystalline solids.
- There are various different crystal structures, and the same substance can have more than one structure (or solid phase). For example, iron has a body-centered cubic structure at temperatures below 912 °C (1,674 °F), and a face-centered cubic structure between 912 and 1,394 °C (2,541 °F).
- Ice has fifteen known crystal structures, or fifteen solid phases, which exist at various temperatures and pressures.
- Glasses and other non-crystalline, amorphous solids without long-range order are not thermal equilibrium ground states; therefore they are described below as nonclassical states of matter.
- Solids can be transformed into liquids by melting, and liquids can be transformed into solids by freezing. Solids can also change directly into gases through the process of sublimation, and gases can likewise change directly into solids through deposition.
Structure of a classical monatomic liquid. Atoms have many nearest neighbors in contact, yet no long-range order is present.
- A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure.
- The volume is definite if the temperature and pressure are constant.
- When a solid is heated above its melting point, it becomes liquid, given that the pressure is higher than the triple point of the substance.
- Intermolecular (or interatomic or interionic) forces are still important, but the molecules have enough energy to move relative to each other and the structure is mobile. This means that the shape of a liquid is not definite but is determined by its container.
- The volume is usually greater than that of the corresponding solid, the best-known exception being water, H2O.
- The highest temperature at which a given liquid can exist is its critical temperature.
The spaces between gas molecules are very big. Gas molecules have very weak or no bonds at all. The molecules in “gas” can move freely and fast.
- A gas is a compressible fluid. Not only will a gas conform to the shape of its container but it will also expand to fill the container.
In a gas, the molecules have enough kinetic energy so that the effect of intermolecular forces is small (or zero for an ideal gas), and the typical distance between neighboring molecules is much greater than the molecular size.
A gas has no definite shape or volume, but occupies the entire container in which it is confined.
A liquid may be converted to a gas by heating at constant pressure to the boiling point, or else by reducing the pressure at constant temperature.
At temperatures below its critical temperature, a gas is also called a vapor and can be liquefied by compression alone without cooling.
A vapor can exist in equilibrium with a liquid (or solid), in which case the gas pressure equals the vapor pressure of the liquid (or solid).
A supercritical fluid (SCF) is a gas whose temperature and pressure are above the critical temperature and critical pressure respectively.
In this state, the distinction between liquid and gas disappears. A supercritical fluid has the physical properties of a gas, but its high density confers solvent properties in some cases, which leads to useful applications. For example, supercritical carbon dioxide is used to extract caffeine in the manufacture of decaffeinated coffee.
In a plasma, electrons are ripped away from their nuclei, forming an electron “sea”. This gives it the ability to conduct electricity.
- Like a gas, plasma does not have definite shape or volume.
- Unlike gases, plasmas are electrically conductive, produce magnetic fields and electric currents, and respond strongly to electromagnetic forces.
- Positively charged nuclei swim in a “sea” of freely-moving disassociated electrons, similar to the way such charges exist in conductive metal, where this electron “sea” allows matter in the plasma state to conduct electricity.
- A gas is usually converted to a plasma in one of two ways, either from a huge voltage difference between two points, or by exposing it to extremely high temperatures.
- Heating matter to high temperatures causes electrons to leave the atoms, resulting in the presence of free electrons. This creates a so-called partially ionized plasma.
- At very high temperatures, such as those present in stars, it is assumed that essentially all electrons are “free”, and that a very high-energy plasma is essentially bare nuclei swimming in a sea of electrons. This forms the so-called fully ionized plasma.
The plasma state is often misunderstood, and although not freely existing under normal conditions on Earth, it is quite commonly generated by either lightning, electric sparks, fluorescent lights, neon lights or in plasma televisions. The Sun’s corona, some types of flame, and stars are all examples of illuminated matter in the plasma state.
This diagram illustrates transitions between the four fundamental states of matter.
A state of matter is also characterized by phase transitions. A phase transition indicates a change in structure and can be recognized by an abrupt change in properties. A distinct state of matter can be defined as any set of states distinguished from any other set of states by a phase transition. Water can be said to have several distinct solid states. The appearance of superconductivity is associated with a phase transition, so there are superconductive states. Likewise, ferromagnetic states are demarcated by phase transitions and have distinctive properties. When the change of state occurs in stages the intermediate steps are called mesophases. Such phases have been exploited by the introduction of liquid crystal technology.
The state or phase of a given set of matter can change depending on pressure and temperature conditions, transitioning to other phases as these conditions change to favor their existence; for example, solid transitions to liquid with an increase in temperature. Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point, boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons are so energized that they leave their parent atoms.
Forms of matter that are not composed of molecules and are organized by different forces can also be considered different states of matter. Superfluids (like Fermionic condensate) and the quark-gluon plasma are examples.
In a chemical equation, the state of matter of the chemicals may be shown as (s) for solid, (l) for liquid, and (g) for gas. An aqueous solution is denoted (aq). Matter in the plasma state is seldom used (if at all) in chemical equations, so there is no standard symbol to denote it. In the rare equations that plasma is used it is symbolized as (p).
Check your knowledge
Particles in Solid are closely packed. The particles of Solid are held tightly by intermolecular force of attraction. Hence particles cannot change their position.
Adding or removing energy from matter causes a physical change as matter moves from one state to another. For example, adding thermal energy (heat) to liquid water causes it to become steam or vapour (a gas). And removing energy from liquid water causes it to become ice (a solid).
The state of matter that has the most energy is gas. In a solid, there is limited room for molecules to move around.