The answer to whether plasma is the fourth state of matter is a definitive yes. Plasma is recognized as a distinct state of matter that exists beyond the more familiar forms of solid, liquid, and gas. This distinction arises when a substance receives so much energy that its atoms begin to break apart, fundamentally changing the material’s properties. Nearly all the visible matter in the universe exists in this energetic state, which behaves differently from any neutral gas.
Defining the Familiar States of Matter
The three classical states of matter are distinguished by the arrangement and energy of their constituent particles. A solid maintains a fixed volume and a definite shape because its atoms are tightly packed and vibrate only in fixed positions. Increasing the kinetic energy causes the substance to melt into a liquid, which retains a fixed volume but conforms to the shape of its container. Further energy addition causes the liquid to boil into a gas, where particles move freely and rapidly, resulting in a state with neither a definite volume nor a definite shape. The transition from solid to gas involves a progressive increase in particle freedom and kinetic energy, which precedes the plasma state.
The Unique Physical Properties of Plasma
Plasma is fundamentally an ionized gas, meaning it is a collection of neutral atoms, positively charged ions, and negatively charged electrons. This mixture contains a roughly equal number of positive and negative charges, making the plasma electrically neutral overall, a condition known as quasi-neutrality. This characteristic of containing unbound charged particles separates plasma from a neutral gas. Unlike a gas, a plasma is an excellent conductor of electricity because of its free-moving electrons and ions.
The presence of these charged particles causes plasma to respond strongly to electric and magnetic fields, a property a neutral gas does not share. The collective behavior of the charged particles is governed by electromagnetic forces, giving plasma fluid-like properties. Although a plasma is quasi-neutral in its bulk, localized regions of charge separation can occur. This collective, electromagnetic behavior, distinct from the simple particle collisions in a gas, is why plasma is considered a separate state of matter.
How Ionization Creates Plasma
Plasma is generated when a gas absorbs enough energy to strip electrons from their atoms, a process called ionization. This requires overcoming the binding energy that holds the electrons to the nucleus. Adding energy, typically heat or strong electromagnetic fields, accelerates gas particles until their collisions are vigorous enough to knock electrons free. This high-energy environment yields the mix of ions and free electrons that defines the plasma state.
There are two primary methods for generating plasma, based on the energy distribution within the gas. Thermal plasma is created when all particles—electrons, ions, and neutral atoms—are in thermodynamic equilibrium and share the same high temperature, often around 10,000 Kelvin. This high temperature ensures that collisions are frequent enough to maintain the ionization.
Non-thermal plasma, often called “cold plasma,” is produced when an electric field preferentially heats only the electrons to very high temperatures. The heavier ions and neutral gas particles remain relatively cool, close to room temperature. This energy difference is possible because the heavier particles do not gain energy as easily from the electric field, and low pressure reduces the frequency of energy-transferring collisions.
Plasma in the Universe and Daily Life
Plasma is the most prevalent state of matter in the observable universe, constituting over 99% of its visible mass. Stars, including our Sun, are massive spheres of hot plasma where the energy is supplied by nuclear fusion. Natural phenomena on Earth also involve plasma, such as the intense heat generated by a lightning strike. The spectacular glow of the auroras occurs when solar wind plasma interacts with the planet’s magnetic field. The solar wind itself is a stream of plasma ejected from the Sun, filling interplanetary space.
Plasma technology is widespread in modern human applications. Common examples include the excited, low-pressure gas inside fluorescent light tubes and neon signs, which are non-thermal plasma. Plasma display panels (PDPs) in televisions use tiny cells containing noble gases that are briefly converted into plasma to emit light. Plasma is also leveraged in advanced industrial processes, such as the precision etching of microchips and the experimental research into controlled nuclear fusion reactors.