Plasma: The Fourth State of Matter Explained

What Is Plasma?

Plasma is called the fourth state of matter after solid, liquid, and gas. If you have studied the states of matter in school, you learned about solids, liquids, and gases. But plasma is a fourth state—and it is by far the most common one in the universe, even though we rarely see it in everyday life on Earth.

It is a state of matter in which an ionized substance becomes highly electrically conductive to the point that long-range electric and magnetic fields dominate its behaviour. In simpler terms, plasma is a gas that has been energized enough to separate electrons from atoms, creating a mixture of charged particles.

How Plasma Differs from Other States of Matter

To understand plasma, think about how the other states of matter work. In a solid, the particles are tightly packed and held in fixed positions, giving the material a definite shape and volume. In a liquid, the particles remain close together but can move past one another, allowing the substance to maintain a fixed volume while adapting to the shape of its container. In a gas, the particles are far apart and move freely, allowing the substance to expand and fill both the shape and volume of its container.

Plasma is similar to a gas in some ways— it does not have a defined shape or volume, instead forming to fill the size and shape of its container, like a gas. However, plasma has a crucial difference: plasma conducts electricity, as opposed to gases, which are electrical insulators.

What Makes Plasma Special: Ionization

The key to understanding plasma is understanding ionization. A plasma is created when one or more electrons are torn free from an atom of gas. Atoms that have lost some or all of their negatively charged electrons are called ions. An ionized atom has a positive charge because it is missing electrons, but still contains positively charged protons and neutrons (with no charge) in its atomic nucleus.

When this happens across many atoms in a gas, you get a mixture of:

• Free electrons (negatively charged)
• Ions (positively charged atoms that lost electrons)
• Neutral atoms (atoms that still have all their electrons)

Plasma is typically an electrically quasineutral medium of unbound positive and negative particles (i.e., the overall charge of a plasma is roughly zero). This means that even though plasma contains lots of charges, the number of positive charges and negative charges is roughly equal, so the plasma as a whole is electrically neutral.

How Is Plasma Created?

Plasma does not occur naturally on Earth very often, but scientists can create it in laboratories, and it forms naturally in space. To make plasma, energy is needed to strip electrons from atoms. The energy can be of various forms – heat, electrical or light (ultraviolet light or intense visible light from a laser).

There are two main ways plasma forms:

Thermal Ionization (Heat)

A plasma may be produced in the laboratory by heating a gas to an extremely high temperature, which causes such vigorous collisions between its atoms and molecules that electrons are ripped free, yielding the requisite electrons and ions. A similar process occurs inside stars.

Electrical Ionization

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.

Hot Plasma vs. Cold Plasma

Not all plasma is the same. Scientists divide plasma into two main types based on temperature.

Hot Plasma (Thermal Plasma)

Thermal plasmas or hot plasmas have electrons and the heavy particles at the same temperature, i.e. they are in thermal equilibrium with each other (Te≈Ti≈Tgas).

Most plasma in the universe is what researchers call high-temperature plasmas. In these high-temperature plasmas, temperatures can be more than 10,000 degrees Fahrenheit, and all the atoms can be fully ionized.

The Sun is made of hot plasma. The intense heat at the center and surface of the Sun (millions of degrees) keeps the atoms completely ionized.

Cold Plasma (Non-Thermal Plasma)

Low-temperature plasmas are different. The atoms are only partially ionized, and they can be incredibly cool—even room temperature. This seems strange—how can electrons be at thousands of degrees while the gas itself is room temperature? The answer is that in cold plasma, electrons gain energy from an electric field much faster than they can pass that energy to heavier particles like ions.

Cold plasma is made up of the same elements, but the temperature of the individual parts are different from each other. Electrons are always at an extremely high temperature, but the neutral atoms remain at room temperature in this case.

This makes cold plasma very useful for medical and industrial applications, since it provides the chemical benefits of plasma without the damage from extreme heat.

Examples of Plasma

Plasma might seem rare, but once you know what to look for, you will see it in many places.

Natural Examples

Above the Earth's surface, the ionosphere is a plasma, and the magnetosphere contains plasma. Within the Solar System, interplanetary space is filled with the plasma expelled via the solar wind, extending from the Sun's surface out to the heliopause.

One of the most beautiful examples is the aurora. Plasma in Earth's magnetosphere sometimes flows along Earth's magnetic field towards the polar regions, creating the colorful light shows in the sky which we call the aurora or Southern and Northern Lights. These beautiful displays occur when energetic plasma particles collide with gases in the atmosphere, causing them to glow in much the same way that fluorescent and neon lights shine.

Lightning is another familiar example. During a lightning strike, the air becomes ionized and produces plasma. The bright white-blue light you see is actually plasma heating the air around it.

Artificial Examples

Humans have learned to create plasma for practical purposes. Electricity in fluorescent lights creates a plasma. Colorful neon lights, often used in signs, also use electricity to convert gas molecules into glowing plasma.

Other artificial plasmas include:

• Plasma televisions
• Welding arcs
• Plasma balls (the novelty toys with glowing filaments inside)
• Industrial plasma torches used for cutting metals

Why Is Plasma Important?

Understanding plasma helps us understand the universe. While rarely encountered on Earth, it is estimated that 99.9% of all ordinary matter in the universe is plasma. Stars are almost pure balls of plasma, and plasma dominates the rarefied intracluster medium and intergalactic medium.

But plasma is not just interesting scientifically. One of the most significant applications of hot plasma lies in fusion research. Scientists are trying to use hot plasma to create fusion reactions—the same process that powers the Sun—to generate clean energy for humanity.

Cold plasma has medical applications. In the field of medicine, cold plasma is being investigated for its potential in accelerating wound healing processes. By delivering a controlled dose of reactive species to the wound site, cold plasma can stimulate cell growth, enhance tissue regeneration, and combat infections, offering a non-invasive alternative to traditional treatments.

The Key Characteristics of Plasma

Several properties make plasma unique and distinct from the other states of matter:

1. Electrical Conductivity: Because it consists of charged particles, plasma has innate electrical conductivity. This is the opposite of gases, which do not conduct electricity well.

2. Magnetic Response: Because plasma particles have an electrical charge, they are affected by electrical and magnetic fields.

Because the charged particles have kinetic energy, plasma always has a magnetic field.

3. No Fixed Shape or Volume: Like the gas state of matter, plasma has neither a definite volume nor a definite shape.

4. Complex Behavior: In plasma, electrons and ions behave and interact in very complex ways, which creates waves and instabilities. This makes plasma more difficult to study than simple gases.

Conclusion

Plasma is the fourth state of matter, formed when a gas becomes ionized (when electrons are separated from atoms). While it is rare on Earth, plasma makes up almost all of the universe. Scientists and engineers are learning to use plasma in many applications—from lighting and welding to medical treatments and energy research. Understanding plasma helps us understand the universe and develop new technologies that improve our lives.