Plasma Definition in Chemistry and Physics

spherical plasma lamp

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Plasma is a state of matter where the gas phase is energized until atomic electrons are no longer associated with any particular atomic nucleus. Plasmas are made up of positively charged ions and unbound electrons. Plasma may be produced by either heating a gas until it is ionized or by subjecting it to a strong electromagnetic field.

The term plasma comes from a Greek word that means jelly or moldable material. The word was introduced in the 1920s by chemist Irving Langmuir.

Plasma is considered one of the four fundamental states of matter, along with solids, liquids, and gases. While the other three states of matter are commonly encountered in daily life, plasma is relatively rare.

Examples of Plasma

The plasma ball toy is a typical example of plasma and how it behaves. Plasma is also found in neon lights, plasma displays, arc welding torches, and Tesla coils. Natural examples of plasma include lightning the aurora, the ionosphere, St. Elmo's fire, and electrical sparks. While not often seen on Earth, plasma is the most abundant form of matter in the universe (excluding perhaps dark matter). The stars, interior of the Sun, solar wind, and solar corona consist of fully ionized plasma. The interstellar medium and intergalactic medium also contain plasma.

Properties of Plasma

In a sense, plasma is like a gas in that it assumes the shape and volume of its container. However, plasma isn't as free as gas because its particles are electrically charged. Opposite charges attract each other, often causing plasma to maintain a general shape or flow. The charged particles also mean plasma may be shaped or contained by electrical and magnetic fields. Plasma is generally at a much lower pressure than a gas.

Types of Plasma

Plasma is the result of the ionization of atoms. Because it's possible for either all or a portion of atoms to be ionized, there are different degrees of ionization. The level of ionization is mainly controlled by temperature, where increasing the temperature increases the degree of ionization. Matter in which only 1% of the particles are ionized can show characteristics of plasma, yet not be plasma.

Plasma may be categorized as "hot" or "completely ionized" if nearly all the particles are ionized, or "cold" or "incompletely ionized" if a small fraction of molecules is ionized. Note the temperature of cold plasma may still be incredibly hot (thousands of degrees Celsius)!

Another way to categorize plasma is as thermal or nonthermal. In thermal plasma, the electrons and heavier particles are in thermal equilibrium or at the same temperature. In nonthermal plasma, the electrons are at a much higher temperature than the ions and neutral particles (which may be at room temperature).

Discovery of Plasma

The first scientific description of plasma was made by Sir William Crookes in 1879, in reference to what he called "radiant matter" in a Crookes cathode ray tube. British physicist Sir J.J. Thomson's experiments with a cathode ray tube led him to propose an atomic model in which atoms consisted of positively (protons) and negatively charged subatomic particles. In 1928, Langmuir gave a name to the form of matter.

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Helmenstine, Anne Marie, Ph.D. "Plasma Definition in Chemistry and Physics." ThoughtCo, Aug. 27, 2020, Helmenstine, Anne Marie, Ph.D. (2020, August 27). Plasma Definition in Chemistry and Physics. Retrieved from Helmenstine, Anne Marie, Ph.D. "Plasma Definition in Chemistry and Physics." ThoughtCo. (accessed February 8, 2023).