States of Matter: Plasma

At some point during your science studies, you would have been introduced to the idea of The Three States of Matter: solid, liquid, and gas. As you may have realised when thinking about, for example, the melting of glass, or contemplating the nature of a flame, this three state model doesn’t tell the whole story. Solid, liquid, and gas are more like three categories into which more specific states of matter fit. This series explores some of these states which perhaps don’t seem to fit neatly into the three states model as you may have learned it.

Plasma. No, we’re not talking about the watery part of blood. Plasma is often called “the fourth state of matter” because it’s what you get when you keep putting energy into a gas. (Recall: in the three states description of matter, solid + enough energy = liquid, liquid + enough energy = gas.) A plasma is a lot like a gas, but where a gas is made of uncharged atoms or molecules, (referred to as “neutrals” in plasma science, for obvious reasons), in a plasma some proportion of those neutrals have shed some of their electrons to become ions. The electrons are free and also form part of the plasma. So, a plasma is made of a mix of neutrals, ions, and electrons, in an equilibrium, in any proportion.

In a plasma, like in a gas, all these components are moving around and colliding with each other. Unlike in a gas, these collisions frequently result in excitation, ionisation, recombination and relaxation – electrons leave and rejoin ions and neutrals, and jump up or down their allowed energy states around those atoms or molecules.

This gives rise to a common – though not universal – property of plasmas: they glow. Any time you see something that seems to have the properties of a gas but which is emitting light, more likely than not, you are looking at a plasma.

Because of the presence of free ions and electrons, plasmas are electrically conductive, unlike gases. So, if you see something that seems to have the properties of a gas, but which is carrying a current, you are looking at a plasma.

A flame is a plasma. The heat from a combustion reaction excites and partially ionises molecules nearby, resulting in light emission as the electrons spontaneously return to their ground state. A yellow or orange flame also produces light through black body radiation; small particles of soot from incomplete combustion also get heated up by the energy being released from the reaction, and they tend to glow orange to yellow. Part of the orange-yellow colour can also be from sodium present in the fuel getting vaporised and excited.

Fireworks, coloured candles, and flame tests for metals also use the heat of combustion to form a plasma, in which the electrons around metal atoms get excited then relax, emitting light at the characteristic spectral line frequencies for the metal.

You also see plasmas created using high voltages every day; this is the principle behind fluorescent lighting and neon signs. A specific gas at low pressure is sealed inside a tube with electrodes built in to it. When a voltage is applied, it provides enough energy to excite and remove electrons from the atoms or molecules of the gas. With collisions and time, the electrons relax back down to the ground state, (while others get excited by the constantly applied voltage – it’s a system in an equilibrium after all!) and light is emitted, just like with the flame. The emission is at the characteristic frequencies for the element(s) in the gas.

In fluorescent tube lighting, the gas used is mercury. Mercury alone gives a bluish looking light with a very high intensity of UV, which doesn’t sound very useful for lighting our homes, nor very safe! You have probably noticed that the glass of a fluoro tube light is white and looks very opaque. This is the special feature that makes it usable as lighting, and which gives it it’s name. There is a coating on the glass of a combination of chemicals called phosphors which convert the UV light emitted by the mercury into a range of visible light frequencies. In most modern fluorescent tube lights, these are a combination of rare earth phosphates and oxides, chosen so their emission ranges combine to give apparent uniform white light. Viewed through a spectroscope, though, you can still see particular emission peaks from both the phosphors and from the visible light part of mercury’s own spectrum.

(As an aside: NSW HSC students are no doubt very familiar with discharge tubes from their Ideas to Implementations unit. These are a classic example of a DC electrically excited plasma, and demonstrate the variability of plasma behaviour depending on vessel shape, gas pressure, method of excitation, and energy delivered to the plasma.)

Another common plasma generated by high voltages is lightning. In lightning, and other arcs and sparks, the voltage between two points becomes high enough to exceed what is called the breakdown voltage of the gas between those two points – for air at atmospheric pressure, that’s around 3 kV per mm. When the breakdown voltage is exceeded, the gas starts to ionise, allowing it to carry a current. At atmospheric pressures, this tends to be limited to a relatively thin path, seen as a bolt of lightning, or a spark.

Stars are big, hot balls of thermally excited plasma, their energy provided by the nuclear fusion reactions going on in their hearts. A nuclear fusion plasma (temperature ~107 – 108 Kelvin) typically doesn’t generate light – it is too highly ionised, the equilibrium strongly disfavours neutral or minimally ionised species, so there’s little or no relaxation of electrons through bound energy states, meaning little or no light emission. Instead, high energy gamma rays are produced by the nuclear fusion reactions. In a star, these are then absorbed and re-emitted as lower energies by the series of outer layers, eventually giving rise to the black body spectrum of the outer photosphere (temperature ~ 104 – 105 Kelvin) which provides the sun- and starlight we see. The moving electrons and ions in a star give rise to the magnetic field of the star.

The earth also has a layer of plasma: the ionosphere, one of the uppermost layers of our atmosphere, has properties like a gas, but with free ions and electrons, and is electrically conductive. This is what allows us to “bounce” radio waves (within an appropriate frequency range) off the atmosphere. Normally, it doesn’t emit light. It is, however, part of the magnetosphere, the region of the atmosphere that gives rise to the polar auroras, which you will no doubt be unsurprised to find are also plasma phenomena.

Plasma is just one state of matter whose story isn’t really told by the simple solid-liquid-gas model. Also in this series: amorphous solids, and liquid crystals. 

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2 thoughts on “States of Matter: Plasma

  1. Pingback: States of Matter: Amorphous Solids | For Science!

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