What is the phase change from plasma to gas called

When you are a liquid and want to become a gas, you need to find a lot of energy. Once you can direct that energy into your molecules, they will start to vibrate. If they vibrate enough, they can escape the limitations of the liquid environment and become a gas. When you reach your boiling point, the molecules in your system have enough energy to become a gas.

The reverse is true if you are a gas. You need to lose some energy from your very excited gas atoms. The easy answer is to lower the surrounding temperature. When the temperature drops, energy will be transferred out of your gas atoms into the colder environment. When you reach the temperature of the condensation point, you become a liquid. If you were water vapor over a boiling pot of water and you hit a wall, the wall would be cool, absorb some of your extra energy, and you could quickly become a liquid. Cooler objects often absorb energy from hotter objects.

Gas to a Plasma and Back to a Gas

Let's finish up by imagining you're a gas like neon (Ne). You say, "Hmmmm. I'd like to become a plasma. They are too cool!" As a gas, you're already halfway there, but you still need to tear off a bunch of electrons from your atoms. The gas needs to ionize. Electrons have a negative charge. Eventually, you'll have groups of positively and negatively charged particles in almost equal concentrations. They wind up in a big plasma ball. Because the positive and negative charges are in equal amounts, the charge of the entire plasma is close to neutral. Neutral happens when a whole bunch of positive particles cancel out the charges of an equal bunch of negatively charged particles.

Plasma can be made from a gas if a lot of energy is pushed into the gas. In the case of neon, it is electrical energy that pulls the electrons off. When it is time to become a gas again, just flip the neon light switch off. Without the electricity to energize the atoms, the neon plasma returns to its gaseous state. We have a special world here on Earth. We have an environment where you don't find a lot of everyday plasma. Once you leave Earth and travel through the Universe, you will find plasma everywhere. It's in the stars and all of the space in between.

Melting Ice

You can watch phase changes at home when you put a piece of ice (solid) on a counter. As long as the temperature is above 0 degrees Celsius, that ice cube will warm and melt. That melted puddle of water (H2O) is a liquid. Heat makes the phase change. Have a towel ready to clean up the mess.

Let’s say you’re cooking when you go camping or at a barbecue at home. When you put the food on the grill, you don’t smell much. As the food heats up, aromatic molecules begin to escape the surface of the food and diffuse through the air. Those volatile molecules are heated up and become a gas when they evaporate. Heat makes the phase change.

In many signs, there are glass tubes filled with neon (Ne) gas. Normally, the gas stays a gas. When you turn on the sign and send an electric current through the tubes, the neon loses its electrons and becomes plasma. Electricity makes the phase change.

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A phase change is a change in the states of matter. For example, a solid may become a liquid. This phase change is called melting. When a solid changes into a gas, it is called sublimation. When a gas changes into a liquid, it is called condensation. When a liquid changes into a solid, it is called solidification. When a gas changes into a solid, it is called deposition . When a liquid changes into gas, it is called evaporation. Phase changes are usually caused by changes in temperature or pressure.

As the states of matter change from solid to liquid to gas, respectively, their composition changes as well. For example, in a solid, the bonds are stronger than hydrogen bonds. That allows the solid substance to have a definite volume and shape. However, when the heat is added to the solid and it melts to become a liquid, the bonds are considerably weaker, and in water, are simply hydrogen bonds. A liquid has a definite volume but not a definite shape, and it thus takes the shape of the container in which it is. When more heat is added, the liquid substance evaporates and becomes a gas, which has no bonds at all. Gas is simply a formless collection of particles that tends to expand in all directions at the same time in order to occupy its full container. If a gas is not confined, the space between the particles will continue to increase. A gas has neither a definite volume nor a definite shape.

In this pattern, solid to liquid to gas, heat is being added in order to provoke the phase change. In the other "direction," gas to liquid to solid, heat is being released through the phase change.

It often helps to think of an ice cube when thinking of phase changes. An ice cube is a solid, and when heated up, it becomes liquid water. When heated up some more, it becomes water vapor, which is a gas.

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This is a very interesting question and since I have thought before about it, I would like to share my answer.

As far as I am concerned, there has been no empirical evidence of coexistence between a fully ionized plasma and a neutral gas as separated phases in contact such as ice and water at $0^{\circ}$C. As @Gotaquestion correctly pointed out, the transition between gaseous and plasma state is continuous and gradual.

However, the heat capacity at constant volume, $C_{V}$, and also the heat capacity at constant pressure, $C_{p}$, exhibit peak values in some temperature intervals where atom/molecule ionization due to energy exchange gets more probable. In the figure below, extracted from an old paper form Phys. Fluids by Drellishak et al, $C_{V}(T)$ and $C_{p}(T)$ curves were calculated using thermodynamics principles and the partition function of diatomic nitrogen and diatomic oxygen.

In these figures, the peaks show the effect of very strong increase in the specific heats mainly due to energy loss by electron ionization. After each peak, the curve decreases to a minimum which is always higher than the previous. This happens because after a peak occurs, the corresponding ionized electrons are introduced in the system, giving their own contribution to the specific heat by increasing the degrees of freedom of the system.

Note that as the peaks get narrower they should resemble more and more a second order phase transition. Second order phase transitions are characterized by continuous $G(T, P), S(T,P), V(T,P)$, but discontinuous constant pressure heat capacity $C_{p}(T,P)$ (a good reference may be find here).

Finally, I must point some caveats in this answer. Here I considered what is called "classic plasma", which uses classical statistical mechanics in the treatment of Debye-Hückel.

I am not a specialist in quantum mechanical plasmas, but maybe in such systems other kind of phase transition may occur.