In terms which might interest an engineer in a given field, how does the gas state differ from the plasma state?
What current technologies would fail if their plasma were replaced with merely hot gas? Which plasma-based technologies can we reasonably expect within the next 5 years*?
Is it commonly accepted that fire is a plasma?
Well, in practical terms, the biggest difference is simply that plasmas are hot. Though of course, different gases ionize at different temperatures, so you could have a hot neutral gas that’s hotter than some plasmas.
Once you control for that, the next biggest differences are that plasmas are conductive, where neutral gases aren’t, and plasmas tend to be more reactive than neutral gases.
Note, by the way, that whenever I use the word “gas”, there, I’m preceding it with the adjective “neutral”. That’s because plasmas and gases are not two distinct categories: Plasmas are themselves gases.
As I understand it, some very hot flames are (partially) ionized, but for the most part the flame is just neutral gas that is hot enough to glow.
As plasma is ionized, it tends to be highly conductive, which is not necessarily true of conventional gases. Plasma arcs can be a huge problem for electronics if a solder whisker causes a short, which can vaporize the solder metal into a plasma that is far more conductive than the whisker was. Kaboom.
ETA: Ninja’d by Chronos an hour ago. Should have read the thread more carefully.
Need not be. Plasma televisions, neon signs, and plasma globes don’t tend to get terribly hot.
By “hot”, I meant “high temperature”, not “containing a lot of thermal energy”. The plasmas in those things are indeed at very high temperatures, but they’re also very diffuse, so they can’t transfer much heat to anything else.
Fire is not a plasma - there are no ionized species in a fire (flame). There are a lot of reactions going on, and a lot of intermediate/ unstable species are formed in a fire (flame). For examples, the number of know reactions happening in a Hydrogen flame are 30, for a natural gas (methane) flame 300, and for gasoline 3000. More here.
Probably all of them that use plasma. You don’t generate a plasma unless you really need it. If you just need gas, you use gas.
Plasma, as I always understood, was a gas so energized that the atoms are losing electrons. If the environment is very close to a vacuum, it does not take a great deal of heat (large number of calories) to energize a gas into a plasma, because with many fewer atoms, much less energy is needed to strip some electrons off a large number of them.
That need for near-vacuum (or extreme temperatures) is what makes plasma the second choice if non-plasma options are also practical - i.e. plasma vs. LED for lights, television, etc.
Plasma TVs radiate lots of heat. If you get within 5-10 feet of a plasma screen, you can feel the heat coming off it.
Not mine (Panasonic VT series). Put my hand above it and yes, I feel the (mild) heat. But not 5-10 feet in front. That would be like a fire.
Also is it correct that plasma can be contained by magnetic fields while gasses cannot?
Last year there was some buzz about using plasmas on aircraft, as ionizing the air say on a wing tip can greatly decrease drag.
However, on googling for this, the first article that came up is from 2000, so perhaps this is another thing that will be perennially 20 or 30 years away. I hope not. Seeing plane wings glow will be cool…they should also fit spinning rims to the wheels 
I thought plasma was defined as the fourth state of matter: solid -> liquid - gas -> plasma.
Since a plasma is electrically conductive, it is subject to all the usual magnetic properties any other conductor is. So if there is a current flowing in it, it has a magnetic field, and will be acted upon by a magnetic field. Further, a moving magnetic field will induce a current in a plasma, which will also give rise to force on the plasma.
A clever arrangement of magnets can create a field that will contain a plasma. The best known are probably the various torus and Tokamak designs used for experimenting with nuclear fusion. There is a lot of cleverness, as getting the geometry stable is not a trivial undertaking.
A gas, doesn’t conduct electricity, so this isn’t going to work. That is really the only difference.
There are arguably lots of states of matter beyond these, but these are the big four. The thing about gasses is that they obey certain laws, (funnily enough the gas laws) and this is what distinguishes them from liquids and solids. PV = nRT and a whole raft of derived useful properties. Plasmas also obey the gas laws, to a reasonable extent. It makes perfect sense to talk about a plasma as a gas, just one with some additional properties. You could probably just as usefully talk about two sorts of gasses, ionised, and un-ionised.
What are the other arguable states of matter?
Would these properties enable us to contain or propel plasma at particularly high temperatures or speeds?
My condition about limiting speculation to 5 years was to prevent a small number of people from sucking up the air in this thread by making predictions which are far more the product of their enthusiasm than their knowledge and reflection. You seem unlikely to do that so I’d be glad to hear what uses you think might be made of plasma technology beyond 5 years.
Wikipedia indicates that even though the electrons in a plasma may be at high temperature, the ions may be quite close to ambient temperature, particularly in the types of devices I mentioned.
Yes, they get warmer than ambient, but not stellar-hot like the plasma in the arc of a spark plug.
It used to be believed that the sun is a mass of incandescent gas, but now the view is that the sun is a miasma of incandescent plasma.
There are an infinite number of possible fluid states. An ideal fluid is described by its pressure and its density. Any given kind of fluid will have something called an equation of state, which gives the relationship between pressure and density. For a lot of fluids (at least, under Earthly conditions), the equation of state is simply “density is a constant” (or something close enough that that’s a good approximation). We call those “liquids”. For a lot of other fluids, the pressure is proportional to the density and to the temperature. We call those fluids “gases”. But there are many other equations of state actually observed in nature. You can have density that’s directly proportional to pressure, you can have density that’s proportional to pressure raised to some power (and the value of that power can be almost anything), and so on.
Plus, if you’re going to say that some kinds of gases are different enough from other kinds of gases to call them a new state of matter (like we do with plasmas), then you can do the same sort of thing with liquids and solids. Water, for instance, has at least a dozen different solid phases, with different crystalline structure, densities, and other material properties, at various values of temperature and pressure.