most materials are solid,liquid or gas.
what is energy defined as?
Physicist here. Can’t pass this one up.
Energy isn’t defined, it’s a derived quantity, meaning that physicists started noticing that if you add up certain combinations of things, you always get the same number before and after something happens.
Example: If I drop a ball, I can add up its kinetic energy (1/2 times mass times speed squared) and its potential energy (mass times height times a constant of proportionality) and I get the same value whether the ball is half-way to the ground, one-quarter of the way, or whatever.
The question in the OP seems to relate to the idea of mass-energy equivalence: good old E equals m c squared. Rephrasing: Since mass is energy, and mass usually comes in the forms gas, liquid, solid, (most people nowadays add “plasma”), which one of these is energy? (Correct me if I misunderstood the question.)
I hope you can see that the question is backwards: All forms of matter are energy, so obviously we can’t say that “energy is blah” (blah = liquid, solid, or gas).
What about energy that doesn’t have mass? For example, light? The answer depends on the particular situation. You wouldn’t call the beam from a flashlight a “gas” (Certainly not a liquid, solid, or plasma, either!), but under some circumstances physicists speak of a “photon gas” - photons being the particles that carry light energy. You can talk about the pressure, temperature, etc., of this gas. So if I had to choose, I’d say “pure energy” comes closest to being a gas.
I know someone will correct if I am wrong, but energy doesn’t have phases. Energy is defined as the ability to cause change or to do work.
energy wouldn’t be the kind of thing that physicists or school children would classify as matter…
but what about fire…is it a solid, liquid or gas ?
thought i’d ask it here rather than waste a thread and precious server load time on the board 
is the SDMB a solid, liquid or mostly gas ?
Fire is usually an example of the fourth state of matter, plasma. Plasma is matter in which the electrons have separated from the atomic nuclei, leaving a lot of ions and a lot of free electrons.
Of course, I think most ‘common’ fire is really a mixture of plasma and gas, since much of it is not ionized. Sort of like fog is a mixture of liquid and gas.
Saltire, you fought ignorance today damn, i’ve been looking for this answer a long time … now i’m off to some more readin’ on them plasmaz…
That’s not right is it? Where’d that physicist go? I didn’t think your standard hydrocarbon combustion fire was plasma at all. I though it was simply a gas phase chemical reaction of oxygen and a hydrocarbon that released (mostly) CO2, water, light, and heat. Sure, in the process atoms are ionizing and recombining, but that doesn’t mean there is a plasma involved. Ions in plasma are stripped of ALL electrons right? not just an outer shell electron or two involved in the chemical reaction. Is it plasma just because there’s a couple H ions running around in the reactions? If so you would have to say you had plasma every time H ions were involved in any chemical reaction right?
Well, brook’s OP was a little on the concise side, but I interpreted it to mean:
Matter can divided into different “types” (phases, really): solid, liquid, and gas. Can energy also be divided into various “types”, and if so, what are they?
So, FriendRob, you also answered this question, at least in part. There is a distinction between kinetic energy, which is energy in the form of motion, and potential energy, which is energy that is stored in some form. Being an engineer and not a physicist, I’m not sure of the proper distinction among other types of energy (err… radiation? stored chemical energy? is that considered potential?), but I imagine that someone else could fill in.
Assuming that I’ve interpreted the OP correctly.
Technically to say what form fire is in, you have to define what you mean by fire. Fire as most people think of it is just heat and light being released as a result of a chemical reaction. When you say fire are you including the matter involved? Plus, some people say the sun is a big ball of fire. It gives off heat, light, and involves matter thats reacting(although in a fusion reaction now). So basically fire is a general term that can refer to lots of reactions and it’s hard to classify it.
Cecil’s columns, the Mailbag, and even GQ are pretty solid (in varying degrees), GD is mostly liquid (with a few solid bits here and there), and MPSIMS, of course, is mostly gas.
What did you expect?