Fire is gas that's hot enough to incandesce (that is, give off blackbody radiation, whose color is due to its temperature). (It is usually not a plasma, though some very hot fires will become partially ionized.) The chemistry of celluose causes it to burn at around 1,000F, which gives off a chracteristic red-orange flame. There is sometimes blue near the bottom, where it's hottest.

In addition to the blackbody radiation, the electron structure of some elements and compounds absorb the energy, exciting the electrons to a higher energy level. They then drop energy levels, emitting photons of a particular color. Sodium gives a very distinctive yellow color; copper compounds tend to the blue-green range, etc.

A stable substance won't burn at room temperature; there is insufficient activation energy. When it's heated enough, it will react with oxygen in the air (if present), forming compounds with lower energy. Wood, for example, is mostly cellulose (C6H10O5)n, which reacts with O2 to form CO2 and water, both very stable compounds. It gives off around 3,000 kJ/mol, which can go into raising the temperature of the cellulose around it. When the energy given off is enough to cause more reactions, the fire self-sustains. If not, the fire burns out.

Paper burns more easily than wood because it has a high surface area, giving it ready access to oxygen. A stack of paper doesn't actually burn all that well, no better than wood. Hydrocarbons like those found in gasoline give off more energy than cellulose, because they lack oxygen, and when you provide it with oxygen and a little heat, they can burn very quickly. Hydrocarbons are also volatile, exposing the fuel to air more effectively: a match dropped into a tank of liquid gasoline will burn out because the liquid will cool the match and stop the fire faster than it can cause the hydrocarbon to burn.

If you're in a gravity well, the hot gas will tend to rise because its density is lower than the cool air around it. In free fall, flames don't rise. It's a classic experiment, and you don't even need to go into space to do it:

http://quest.nasa.gov/space/teachers/microgravity/9flame.html

Ancient Greeks thought considered fire as well as earth, water, and air were elements. But now, as we know, the elements are defined by how many protons that the pure substances posses. Fire is made up of different substances, so it is not an element.

Generally, fire is a mixture of hot gasses. Flames, are produced by a chemical reaction, called combustion. At a certain process of combustion, there is a state, called ignition point, where flames are produced. Flames are primarily consisted of carbon monoxide (CO), water vapor(H20), oxygen(O2), and nitrogen(N2). Flames emit heat and light. If the flames are hot enough, the gasses are ionized and yet become another state of matter: plasma.

Source: paraphrase from What is the State of Matter of Fire or Flame? Is it a Liquid, Solid, or Gas? and What Is Fire Made Of? created by Anne Marie Helmenstine, Ph.D.

Hope this help.

Fire is an oxidation/reduction reaction. Simply put, this means that electrons are being transferred from one molecule to another. In the case of a gas stove, methane is being oxidized (giving away its electrons) and oxygen is being reduced (soaking up the electrons). Overall the reaction is CH4 + O2 -> 2H20 + CO2, but it is the process of electron transfer that you are watching when you watch a flame.

As for why some things burn easier than others, it is because this electron transfer is, in some cases, easier than in others. A cool example is human skin. How come we don't just burst into flames? Because ground state O2 's (a free radical!) electron structure is such that it cannot easily trade its electrons with our skin. Electrons have to be lined up in a very specific way in order to react with ground state O2. It is thermodynamically favorable but kinetically very slow.

And things can absolutely burn at room temperature. If you mix enough Methane with enough O2 at room temperature....BOOM! fire.

If we don't want to explain fire using chemistry but in common language, we can say fire is oxidation(reaction of oxygen with a substance) taking place really fast in the presence of a lot of heat(thus, exothermic) with a fuel, usually characterized by a flame.

If you ask what a flame is, then a flame is "body of incandescent light" which quite means luminous with intense heat. So, flame is the fire and not just mere result of the fire; the fire is sustained because of the heat from flame.

If there is anything unclear or you don't understand please comment and I'll answer it!

Thanks for the A2A Anurag

I'm not a physicist, so I'll give you the layman's answer. Fire is heat, fuel, and oxygen combined in appropriate proportions to create a self-sustaining cycle of exothermic chemical reaction.

If you're trying to make fire or extinguish it, you need to understand fire needs three things:
Enough heat concentrated in one place to vaporize the fuel.
Fuel: a substance which chemically combines with oxygen in an exothermic reaction.
And air or another substance able to provide a source of oxygen.

Remove any of these three things and the fire goes out.

The heat comes from the exothermic chemical reaction.
The light comes from electrons changing their orbital shell level. This is one way that atoms shed heat.

It's possible to have oxidation (exothermic chemical reactions) without giving off light. Rust (iron atoms oxidizing) is one such example. The process is slow enough that there is insufficient heat to excite the electrons to their higher positions. Electrons give off light only when falling to a lower a lower level.

The same chemical reaction at a higher speed will give produce a bright red light. (Try lighting some fine steel wool with a 9-volt battery. Wear safety glasses and thick gloves. If you try this, do it outside and don't breathe the fumes. The result will be a small pile of rust.)

It's possible to have a chemical reaction which gives off light but not heat. Glow sticks (which mimic photo-luminescent animals) is an example. Once again, electrons falling to a lower shell level give off specific wavelengths of light. The entire reaction is neither exothermic nor endothermic.

Fire is generally considered to be a plasma, which means a hot ionized gas with free electrons. Being a plasma, the shape of a fire can be manipulated with magnets or electrical charges.

A bunch of atoms get excited by absorption of heat from an exothermic chemical reaction.

Later on , they release this excess energy in the form of light giving us the flames of fire.

Every element actually produces a different colour light when it returns to it's ground state. This colour is dependent on the energy gap in electron’s excited and ground state. Example:carbon- yellow flame, arsenic-blue, boron-bright green, barium- apple green etc.

This property of giving of different colour by different elements is used to determine the presence of elements in mixtures.

Fact: We found the elements in our sun by analysing it's radiation spectrum.

Such a wonderful question!

We see a fire (luckily!) thanks to very tiny particles that are left unburned and when they get really hot they emit light of all wavelengths (or colors) but slightly less blue than the other colors and that's why we see them red-yellowish. The hotter the fire the more yellow the flame. This phenomenon is called Black-body radiation. This is the same phenomenon that happens when we heat a metal, it emits light (initially red, but becomes whiter as we heat it more and more).

(Taken from Wikipedia, By: Fir0002/Flagstaffotos

On the other hand, the blue color we normally see at the bottom of a candle fire or a normally functioning kitchen stove* is produced when the combustion is complete, leaving only small molecules (not tiny particles, like in the yellow part of the flame) that are electronically excited by the heat and when the electrons go back to it's basal level they emit this energy as light (and this light falls normally in the vicinity of blue colors).

*: This is why it's very important to always check we get a blue flame when using the stove, otherwise it means it's not working properly and can be producing dangerous substances due to incomplete combustion of the gas.

Because something is burning.

Just kidding. A better way to ask this question is - "Why does fire give off a particular color of light?" Why is fire usually orange and not, say, purple or bright green?

Another way to say "what color is this substance?" is to say "what frequency of *visible photons are being emitted?" Color is another name for light of a certain frequency. "Green" light means photons with a frequency of oscillation of in the ballpark of 550nm are being emitted. Yellow corresponds to around 590nm, red to 700-740 or so nm. Visible color is just photons being emitted at characteristic frequencies.

An Experiment:

You could do the above experiment at home. From left to right: Lithium Chloride (harder to find), Borax (laundry detergent), KCl (a salt substitute you can find in the grocery store), and on the right is NaCl (table salt). Photo credit [1] and [2]

What Creates Visible Colors?
Just burning a bunch of stuff is easy, but explaining why the colors are different is more in-depth.

In terms of chemistry, visible color is a phenomenon which occurs when a previously excited electron returns to its ground state. The same explanation for why anything emits color at all suffices whether a substance is burning, or just sitting on your desk.

When a source of energy (such as heat, light, whatever) is applied to a substance, it causes some electrons to move to a higher energy state. Higher energy states are unstable, so the electrons are constantly moving up and back down. When they move to a lower energy state, a photon is emitted which corresponds to the difference in energy states.

WARNING: More In-depth answer below

Molecular Orbitals: Absorbance and Fluorescence

Every molecule has a unique electron configuration. Remember that the light corresponds to the energy difference between the excited and ground states. Imagine an electron jumping from one electron shell to the next highest one. Then, when it jumps back down, light is emitted that is the same energy as the energy difference between shells.

In a nutshell, you should understand the concepts of Absorbance and Fluorescence.

Some terms:
HOMO: Highest Occupied Molecular Orbital
LUMO: Lowest Unoccupied Molecular Orbital

Radiation in the form of heat causes an electron to "jump" from the highest occupied orbital to the next unoccupied orbital [Absorbance]. But that energy configuration is unstable, so the electron "jumps" back to its rest state [Fluorescence]. The energy has to go somewhere, and it is emitted as light [Fluorescence], the frequency (color) of the light emitted depends entirely on the substance to which radiation in the form of light, heat, or whatever, is applied.

Photo credit [3]

You could draw the Molecular Orbital diagram for any substance being burned. HOWEVER, the difference in energy between the HOMO and LUMO only corresponds to the color of the substance when it's sitting on the table. When a substance actually combusts, the substances emitting light are combustion products of the original substance, with a variety of electron configurations. For example, when you burn wood, some of it changes into an amalgamation of carbon and organic molecules, free radicals, etc, and these products, respectively, emit radiation from electronic excitations induced by the absorption of heat.

*I should note that when you burn something, a large range of electromagnetic radiation is being emitted, but humans can only see a small part of the electromagnetic spectrum.

A few sources:
[1] Colored fire
[2] Flame Colors
[3] Electronic Spectroscopy: Interpretation

Standing in front of a fire, we can sense both its heat and its light.
(1) How are the light and heat radiated by the fire the same?
(2) How are they different?

Short answers:

1. Both visible light and radiant heat are parts of the electromagnetic spectrum.
2. Light and heat use different wavelengths (carry different energies).

Thanks for asking, Elizabeth, and welcome to Quora!

In the mythology of virtually every culture, fire is a sacred substance that gives life or power - and considered as one the four basic elements that composed all things in the universe. Fire is not a substance; it is a process – a chemical reaction. It’s the same chemical reaction that occurs when an iron nail rusts - that process is oxidation - combining oxygen with another substance.

The defining difference between a fire and the rusting nail is speed. Fire is an oxidation process that happens very fast, so that light, heat and sound are released — often with a lot of force. When an object (wood, for instance) is heated it releases volatile gases. and when the gas is hot enough - say about 360° C - the compound molecules break apart, and the atoms recombine with the oxygen to form water, carbon dioxide and other products. In other words, they burn.

The sudden release of energy causes temperatures to rise, sometimes by thousands of degrees. And it also results in smoke, the toxic waste of fire’s leftovers. [When wood burns, some of the decomposed material is released as "smoke" - or volatile gases. Smoke is a compound of hydrogen, carbon and oxygen. The rest of the material forms char, which is nearly pure carbon, and ash, which is all of the nonburnable minerals in the wood (calcium, potassium, and so on). Charcoal is wood that has been heated to remove nearly all of the volatile gases and leave behind the carbon. That is why a charcoal fire burns with little or no smoke.

A fire needs three components:

A combustible material - or fuel - something that will burn
Heat - enough to make the fuel burn
Oxygen

All three components must be present to start a fire. Fire will burn until one or more of the components are exhausted or removed. Traditional fire extinguishing methods involve removing either the fuel, or heat, or the oxygen, depending on what is burning.

In more recent years, a fourth component – the uninhibited chain reaction – has been added to explain fire. This chain reaction is the feedback of heat to the fuel to produce the gaseous fuel used in the flame. In other words, the chain reaction provides the heat necessary to maintain the fire. The addition of this fourth component (which forms what is called the "fire tetrahedron ") more accurately describes the mechanism for fire suppression by clean agent halon replacements which break up the uninhibited chain reaction of combustion. [Halon is a liquefied, compressed gas that stops the spread of fire by chemically disrupting combustion. It is electrically non-conducting. Halon 1211 - a liquid streaming agent - and Halon 1301 - a gaseous flooding agent - leave no residue and are remarkably safe for human exposure.] What is Halon? How does Halon Work?

The flame that we see dancing and glowing when something is burning is gas that is still reacting and giving off light. Plasma is a gas in which a good fraction of the molecules are ionized. Ordinary flames ionize enough molecules to be noticeable, but not as many as some of the much hotter things that we usually call plasma - like in arc-welding where the temperature can go up to 12,000°C. What is the State of Matter of Flame?

There are four stages of fire:

Ignition: Fuel, oxygen and heat join together in a sustained chemical reaction.

Growth: With the initial flame as a heat source, additional fuel ignites. Convection and radiation ignite more surfaces. The size of the fire increases and the plume reaches the ceiling. Hot gases collecting at the ceiling transfer heat, allowing all fuels in a room to come closer to their ignition temperature at the same time.

Development: Fire has spread over much if not all the available fuel; temperatures reach their peak, resulting in heat damage. Oxygen is consumed rapidly.

Decay (Burnout): The fire consumes available fuel, temperatures decrease, fire gets less intense and eventually dies out.

Fire spreads by transferring the heat energy from the flames in three different ways.

Conduction: The passage of heat energy through or within a material because of direct contact, such as a burning house, when even a wad of paper on fire can heat books on a nearby window, igniting them which in turn heats the drapes hanging behind, until they too burst into flames and so on.

Convection: The flow of fluid or gas from hot areas to cooler areas. The heated air is less dense, and rises, while cooler air descends. A large fire in an open area produces plume or column of hot gas and smoke high into the air. But inside a room, those rising gases encounter the ceiling. They travel horizontally along the ceiling forming a thick layer of heated air, which then moves downward.

Radiation: Heat traveling via electromagnetic waves, without objects or gases carrying it along. Radiated heat goes out in all directions, unnoticed until it strikes an object. Burning buildings can radiate heat to surrounding structures, sometimes even passing through glass windows and igniting objects inside.

The Chemistry of Fire

You have to first understand what light is and how it is emitted. Light is nothing but an electromagnetic wave, i.e. a disturbance in electric and magnetic fields, such that it can sustain the changes itself and propagate. Any charge while accelerating emits electromagnetic radiation, I'm sure you can see why. Any substance which is hot, will have atoms jiggling. Electrons are the major component of this jiggling and thus contribute to emission of light. For the temperatures we see, the light emitted is mostly in Infrared region thus invisible to the naked human eye.

Now you need to understand what fire is. Fire is simply a lot of fuel being burnt at a high temperature. A lot of particles are being heated and are thus jiggling more and more. As they jiggle vigorously, they produce light of higher frequencies, a substantial part of which is in the visible spectrum and thus you're able to see it. You'll notice that different materials have differently colored flames. Hydrogen, for example, has blue flames. This is because of the temperature of the flame. The hotter the flame, the more bluish the color.

So, the summed up answer is, everything emits light. Fire is hot thus emits more visible light.

When matter gets hot, there is greater motion of atoms within it. This thermal motion causes an emission of thermal radiation. When the matter gets hot enough, that emission will include visible light. This is called incandescence.

When you look at a fire you are looking at volatile gas molecules that arise from the burning and vaporization of the material being burned. Those atoms are incandescent.

Glowing embers do not give off as much light because they are not as hot. There is a process called pyrolysis by which the outer hottest layer of a substance gets hot enough to vaporize. The vapor carries away its heat (fire) leaving behind an exposed cooler layer of the substance. This is the same principle by which an ablative heat shield works, on a spacecraft.