What Is Fire? The Physics and Chemistry of Combustion

Fire looks like a thing, but it is really an event. A flame is the visible part of a chemical reaction between a fuel and oxygen that releases energy as heat and light. Our thermodynamics tees celebrate exactly this kind of energy release. This post builds on our explainer Why Does Time Only Go Forward?, because a burning log is one of the clearest one-way processes in everyday life. Ash never un-burns.

What is combustion?

Combustion is a reaction in which a fuel combines with an oxidiser, usually oxygen in the air, and releases energy. For methane, the main component of natural gas, the balanced equation is

CH₄ + 2 O₂ → CO₂ + 2 H₂O ΔH°c = −890.8 kJ/mol

Here CH₄ is methane, O₂ is oxygen, CO₂ is carbon dioxide and H₂O is water (liquid, in this standard value). ΔH°c is the standard enthalpy of combustion: the heat released when one mole of fuel burns completely under standard conditions, in kilojoules per mole. The negative sign means energy leaves the reacting chemicals, so the reaction is exothermic. OpenStax Chemistry lists −890.8 kJ/mol for methane (OpenStax Chemistry, §5.3^[1]), and the NIST Chemistry WebBook gives −890.7 ± 0.4 kJ/mol (NIST WebBook^[2]).

Where does the energy in fire come from?

The energy is stored in chemical bonds, or more exactly in the difference between the bonds you break and the bonds you make. Breaking a bond always costs energy; forming one always releases energy. OpenStax gives the rule of thumb as

ΔH ≈ ΣD(bonds broken) − ΣD(bonds formed)

where D is the average bond energy in kJ/mol and Σ means “the sum of” (OpenStax Chemistry, §7.5^[3]). For methane, OpenStax gives the sum of its four C–H bonds as 1660 kJ, an average of 415 kJ/mol each, and lists O=O at 498, C=O at 741 and H–O at 464 kJ/mol.

Our worked estimate, making water vapour: breaking four C–H bonds and two O=O bonds costs 4 × 415 + 2 × 498 = 2,656 kJ. Forming two C=O bonds and four O–H bonds releases 2 × 741 + 4 × 464 = 3,338 kJ. The difference is about −680 kJ per mole of methane. That is smaller than the measured −890.8 kJ/mol, for two reasons we can state with confidence: the standard value counts the extra heat released when the water condenses to liquid, and average bond energies are only averages, so this method is a rough estimate. The sign and the size are right, though. The products are held together more tightly than the fuel and oxygen were, and the difference comes out as heat.

Which fuels carry the most energy per kilogram?

Dividing OpenStax’s molar values by each fuel’s molar mass gives energy per kilogram (our calculation): hydrogen about 142 MJ/kg, methane about 55.5 MJ/kg, isooctane (a model for petrol) about 47.8 MJ/kg and ethanol about 29.7 MJ/kg (OpenStax Chemistry, §5.3^[1]). Hydrogen wins per kilogram because its molecules are so light.

Why does fire need a spark to start?

If burning releases energy, why does a log not burst into flame by itself? Because even a downhill reaction has a hill to climb first. Molecules must collide hard enough to start rearranging their bonds. OpenStax calls that minimum the activation energy, Eₐ, and gives the Arrhenius equation for how the rate constant k depends on temperature (OpenStax Chemistry, §12.5^[4]):

k = A e^−Eₐ/RT

Here A is a frequency factor (same units as k), Eₐ is the activation energy in J/mol, R = 8.314 J/(mol·K) is the gas constant and T is the absolute temperature in kelvin.

Take an illustrative activation energy of 100 kJ/mol (a round number for the sake of the example, not a measured value for any particular fuel). Raising the temperature from 300 K, room temperature, to an illustrative flame-like 1,500 K multiplies k by e^{(Eₐ/R)(1/300 − 1/1500)}, which is about 8.5 × 10¹³ (our calculation). That huge factor is the secret of fire. A match supplies the first heat. The reaction then releases more heat than it needs to keep itself going, so it heats the next layer of fuel, which reacts and heats the next. Once it starts, it feeds itself until the fuel or the oxygen runs out.

Why is fire hot and bright?

The energy released goes into the motion of the product molecules, which is what high temperature means. In a candle, some of the carbon does not burn completely and forms tiny solid particles of soot. Heated to high temperature, those particles glow, which gives a candle flame its yellow light. NASA puts it this way: on Earth, gravity-driven buoyant convection makes a candle flame teardrop-shaped and carries soot to its tip, which makes it yellow (NASA, 2020^[5]).

Why does a flame point upward, and what happens in space?

Hot gas is less dense than the cool air around it, so it rises, and cooler air flows in at the bottom to replace it. That loop, called buoyant convection, stretches the flame upward and keeps feeding it fresh oxygen (NASA Science^[6]).

Take gravity away and the loop stops. NASA reports that in microgravity, where those convective flows are absent, a candle flame becomes spherical, soot-free and blue (NASA, 2020^[5]). On the International Space Station, NASA notes that low-momentum flames tend to be rounded or even spherical, which lets researchers study flame behaviour without buoyancy getting in the way (NASA, ISS research^[7]).

Why does fire only go one way?

A burning log turns ordered fuel and oxygen into hot gas, water vapour and light spreading out in every direction. Running that film backwards, with the smoke and heat flowing back to rebuild the log, would need a vast and unlikely conspiracy of molecules. That is the second law of thermodynamics in action, which our post What Is Entropy? explains with Boltzmann’s formula. More characters for heat and energy live in our thermodynamics t-shirt collection.

Quick FAQ

Is fire a solid, liquid, gas or plasma?

A flame is mostly hot gas, with glowing soot particles in many flames. It is best described as a reaction zone rather than a state of matter.

What are the three things a fire needs?

Fuel, an oxidiser such as oxygen, and enough heat to get over the activation energy. Remove any one and the fire stops.

Why are some flames blue?

A flame with little or no soot does not have the yellow glow of hot particles. NASA’s microgravity candle flames are soot-free and blue.

Where does the energy of fire come from?

From chemical bonds: the bonds in the products are stronger overall than the ones broken in the fuel and oxygen, and the difference is released as heat.

References

1. P. Flowers, K. Theopold, R. Langley & W. R. Robinson, Chemistry 2e (OpenStax, Rice University), §5.3 “Enthalpy”. https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy
2. NIST Chemistry WebBook, SRD 69, “Methane” (gas-phase thermochemistry data). https://webbook.nist.gov/cgi/cbook.cgi?ID=C74828&Mask=1
3. P. Flowers, K. Theopold, R. Langley & W. R. Robinson, Chemistry 2e (OpenStax, Rice University), §7.5 “Strengths of Ionic and Covalent Bonds”. https://openstax.org/books/chemistry-2e/pages/7-5-strengths-of-ionic-and-covalent-bonds
4. P. Flowers, K. Theopold, R. Langley & W. R. Robinson, Chemistry 2e (OpenStax, Rice University), §12.5 “Collision Theory”. https://openstax.org/books/chemistry-2e/pages/12-5-collision-theory
5. NASA (30 April 2020), “Candle Flame in 1g vs Microgravity”. https://www.nasa.gov/image-article/candle-flame-1g-vs-microgravity/
6. NASA Science, “Why NASA Is Studying Flames in Space”. https://science.nasa.gov/biological-physical/resources/explainers-infographics/why-nasa-is-studying-flames-in-space/
7. NASA, “Studying Combustion and Fire Safety” (ISS research). https://www.nasa.gov/missions/station/iss-research/studying-combustion-and-fire-safety/

Written by Pixelated Physics. Every factual claim is linked to the source we checked; points that are still debated are labelled open.