Intuition The ONE core ideaBurning is rearranging atoms without losing any — the fuel's carbons and hydrogens get glued to oxygen from the air. If you can count each kind of atom on both sides of the arrow, you can predict exactly how much oxygen you burn and how much CO 2
Before you can read the parent note Combustion Stoichiometry , you need every symbol it quietly assumes. We build each one from nothing, in an order where each idea rests on the one before it.
Definition How to read the::: reveal lines on this page
Several lines below (and the whole checklist at the end) have the shape Question ::: Answer. In this vault the part before the ::: is a prompt and the part after is the hidden answer — cover the right side, try to answer, then check. It is a built-in flashcard, nothing more.
An atom is the smallest ball of a chemical element that still counts as that element. A carbon atom, a hydrogen atom, an oxygen atom — three different kinds of ball, each with its own colour in our pictures.
Intuition What Figure 1 shows and whyFigure 1 lays the three players side by side: a dark ball labelled C (carbon), a small teal ball H (hydrogen), an orange ball O (oxygen). We use one fixed colour per element for the whole page so that in every later picture you can track a ball by its colour and confirm it never vanishes. Everything in combustion is bookkeeping of these balls — if a ball exists before, it must exist after.
A molecule is a group of atoms stuck together. Water is two hydrogen balls glued to one oxygen ball.
Definition Chemical formula and subscriptsWe write a molecule as letters with small subscript numbers. The subscript is how many of that atom sit inside one molecule.
H 2 O 2 hydrogen + 1 oxygen (no subscript means 1).CO 2 1 carbon + 2 oxygen.O 2 2 oxygen glued together (this is what "oxygen gas" in the air actually is).
Intuition What Figure 2 shows and whyFigure 2 draws three molecules as clusters of balls with their formula card underneath. Look at H 2 O is the subscript. This is why the topic needs formulas: a formula is a recipe card of balls per molecule , and to count atoms you must first know how many of each colour hide inside every molecule.
Common mistake A subscript is stuck to the letter on itsleft only
In CO 2 2 O , not the C . So it is 1 carbon and 2 oxygens — not 2 of each. In Figure 2, notice the dark C ball is alone while two orange O balls flank it.
The big number written in front of a whole molecule is a coefficient . It multiplies everything in that molecule. It is the "how many copies" number.
2 H 2 O 2 × 2 = 4 2 × 1 = 2
Common mistake Coefficient (front) vs subscript (below)
Subscript = balls inside one molecule → you may never change it (that would make a different substance).Coefficient = number of molecules → this is the knob you are allowed to turn when balancing.
Turning a subscript to fix an equation is like fixing a recipe by redefining what "water" means. Not allowed.
Definition Reactants, arrow, productsA chemical equation reads left-to-right:
reactants (before) fuel + O 2 ⟶ products (after) CO 2 + H 2 O + ⟶ Reactants are what you start with (left); products are what you end with (right).
Intuition Why the arrow, not an equals signChemists use an arrow because a reaction is a direction in time — before on the left, after on the right. But for counting atoms it behaves like an equals sign: the same balls must appear on both sides. That equality is the whole game.
Definition A balanced equationAn equation is balanced when, for each element separately , the number of that element's atoms on the left equals the number on the right. Count carbons: left = right. Count hydrogens: left = right. Count oxygens: left = right.
Intuition What Figure 3 shows and whyFigure 3 puts CH 4 + 2 O 2 CO 2 + 2 H 2 O left = 1 C, 4 H, 4 O and right = 1 C, 4 H, 4 O . The two tallies match colour-for-colour — that is what "balanced" looks like . Nothing was created; the same balls were re-glued.
Intuition Why balancing isforced on us
This isn't a preference — it's Law of Conservation of Mass : atoms are neither created nor destroyed in a chemical change. So every ball you had before must still exist after, just re-glued. Balancing is simply obeying that law with a pencil . See Balancing Chemical Equations for the general method; combustion is a friendly special case because it has only three product "sinks".
Combustion = a fuel reacting fast with oxygen, releasing heat and light (fire). The carbons and hydrogens of the fuel grab oxygen.
Definition Oxidation state — a "charge scorecard" for an atomAn oxidation state (or oxidation number) is a bookkeeping number that says how many electrons an atom has effectively lost (+) or gained (−) when it bonds. The rule of thumb: in a bond, the greedier atom (oxygen, very greedy for electrons) is counted as taking the shared electrons, so it scores negative; the atom that gave them up scores positive.
Oxygen almost always scores − 2
A neutral, unbonded element scores 0 O 2
Every molecule's oxidation states must add to the molecule's total charge (0 for neutral molecules).
Intuition Why carbon climbs to+ 4 CO 2 CO 2 − 2 2 × ( − 2 ) = − 4 0 + 4 highest score carbon can reach — it has "given up" all four of its bonding electrons to oxygen. So "fully oxidised carbon" literally means "carbon at its top oxidation score, + 4 0 CO + 2 climbing this scoreboard . Deep dive: Oxidation States .
Definition Complete vs incomplete combustion
Complete = there is enough oxygen, so every carbon reaches + 4 CO 2 H 2 O fully oxidised — grabbed as much oxygen as they can.Incomplete = oxygen ran short, so carbon stops early: CO + 2 C 0
Definition Standard atomic mass and the atomic mass unitAtoms are far too tiny to weigh one at a time, so chemists use a relative scale. They picked one atom as a ruler and measure every other atom against it. The unit on this scale is the atomic mass unit (symbol u ), and the standard atomic mass of an element is roughly how many times heavier its atom is than a hydrogen atom.
Hydrogen ≈ 1 ≈ 12 ≈ 16 These are the "12, 16, 1" numbers used everywhere below — they are not random; they are each atom's weight on the shared ruler.
A mole is a counting word for a huge fixed number of molecules (about 6.022 × 1 0 23
Intuition The magic bridge: u becomes gramsThe mole is chosen so cleverly that the atomic mass in u equals the mass in grams of one mole of those atoms. Carbon is 12 u per atom → one mole of carbon weighs 12 g. That is why we can just add the atomic masses to get grams per mole.
M molar mass M mass in grams of one mole of it, in units g/mol. You get it by adding the standard atomic masses of every atom in one molecule:
M ( O 2 ) = 2 × 16 = 32 M ( CH 4 ) = 12 + 4 × 1 = 16 M ( CO 2 ) = 12 + 2 × 16 = 44
Intuition Why the topic needs molesA balanced equation's coefficients are ratios of molecules (1 methane : 2 oxygen). But you weigh things in grams , not in molecules. The mole and molar mass are the exchange rate that converts "2 molecules of O₂ per methane" into "64 grams of O₂ per 16 grams of methane." Deep dive: Mole Concept and Molar Mass .
Definition The generic fuelC x H y O z letters as stand-ins for numbers you'll fill in later :
x y z a , b , c C x H y O z + a O 2 → b CO 2 + c H 2 O
Intuition Why letters instead of numbersThis is the algebra trick: solve it once with symbols, then plug in x = 1 , y = 4 , z = 0 x = 2 , y = 6 , z = 1 every fuel forever.
Worked example Reading fractions like4 y 2 y y = 18 2 y = 9 2 25 = 12.5 Why is 12.5 Because a coefficient here really means a ratio, not a literal molecule count — "12.5 O₂ per 1 fuel" is the same recipe as "25 O₂ per 2 fuel." Chemistry happens in moles (trillions of molecules), so half a mole is perfectly real even though half a single molecule is not. To display whole numbers we simply multiply the entire equation by 2 , scaling every coefficient together so the ratios stay identical.
Here is how the pieces stack, in words, so you don't need to render the diagram: the atom (a countable ball) is the root. Gluing atoms makes a molecule , written as a formula with subscripts ; putting a coefficient in front counts copies of that molecule. Separately, the atom leads to conservation of atoms . A chemical equation with an arrow plus that conservation rule gives a balanced equation , which underpins complete combustion and hence the general formula for C x H y O z atomic mass → mole and molar mass → grams calculations . The general formula and the grams calculation together feed the parent combustion stoichiometry topic.
Coefficient copies of molecule
Chemical equation with arrow
(Cover the text after each ::: and answer, then check — see the reveal-line note at the top.)
How many hydrogen atoms are in one molecule of H 2 O 2 (the subscript on H)
How many oxygen atoms in one CO 2 2
What does the big number in front of a molecule (a coefficient) do? Multiplies the count of that whole molecule
Which number may you NEVER change to balance an equation — subscript or coefficient? The subscript (it defines the substance)
What does "balanced" mean? Each element has equal atom count on left and right
Which law forces equations to balance? Complete combustion turns all carbon into ___ and all hydrogen into ___ CO 2 H 2 O
What does the atomic mass unit (u) measure? How heavy an atom is relative to hydrogen (H≈1, C≈12, O≈16)
Why does adding atomic masses give grams per mole? The mole is chosen so mass in u per atom = grams per mole
What is a mole? A counting word for ~6.022 × 1 0 23
What is the molar mass of O 2 32 g/mol (2 × 16
Formula linking mass, moles and molar mass? moles = mass / M
Why does carbon reach oxidation state + 4 CO 2 Two O each score −2 (total −4); molecule is neutral, so C must be +4
Solve for b, c, a in C x H y O z b = x c = y /2 a = x + y /4 − z /2
Why is a coefficient like 12.5 acceptable on paper? It's a ratio (moles), and you multiply the whole equation by 2 for whole numbers
In C x H y O z x , y , z Number of carbon, hydrogen, oxygen atoms in the fuel
