Balancing equations.

Step 1 of 5

Coefficients

A chemical equation writes the reactants on the left, an arrow, and the products on the right: hydrogen plus oxygen gives water. But $\mathrm{H_2 + O_2 \rightarrow H_2O}$⁠ is wrong: two oxygen atoms on the left, one on the right. An equation like that claims an oxygen atom has vanished, and conservation of mass says that does not happen.

To balance the equation you write coefficients: numbers in front of the formulas that say how many molecules take part. The formulas themselves stay fixed. Move Run the reaction and watch the readings: how many hydrogen molecules disappear for each oxygen molecule?

Step 2 of 5

Two from one

A coefficient can appear on the products’ side too. Hydrogen and chlorine react to hydrogen chloride, $\mathrm{HCl}$⁠, a gas that dissolves in water to make hydrochloric acid. Each $\mathrm{HCl}$⁠ molecule holds just one hydrogen atom, and each $\mathrm{H_2}$⁠ molecule brings two.

Here Compound, Oxygen or chlorine supplied and Run the reaction are fixed, and the reaction has not started. Count the atoms.

Step 3 of 5

Just enough chlorine

A balanced equation is also a recipe: it says in what ratio the reactants combine. Sodium, a soft metal, and chlorine molecules react by $\mathrm{2Na + Cl_2 \rightarrow 2NaCl}$⁠: each chlorine molecule gives two atoms, one for each sodium atom.

Now Compound is fixed at Table salt. Change Oxygen or chlorine supplied, and look for the amount at which the sodium and the chlorine run out together.

Step 4 of 5

Formulas set by charges

You may not balance an equation by changing a formula, because the formula is set by the substance. In salts it is set by the charges of the ions: sodium gives one electron and becomes $\mathrm{Na^+}$⁠, magnesium gives two and becomes $\mathrm{Mg^{2+}}$⁠, and chlorine takes one and becomes $\mathrm{Cl^-}$⁠. The salt is neutral, so the formulas are $\mathrm{NaCl}$⁠ and $\mathrm{MgCl_2}$⁠.

Step 5 of 5

Balancing by counting

You can balance almost any equation one way: start from an element that appears in one substance on each side, balance it with a coefficient, move to the next element, and finish by counting each element on each side. A coefficient multiplies the whole formula after it: $\mathrm{2H_2O}$⁠ holds four hydrogen atoms and two oxygen atoms.

The next question has no Scene: the burning of methane, the gas in a cooker.