Write the chemical formulae for the reactants on the left side of the equation with a plus sign (+) between each to indicate their interaction. For example, write the skeleton equation for rusting (iron plus oxygen yields iron oxide) as follows:
Fe + O2
Write the skeleton equation for iron oxide plus carbon monoxide yields iron plus carbon dioxide like this:
Fe2O3 + CO
Write an arrow pointing right, just to the right of the reactants to symbolize "yields." Now the first example should look like this:
Fe + O2 ->
And the second example should look like this:
Fe2O3 + CO ->
Write the formula for the product to the right of the "yields" symbol (->) to complete the skeleton equation. Write a plus sign in between each if there is more than one product. Now the first example should look like this:
Fe + O2 -> Fe2O3
And the second example should look like this:
Fe2O3 + CO -> Fe + CO2
Add the notation for the physical state of each substance (solid, liquid, or gas) to impart more specificity, if desired. This step is not necessary to complete a skeleton equation, but does add clarity regarding the reaction being represented. These notations are solid state (s), liquid state (l) and gaseous state (g).
Write these as subscripts if possible. Now the first example would end up looking like this:
Fe(s) + O2(g) -> Fe2O3(s)
And the second example would look like this:
Fe2O3(s) + CO(g) -> Fe(s) + CO2(g)