Chemical equations are read from left to right, like a sentence. Everything on the left of the arrow is a reactant. Everything on the right is a product.
The arrow is not an equals sign; it is read aloud as yields or gives, and it points in the direction the change runs. A plus sign between two formulas just means and. Burning methane in a gas stove is written like this: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) Methane and oxygen are the reactants.
Carbon dioxide and water are the products. The letters in parentheses are state symbols, and a complete equation includes them: (s) for solid, (l) for pure liquid, (g) for gas, and (aq) for aqueous, meaning dissolved in water. States matter, because the same chemicals often behave very differently as solids than they do in solution.
Coefficients are not subscripts
This mix-up costs students more points than almost anything else. A coefficient is the full-size number in front of a formula, and it counts whole units. A subscript is the small lowered number inside a formula, and it counts atoms within one unit.
In 2H2O, the coefficient 2 means two water molecules, holding 4 hydrogen atoms and 2 oxygen atoms between them. When you balance an equation you may change coefficients as much as you like. You may never change a subscript, because H2O2 is hydrogen peroxide, a different substance with different chemistry.
Why the mass always balances
Reactants do not vanish. Their bonds break and their atoms get reassembled, so every atom present at the start is still present at the end. Check the methane equation: 1 carbon, 4 hydrogens, and 4 oxygens on the left, and 1 carbon, 4 hydrogens, and 4 oxygens on the right.
Masses balance for the same reason. Heating limestone to about 900 degrees C drives off carbon dioxide and leaves lime behind: CaCO3(s) -> CaO(s) + CO2(g). The reactant has a molar mass of 100.09 g/mol.
The products are calcium oxide at 56.08 g/mol and carbon dioxide at 44.01 g/mol, and 56.08 + 44.01 = 100.09. Decompose 100.09 g of limestone and you recover 56.08 g of solid lime plus 44.01 g of gas. Nothing was destroyed; part of the mass simply left as a gas, which is why open containers appear to lose weight.
Limiting and excess reactants
Reactants are hardly ever mixed in the exact ratio an equation calls for. The one that runs out first is the limiting reactant, and it fixes the maximum amount of product you can make. Whatever remains when the reaction stops is in excess.
Try ammonia synthesis, N2 + 3H2 -> 2NH3, starting with 28.0 g of nitrogen and 4.00 g of hydrogen. In moles that is 28.0 / 28.02 = 1.00 mol N2 and 4.00 / 2.02 = 1.98 mol H2. One mole of nitrogen would need 3.00 mol of hydrogen, and only 1.98 mol is on hand, so hydrogen is limiting.
That hydrogen produces 1.98 x 2/3 = 1.32 mol of ammonia, which is 22.5 g. It consumes 1.98 / 3 = 0.660 mol of nitrogen, or 18.5 g, leaving 9.5 g of nitrogen unreacted. Check the books: 22.5 g of ammonia plus 9.5 g of leftover nitrogen is 32.0 g, exactly what went in.
Things in the mixture that are not reactants
A catalyst may be consumed in one step of a reaction and regenerated in a later one, so its amount is unchanged once the dust settles. That makes it a helper rather than a reactant, and it is normally written above the arrow instead of beside the starting materials.
Spectator ions are the other trap. Mixing silver nitrate and sodium chloride solutions gives AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq). Only the silver and chloride ions actually change, joining to form a solid.
Sodium and nitrate ions drift through the beaker untouched, so they are dropped from the net ionic equation, Ag+(aq) + Cl-(aq) -> AgCl(s). In that stripped-down version the reactants are just those two ions.
Conditions, and reactions that run both ways
Anything written above or below the arrow is a condition rather than an ingredient: a temperature, a pressure, a catalyst formula, a solvent, or the symbol Delta , which simply means heat is applied. Reactant is a role, not a permanent label. A double arrow ( ) marks a reversible reaction in which products turn back into starting materials.
Read that reaction backward and the roles swap: in the reverse of N2 + 3H2 2NH3, ammonia is the reactant. At equilibrium both directions run at the same rate, so each substance is being consumed and made at once. The same swap shows up in multi-step mechanisms, where the product of step one is the reactant of step two.