When substances react, we do more than notice clues such as heat, color, precipitates, or gas. We can also read the reaction type from the pattern of reactants and products.
This lesson uses four common examples: combustion, rusting, precipitation, and gas formation. OpenStax Chemistry 2e also groups reactions through examples such as precipitation, acid-base reactions, and oxidation-reduction in Classifying Chemical Reactions.
Combustion happens when a substance reacts with oxygen and usually releases energy as heat or light. For burning phosphorus, a common school-level equation is:
P4+5O2→2P2O5
Some chemistry sources write the phosphorus oxide as P4O10. That does not contradict the equation above because P2O5 is its empirical formula. For coefficient practice here, 2P2O5 keeps the numbers of P and O atoms balanced.
Rusting is an example of oxidation in metals. Iron reacts with oxygen and water, then forms rust, often written as hydrated iron(III) oxide.
4Fe+3O2+2xH2O→2Fe2O3⋅xH2O
The letter x means the amount of water in rust is not always one fixed number. Because the product has coefficient 2, the water on the left is written as 2xH2O so the equation stays balanced. The key pattern here is that iron changes into a new reddish-brown substance.
Precipitation happens when a newly formed substance does not dissolve well in the mixture. For example, limewater can react with dissolved baking soda and form calcium carbonate as a white precipitate.
Ca(OH)2+NaHCO3→CaCO3+NaOH+H2O
The product CaCO3 matters because this compound appears as the solid. A precipitate is a newly formed product with low solubility, not just material that falls to the bottom.
A coefficient is the number placed before a chemical formula. We may change coefficients so the number of atoms before and after the reaction matches.
P4+5O2→2P2O5
In this equation, 5 before O2 and 2 before P2O5 are coefficients. The formula O2 must not be changed into O5, because that would change the substance.
Names such as combustion, precipitation, or gas formation help us predict the reaction pattern. The final check still comes from the balanced chemical equation.
Use this quick check.
Reaction type
Nearby clue
What to check
Combustion
O2 appears and energy is released.
Atoms on the left and right are balanced.
Rusting
A metal changes color into rust.
The new product contains metal and oxygen.
Precipitation
A new solid appears from solution.
The solid product is written as a new compound.
Gas formation
Bubbles or gas leave the mixture.
A gas product such as CO2 appears.
Once the reaction type is readable, the next move is to write the reaction neatly. Put reactants on the left, products on the right, then adjust coefficients until the atoms balance.
