A chemical reaction is not a magical creation; it is an atomic remodeling project. Bonds break, elements separate, and new structures assemble—but the starting building blocks never vanish. The Law of Conservation of Mass dictates that every single atom present before a reaction must remain afterward. To model this, we balance chemical equations with molecular coefficients. In this lesson, you will learn to categorize chemical changes into five distinct patterns and balance molecular equations visually.
the atoms are rearranged, not created or destroyed · ledger counts stay identical
Rather than memorizing millions of unique chemical reactions, chemists categorize reactions into a few core blueprints. By looking at how the reactants are combined, you can predict what kind of products will form.
A + B → AB
Example: 2 Mg + O₂ → 2 MgO
Signature: Two things walk in, one thing walks out. Separate elements or simple compounds combine to make a single final compound.
AB → A + B
Example: 2 H₂O₂ → 2 H₂O + O₂
Signature: One reactant, multiple products — usually needs heat, electricity, or a spark to kick it apart.
A + BC → AC + B
Example: Zn + CuSO₄ → ZnSO₄ + Cu
Signature: A lone element on each side of the arrow — one element shoves another out of its compound to take its place.
AB + CD → AD + CB
Example: AgNO₃ + NaCl → AgCl + NaNO₃
Signature: Two compounds swap partners — positive ions change negative partners. Often watch for a solid precipitate dropping out of solution.
fuel + O₂ → CO₂ + H₂O
Example: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O
Signature: O₂ on the left, CO₂ and H₂O on the right. If those three appear and heat is released, it is combustion, full stop.
acid + base → salt + water
Example: HCl + NaOH → NaCl + H₂O
Signature: An H-first compound (acid) meets an -OH compound (base); water (H₂O) always forms along with an ionic salt.
Naming this pattern "neutralization" is beyond this course's assessment boundary. You won't be asked to classify a reaction as neutralization specifically — but you should always recognize it as a double replacement underneath.
To conserve atoms, we might be tempted to change the subscripts inside formulas. Never do this. Subscripts define the chemical identity of a substance. If you change the subscript of water (H2O) to H2O2 to get more oxygen, you are no longer modeling water—you are modeling hydrogen peroxide, a corrosive bleach. You cannot change the recipe ingredients to balance the scale.
Instead, we adjust the coefficients (the numbers placed in front of formulas). A coefficient scales the entire molecule. Placing a "2" in front of H2O (2 H2O) means you have two distinct water molecules, doubling the counts of both hydrogen and oxygen atoms without changing what the substance is.
Think of each compound as being inside a sealed plastic box. You can buy more boxes (coefficients), but you cannot open the box to alter the atoms inside (subscripts).
Select a chemical equation. Use the plus and minus buttons to change the coefficients. Watch the molecule drawings populate the balance beam. When the atom counts match on both sides, the beam will level out.
Fill in the blanks to lock in the core terms. Matches are case-insensitive.
Automobile airbags rely on precise stoichiometry. In a crash, a sensor triggers the rapid decomposition reaction of solid sodium azide (NaN₃). Balancing the equation shows how a small, dense pellet of reactant rapidly yields a massive volume of nitrogen gas (N₂) in milliseconds, cushioning the impact.