Keep the atoms, balance the reaction.
A chemical reaction does not create atoms. It remodels them. Bonds break and new bonds form, but the building blocks never vanish. The Law of Conservation of Mass says every atom present before a reaction is still there afterward. You show this by balancing the equation with coefficients. By the end, you can sort reactions into five patterns and balance an equation.
atoms are rearranged, never created or destroyed · the ledger counts stay identical
Core Claims
- Reaction types: Reactions fall into five patterns: synthesis, decomposition, single replacement, double replacement and combustion. An acid-base reaction is a double replacement.
- Law of conservation: Atoms are never created or destroyed. Every atom in the reactants appears in the products.
- Balancing rules: Change coefficients to level the atom count. Never change a subscript, because it defines the substance.
The five reaction blueprints
Retrieval Checklist
- Sort a reaction into one of the five patterns.
- Balance an equation by changing coefficients.
- Check that the atoms match on both sides of the arrow.
Five blueprints of rearrangement.
Chemists do not memorize millions of reactions. They sort reactions into a few blueprints. Look at how the reactants combine and you can predict what kind of products form.
Synthesis
A + B → AB
Example: 2 Mg + O₂ → 2 MgO
Signature: Two things go in and one thing comes out. Elements or simple compounds combine into a single compound.
Decomposition
AB → A + B
Example: 2 H₂O₂ → 2 H₂O + O₂
Signature: One reactant, several products. It usually needs heat, electricity or a spark to break it apart.
Single Replacement
A + BC → AC + B
Example: Zn + CuSO₄ → ZnSO₄ + Cu
Signature: A lone element sits on each side of the arrow. One element pushes another out of its compound and takes its place.
Double Replacement
AB + CD → AD + CB
Example: AgNO₃ + NaCl → AgCl + NaNO₃
Signature: Two compounds swap partners. Each positive ion pairs with the other negative ion. A solid precipitate often drops out of solution.
Combustion
fuel + O₂ → CO₂ + H₂O
Example: CH₄ + 2 O₂ → CO₂ + 2 H₂O
Signature: O₂ is on the left. CO₂ and H₂O are on the right. If those three appear and heat is released, the reaction is combustion.
Acid-Base Neutralization
acid + base → salt + water
Example: HCl + NaOH → NaCl + H₂O
Signature: An acid (written with H first) meets a base (written with -OH). Water and an ionic salt form.
The name "neutralization" is beyond this course's assessment boundary. You will not be asked to classify a reaction as neutralization. Recognize it as a double replacement.
Hunting for a lone element in every reaction. If both reactants are compounds, the reaction cannot be single replacement. The two compounds swap partners, so it is double replacement.
Practice classifying reactions
Adjust coefficients, lock the subscripts.
To keep the atoms equal, you might want to change a subscript inside a formula. Never do this. Subscripts define the chemical identity of a substance. Change H2O to H2O2 and you model hydrogen peroxide, a bleach, instead of water. You cannot change the ingredients to balance the scale.
Instead, change the coefficients, the numbers in front of formulas. A coefficient multiplies the whole molecule. A 2 in front of H2O (2 H2O) means two separate water molecules. That doubles the hydrogen and oxygen atoms and keeps the identity.
Walkthrough: balancing methane combustion
Think of each compound as a sealed plastic box. You can add more boxes (coefficients). You cannot open a box and change the atoms inside (subscripts).
Changing a subscript to balance an equation. H2O to H4O matches the hydrogens, but it names a substance that is not water.
Balance the beam. Conserve the atoms.
Pick a reaction. Move the coefficient sliders and watch the molecules fill the pans. When the atom counts match on both sides, the beam levels.
Which coefficients make the atoms match on both sides?
Oxygen: 2 on the left and 1 on the right. Change a coefficient, never a subscript.
- Hydrogen atom
- Oxygen atom
- Nitrogen atom
- Carbon atom
Predict the coefficients.
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
A reaction where a fuel reacts with oxygen to make CO2 and H2O is . A reaction where one reactant breaks into several products is . Equations must be balanced because of the Law of . The numbers in front of formulas that balance an equation are . The subscripts inside a formula fix the substance's .
Why lock the subscripts?
Balance a thermal decomposition.
Give yourself 1 mark for each idea you wrote down.
An airbag depends on a balanced equation. In a crash, a sensor triggers the decomposition of solid sodium azide (NaN₃). Balancing the equation shows how a small pellet makes a large volume of nitrogen gas (N₂) in milliseconds.
Adaptive Competency Practice: HS-PS1-2
Master reaction classification, balancing coefficients, and conservation of matter one question at a time. The adaptive engine detects individual misconceptions and provides immediate step-by-step coaching.