The big idea: atoms are never created or destroyed

In a chemical reaction, atoms rearrange into new groups. Nothing appears and nothing vanishes. That is the Law of Conservation of Matter, and it means a correct equation has the same number of each kind of atom on both sides. Balancing is how we make the equation obey that law.

Not balanced yet

Count the red spheres: 2 oxygen atoms on the left, only 1 on the right. An oxygen atom went missing. Nature does not allow that.

Balanced

Two hydrogen molecules and one oxygen molecule become two water molecules. 4 H and 2 O on each side. Every atom is accounted for.

The one rule: change coefficients, never subscripts

Coefficient = how MANY molecules

The big number in front. 2H2O means two water molecules. Multiply every atom in the formula by it.

2 × (2 H + 1 O) = 4 H and 2 O

Subscript = what the substance IS

The small number after a symbol. Change it and you change the substance. H2O2 is not "water with extra oxygen." It is hydrogen peroxide, the stuff in the brown bottle.

A different molecule entirely. You can never fix an equation by rewriting a formula.

Count with a table

Before you touch a coefficient, count. Make a table with one row per element and two columns: reactants (left of the arrow) and products (right). If every row matches, it is balanced. If a row does not match, that row tells you what to fix.

Aluminum: 1 on the left, 2 on the right. Oxygen: 2 and 3. Both rows are off, so this equation is not balanced yet. Keep the table next to you and recount after every change.

The balancing routine

  1. Count first. Build your table for the equation exactly as written. Blank coefficient means 1.
  2. Start with an element that is not H or O, ideally one that shows up in only one formula on each side. Metals are usually easiest.
  3. Fix one element at a time. Find which side has fewer, find the formula on that side that holds it, and raise that coefficient. If the numbers do not divide evenly, use the smallest number both go into (like 2 and 3 → 6).
  4. Recount everything. Changing one coefficient changes every atom in that formula, so other rows can shift. Fix them too.
  5. Save hydrogen and oxygen for last. They usually show up in several formulas, so they settle once the rest is done.
  6. Polyatomic ions (like SO4, NO3, OH) that stay together on both sides can be counted as one unit. Much less counting.
  7. Check for smallest whole numbers. If every coefficient can be divided by the same number, divide.

Watch it happen

Step through three worked examples. Watch the molecules and the table change together.

The four reaction types

Every equation you will see here fits one of these patterns. Count what is on each side and look for a lone element.

Spot the type: flow chart

Click your way down. Then hit Quiz me to walk the chart with a real equation.

Real-world reactions

Every one of these has happened in a kitchen, a garage, or a classroom lab.

Common-mistake spotter

A student tried to balance the equation. What did they do? Sometimes the answer is "nothing wrong."

How are you feeling about this today?

Be honest. Nothing here is graded, and you can switch levels any time.

Printable worksheets

Each sheet prints the student page first and the answer key on the next page. Shuffle to pull a fresh set from the bank. Levels match the practice levels.

Tip: the coach set is a good exit ticket for students who used Level 1, since it holds the same kind of equations they just practiced. The reaction-types sheet works as a quick warm-up any day.

Standards

Georgia Physical Science SPS3 — Obtain, evaluate, and communicate information to support the Law of Conservation of Matter.

  • SPS3b — Develop and use a model of a chemical equation to illustrate how the total number of atoms is conserved during a chemical reaction. The molecule pictures and the atom table are that model; every check compares atoms on both sides.
  • SPS3a support — the real-world gallery connects each reaction type to something students can see, which pairs well with a conservation-of-mass lab.
  • SPS2c reinforcement — students read formulas and polyatomic ions correctly to count them. Send anyone who struggles with subscripts and parentheses back to Counting Atoms first.

Reaction types covered: synthesis, decomposition, single replacement, double replacement. Combustion is deliberately left out.

How the three levels differ

Level 1 · Coach mode
Two-element equations, coefficients of 1 to 3, no polyatomic ions. A coach panel updates live and always names the next element to look at and why. The hint button gives the exact number. "Show atom counts" is one click away. Built for students who have shut down.
Level 2 · Guided practice
Three-element equations, coefficients up to 4, simple polyatomic ions (OH, NO3, SO4). No coach. Three hint layers: which element, the reasoning, then the number. Feedback names the specific element that is off and by how much.
Level 3 · Challenge
Coefficients up to 6, polyatomic ions in parentheses, four elements, plus an option to count polyatomic ions as units. Hints still exist but the page tracks no-hint solves and streaks. Built for students who finish early.
After five in a row at Level 1 or 2 the page nudges the student up a level. It never forces it.

Running it in class

  • Nothing is collected. No names, no codes, no sign-in. Progress counts live only on the device until the tab closes. It is practice.
  • The atom table matches the paper method. One row per element, reactants and products columns. Students choose "On paper" or "On this page"; the page remembers the choice. Either way, the feedback expects them to recount after every coefficient change.
  • Project the Learn tab first. "Watch it happen" steps through three examples with the molecules and the table moving together, including the smallest-common-multiple trick for Al + O2.
  • Mistake spotter as a warm-up. Ten wrong attempts (and a couple of correct ones) that surface the classic errors: changed subscript, balanced one side only, not lowest ratio, invented a leftover atom.
  • Speed drill for the last five minutes. Balanced equations only, type identification only, sixty seconds. Best score stays on the device.
  • Print tab gives leveled worksheets with keys and a blank atom table beside each equation.
  • Works offline once loaded. No accounts, no ads, nothing to install.

Quick projector examples

Click any equation to load it into Practice at Level 3 with the coach turned on, so you can talk through it on the board.