The Balancing Chemical Equations simulation from PhET lets you find out how to tell whether a chemical equation is balanced and what you can change to balance it. You can see the atoms on both sides, test your ideas, and play a game to check what you have learned. It is free, runs in any browser on a phone, tablet or laptop, and it is a great fit for the chemical reactions and equations lessons in Tamil Nadu State Board (Samacheer Kalvi) Science and Chemistry.
What is the Balancing Chemical Equations simulation?
This interactive simulation lets you balance chemical equations by changing the numbers in front of each substance. You can compare the atoms on the left and right sides and test your ideas in a game. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
Middle school to university
Equations, coefficients and conservation of mass
Phone, tablet, Chromebook, PC
English
PhET’s learning goals are to balance a chemical equation, to recognize that the number of atoms of each element is conserved in a reaction, to describe the difference between coefficients and subscripts, and to translate between symbols and pictures of molecules.
Launch the Balancing Chemical Equations simulation online
Tap or click inside the player and start exploring. Use the full-screen button in the player for a bigger view.
Credit: University of Colorado Boulder
How to use the Balancing Chemical Equations simulation, step by step
- Predict first. Before you change a number, say whether the equation looks balanced.
- Count the atoms. Count each kind of atom on the reactant side and on the product side.
- Change the coefficients. Change the numbers in front of the formulas, one at a time, and watch the atom counts change.
- Check the balance. The equation is balanced when every element has the same count on both sides.
- Play the game. Balance new equations in the game to test your skills.
Tip for students: balance the elements that appear in only one substance on each side first, and leave hydrogen and oxygen for last.
The idea behind the simulation: atoms are conserved
Atoms in = Atoms out
In a chemical reaction atoms are not created or destroyed, they are only rearranged. So the number of atoms of each element must be the same on both sides of the equation. This is the law of conservation of mass. To balance an equation you change the coefficients (the big numbers in front of the formulas). You never change the subscripts (the small numbers inside a formula), because that would change the substance.
Worked example: In 2H₂ + O₂ → 2H₂O, the left side has 2 × 2 = 4 hydrogen atoms and 2 oxygen atoms. The right side has 2 × 2 = 4 hydrogen atoms and 2 × 1 = 2 oxygen atoms. Every element matches, so the equation is balanced.
Chemical equation key terms explained
These are the words you will meet while using the Balancing Chemical Equations simulation, with simple meanings.
Chemical equation
A short way to show a reaction using formulas and an arrow.
Reactants
The substances you start with, on the left of the arrow.
Products
The new substances that form, on the right of the arrow.
Coefficient
The big number in front of a formula. It tells how many molecules there are.
Subscript
The small number inside a formula. It tells how many atoms of that element are in one molecule.
Conservation of mass
Matter is not created or destroyed in a chemical reaction.
Atom
The smallest unit of an element.
Molecule
Two or more atoms joined together.
Common mistakes: Changing the subscripts instead of the coefficients, forgetting that a coefficient multiplies every atom in the formula, and balancing one element but not checking the others again.
Try the equation balance checker (practice what you explore)
Choose a reaction and type a coefficient for each substance. The checker counts the atoms on both sides. Then tap a “what if” button.
What happens when you change a number
| Change | Effect |
|---|---|
| Change a coefficient | Every atom in that formula is multiplied |
| Change a subscript | You make a different substance (not allowed when balancing) |
| Same atom counts on both sides | The equation is balanced |
| More atoms on the left than the right | Raise the coefficient on the product side |
| Double every coefficient | The equation stays balanced |
Classroom activities for teachers
The Balancing Chemical Equations simulation fits a single 40-minute period.
- Predict and test (10 min): students say whether an equation is balanced, then check the atom counts.
- Atom count table (10 min): students record the atoms of each element on both sides before and after balancing.
- Coefficient or subscript (10 min): students explain why only coefficients can be changed.
- Exit ticket (10 min): students explain in two sentences why equations must be balanced.
Looking for the matching chapters? Browse the Science and Chemistry books on TN SchoolBooks and download the Samacheer Kalvi PDFs, then use this simulation to make the chemical reactions lessons visual. More free science simulations and textbook PDFs are on the home page.
Quick quiz: test your understanding of balancing equations
Score: 0 / 5
Balancing Chemical Equations simulation: frequently asked questions
Is the Balancing Chemical Equations simulation free?
Yes. PhET simulations are free to run online in a browser. Check PhET’s licensing page for reuse terms.
Does it work on a mobile phone?
Yes. It is built with HTML5 and works on phones, tablets and Chromebooks. Use landscape mode or the player’s full-screen button for a bigger view.
Why must a chemical equation be balanced?
Atoms are not created or destroyed in a reaction. So the number of atoms of each element must be the same on both sides. This is the law of conservation of mass.
What is the difference between a coefficient and a subscript?
A coefficient is the big number in front of a formula and tells how many molecules there are. A subscript is the small number inside a formula and tells how many atoms of an element are in one molecule. You change coefficients to balance, never subscripts.
The Balancing Chemical Equations simulation turns atom counting into a puzzle you can try, test and see. Explore it, take the quiz above, and keep playing until balancing an equation feels easy.