The Reactants, Products and Leftovers simulation from PhET starts with a sandwich and ends with chemistry. You build sandwiches from different amounts of ingredients, then do the same with chemical reactions to find how many products you can make and what is left over. A game tests 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 limiting reactant lessons in Tamil Nadu State Board (Samacheer Kalvi) Chemistry.
What is the Reactants, Products and Leftovers simulation?
This interactive simulation uses everyday sandwich making to explain what a limiting reactant is, and then moves on to real chemical reactions. You can see how many products form from the amounts of reactants you choose and which reactant is left over. Because it runs on HTML5, there is nothing to install, and it works on phones, tablets, Chromebooks and computers.
Middle school to university
Limiting reactants, products and leftovers
Phone, tablet, Chromebook, PC
English
PhET’s learning goals are to recognize that atoms are conserved in a reaction, to use everyday experiences like making sandwiches to explain a limiting reactant, to identify the limiting reactant, to predict products and leftovers from the quantities and the balanced equation, to work backward from products and leftovers to the starting amounts, and to translate between formulas and molecule pictures.
Launch the Reactants, Products and Leftovers 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 Reactants, Products and Leftovers simulation, step by step
- Start with the sandwich. Build a sandwich and find how many you can make from the ingredients you have.
- Predict first. Before you change the amounts, say how many products you expect and what will be left over.
- Move to a reaction. Choose different amounts of the reactants and see how many products form.
- Find the limiting reactant. Find the reactant that runs out first, and note what is left over.
- Play the game. Test your skills across the levels, including working backward from products and leftovers.
Tip for students: always divide each reactant amount by its coefficient. The smaller answer points to the limiting reactant.
The idea behind the simulation: the limiting reactant
Number of reactions = smallest of (amount ÷ coefficient)
In a balanced equation the coefficients give the recipe. If you have more of one reactant than the recipe needs, the extra is left over. The reactant that runs out first is the limiting reactant, and it decides how much product you can make. The reactant that is left over is the excess reactant.
Worked example: In 2H₂ + O₂ → 2H₂O, suppose you have 5 H₂ molecules and 3 O₂ molecules. H₂: 5 ÷ 2 = 2.5. O₂: 3 ÷ 1 = 3. The smaller number is 2.5, so H₂ is limiting and the reaction can run 2 full times. That makes 2 × 2 = 4 H₂O molecules. Left over: 5 − 4 = 1 H₂ and 3 − 2 = 1 O₂.
Limiting reactant key terms explained
These are the words you will meet while using the Reactants, Products and Leftovers simulation, with simple meanings.
Reactants
The substances you start with in a reaction.
Products
The new substances that form.
Leftovers
The reactant that remains after the reaction has finished.
Limiting reactant
The reactant that runs out first and limits how much product forms.
Excess reactant
The reactant that is not used up completely.
Coefficient
The number in front of a formula that tells how many molecules the recipe needs.
Balanced equation
An equation with the same number of each atom on both sides.
Conservation of mass
Atoms are not made or destroyed in a chemical reaction.
Common mistakes: Picking the reactant with the smaller number of molecules as limiting without dividing by the coefficient, forgetting to count the leftovers, and ignoring the balanced equation.
Try the limiting reactant calculator (practice what you explore)
Choose a reaction and the number of molecules of each reactant. The calculator finds the limiting reactant, the products and the leftovers. Then tap a “what if” button.
What happens when you change a variable
| Change | Effect |
|---|---|
| Add more of the limiting reactant | More product forms |
| Add more of the excess reactant | No more product, just more leftovers |
| Amounts exactly match the recipe | No leftovers |
| Double both reactants | The product doubles |
| Zero of one reactant | No reaction and no product |
Classroom activities for teachers
The Reactants, Products and Leftovers simulation fits a single 40-minute period.
- Sandwich first (10 min): students work out how many sandwiches they can make from given ingredients and name the limiting ingredient.
- Predict and test (10 min): students predict the products and leftovers of a reaction, then check.
- Work backward (10 min): students are given products and leftovers and find the starting amounts.
- Exit ticket (10 min): students explain in two sentences what a limiting reactant is, using the sandwich idea.
Looking for the matching chapters? Browse the 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 limiting reactants
Score: 0 / 5
Reactants, Products and Leftovers simulation: frequently asked questions
Is the Reactants, Products and Leftovers 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.
What is a limiting reactant?
The limiting reactant is the reactant that runs out first. It decides how much product can form. Any reactant that is not used up is called the excess reactant.
How do you find the limiting reactant?
Divide the amount of each reactant by its coefficient in the balanced equation. The reactant with the smallest result is the limiting reactant.
The Reactants, Products and Leftovers simulation turns a tricky idea into something as simple as making sandwiches. Explore it, take the quiz above, and keep playing until finding the limiting reactant feels easy.