Reactants, products and leftovers

The title of the Project: Reactants, products and leftovers

This virtual laboratory is intended for use in chemistry classes on the following topics:

  • 8th grade. Chapter II. ““Formulas of substances and equations of chemical reactions””


  • Understand the difference between reactants and products in a chemical reaction.
  • Identify leftover materials (unreacted reactants) after a reaction.
  • Gain a conceptual understanding of the Law of Conservation of Mass.

Practical part

  1. Launch the simulation. You will be given three different screens: 1)Sandwiches; 2)Molecules; 3) Game screen. Choose the first screen.

The “Sandwiches” screen

This screen uses a simplified approach to introduce the concepts. Instead of chemical formulas, it uses familiar objects like bread and cheese to represent reactants and sandwiches to represent the products. By manipulating the quantities of bread and cheese, you can observe how a “sandwich” (product) is formed and how leftover ingredients (unreacted reactants) might remain. This helps grasp the basic idea of reactant ratios needed for a reaction.

2. On the left side of the screen the starting materials for reaction are displayed. You can adjust the quantities of each reactant.

    3. The right side shows the products and leftover reactants after the reaction is complete. The number of product molecules is automatically calculated based on the chosen reactants.

    4. The reactants and products are separated by an arrow in a chemical reaction.

    5. For example, at the top of the screen you can see a sandwich recipe or equation. According to this equation, you need 2 slices of bread and 1 slice of cheese to make one sandwich.

    6. Let’s add one more piece of cheese. You will see that it is listed as a leftover, because there should be two more bread slices to make a second sandwich. 

    7. You can also select a “Meat and cheese” section to make your “sandwich” more complicated.

    8. In a “Custom” section you can experiment with different ingredients and make your own equation. You can choose from a variety of bread, cheese, and meat options to create unique sandwich combinations. Note that no matter how many ingredients you  add, the reaction won’t go unless you first define a recipe with at least two different ingredients.

    The “Molecules” screen

    Now, switch to the next screen. While the overall layout of the screen stays the same in the “Molecules” section, you now have a menu at the top where you can pick one of three common chemical reactions to explore. The default scenario shows the production of water.

    9. On the left, you’ll see hydrogen (H₂) and oxygen (O₂) molecules as reactants. On the right, you’ll see water molecules (H₂O) formed as a product.

    10. The coefficients in front of the chemical formulas indicate the number of molecules needed. (e.g., 2 H₂ for every 1 O₂). On the “sandwiches” screen, each ingredient was like a single piece. In real chemical reactions, reactants are usually composed of multiple atoms. Based on this, explain why using just one molecule of hydrogen (H₂) and one molecule of oxygen (O₂) doesn’t produce water and is shown as leftovers.

    The “Game” screen

    11. Move to the “Game” tab. This tab allows you to practice balancing chemical reactions and predicting products. There are three levels with increasing difficulty. You can also challenge yourself by adding a time limit and hide the molecules or numbers by changing the radio button.

    12. Level 1: Balancing the Reaction. In this level, the balanced chemical equation is provided at the top.You can adjust the number of reactant molecules using the sliders.

    13. Your goal is to manipulate the reactants to achieve a balanced reaction, where the number of atoms of each element is equal on both sides of the equation (represented by the circles above the beakers). Once you think you have achieved a balanced reaction, click “Check” to see if it’s correct.

    14. Level 2: Predicting Products: This level provides a set amount of reactants and asks you to predict the number of product molecules formed based on the balanced chemical equation displayed at the top. Use your understanding of the reactant ratio to predict the number of products. Click “Check” to see the actual number of products generated by the simulation.

    15. Level 3: Leftover Reactants: This level presents a scenario with a set amount of reactants. The challenge is to predict which reactant will be completely used up (limiting reactant) and which may have leftover molecules after the reaction. Analyze the balanced equation and the initial quantities of reactants to make your prediction. Click “Check” to observe the reaction and see if there are any leftover reactants.


    The PhET simulation “Reactants, Products, and Leftovers” provides a valuable tool for exploring the fundamentals of chemical reactions. Through the interactive screens “Sandwiches” and “Molecules”, students gained a simplified understanding of reactant ratios and how they influence product formation. They also applied their knowledge of balancing chemical equations through the “Game” screen.