Balancing Chemical Equations (11 Favorites)

SIMULATION in Balancing Equations, Conservation of Mass. Last updated November 14, 2018.


In this activity, students will learn how to count atoms and how to balance chemical equations using a simulation and games from PhET Interactive Simulations.

Grade Level

Middle and High School


By the end of this activity, students should be able to

  • Balance a chemical equation.
  • Understand that the number of atoms of each element is conserved in a chemical reaction.
  • Recognize and explain the difference between coefficients and subscripts in a chemical equation.
  • Translate between the written description of a chemical reaction and a symbolic representation.

Chemistry Topics

This activity supports students’ understanding of

  • Chemical reactions
  • Balancing Equations
  • Reactants
  • Products
  • Conservation of Matter
  • Chemical formula
  • Coefficients
  • Subscripts


Teacher Preparation: 10 minutes

Lesson: 45 minutes



  • No specific safety precautions need to be observed for this activity.

Teacher Notes

  • It is suggested that students work individually, and at their own pace through this activity. Instructions for working through the simulation are included on the student activity sheet.
  • The accompanying activity also includes a “PhET Tips for Teachers” download with suggestion for using the simulation.
  • Background information and vocabulary:
    Example: Silver nitrate and magnesium chloride solutions react to form solid silver chloride and magnesium nitrate solution.

2AgNO3 (aq) + MgCl2 (aq) → 2AgCl (s) + Mg(NO3)2 (aq)

    • A reactant is a chemical substance that is present at the start of the reaction. In the reaction above, silver nitrate and magnesium chloride are reactants. There can be one or more reactants involved in a chemical reaction.
    • A product is a new substance that is formed during a chemical reaction. The example shows silver chloride and magnesium nitrate as products.
    • A chemical reaction is a process that involves the rearrangement of the atoms in reactant molecule(s) to form new molecules, which are referred to as the product(s). As you can see in the reaction, the reaction are rearranged to form the products.
    • When a chemical reaction occurs you will observe one or more of the following: energy is produced or used, a color change, the formation of a precipitate, or the formation of a gas. In this example, the formation of a precipitate serves as evidence that a reaction occurred.
    • A chemical formula is a symbolic representation that uses element symbols and numbers to show the composition of a chemical substance.
    • A chemical equation is a symbolic representation of what occurs when reactants and converted to products during a chemical reaction. As shown above, the reactants and products of a reaction can be converted into chemical formulas and a symbolic equation.
    • Subscripts are used to show the number of each type of atoms in a chemical substance. They are written as a small number after the element symbol. The equation above shows that there is one magnesium atom and two chlorine atoms present in magnesium chloride (MgCl2).
    • In some cases, the subscript is outside of a group of atoms that are surrounded by parenthesis. The reaction above includes magnesium nitrate (Mg(NO3)2) as a product. The parenthesis indicate that the nitrate is a group of atoms bonded together – one nitrogen and three oxygen. The subscript after the parenthesis tells us that there are two nitrates involved in the reaction.
    • A coefficient is a number written before a chemical formula to show how many molecules (or moles) are present. The coefficient of 2 in front of the silver nitrate and silver chloride indicates that there are two of each involved in the reaction.
    • After each formula there is a symbol that indicates the state of the substance: solid (s), liquid (l), gas (g), aqueous solution (aq).
    • The Law of Conservation of Mass states that the mass of the reactants must equal the mass of the products. Additionally, the number of each type of atom on the reactant side of the equation will equal the number of each type of atom on the product side of the equation.
  • Rules for balancing chemical equations:
  1. Mentally, draw a box or circle around chemical formulas – you cannot change any symbol or subscript in the formula to balance the equation.
  2. Example 1: You cannot change a subscript. H2O is different than H2O2!!
    Example 2: You cannot insert coefficients in middle of formula. H22O is not correct.

  1. Count the number of each type of atom on both sides of the equation. You might want to make a simple table to keep track as you learn how to balance equation.
  2. Add coefficients to the front of the boxes to balance the equation and update your element count. Coefficients must be whole numbers.
  3. These tips will help you balance equations
    Remember to write the seven diatomic elements (H2, N2, O2, F2, Cl2, Br2, I2) with the subscript2. Once they react, they will exist as individual atoms in a molecule.
  • If the same polyatomic ion appears both side of the reaction, put a mental box around it and treat it as a single unit
  • In some types of ionic reactions it will help to write water as H–OH instead of H2O.
  • Balance the elements in compounds first. Start with metals and then balance nonmetals.
  • Leave the reactants and products that are elements until the end.
  1. When the number of atoms of each element is the same before and after the reaction, equation is balanced.

For the Student

Download all documents for this activity, including the teacher guide, from the "Downloads box" at the top of the page.