You may remember that the law of conservation of mass says that matter is neither created nor destroyed during a chemical reaction. This means that all chemical reactions must be balanced—the number of atoms, moles, and ultimately the total mass must be conserved during a chemical process. Here are the rules to follow when balancing equations:

When balancing reactions, keep your hands off the subscripts! Use only coefficients to balance chemical equations. Now let’s try an example. When you solve it yourself, make sure to follow the steps!

Write the balanced equation for the reaction between chlorine and sodium bromide, which produces bromine and sodium chloride.

First write the chemical formulas—be on the lookout for the diatomic elements (such as Cl₂):

Next, find the reagent with the scariest subscripts. In this case, start with Cl₂. You need a coefficient of 2 in front of NaCl, which then requires a coefficient of 2 in front of NaBr. The balanced equation becomes

Finally, count up everything to make sure you balanced the equation correctly. You have two chlorine atoms, two sodium atoms, and two bromines on the reactants side and two bromines, two sodiums, and two chlorines on the products side. You’re done.

Write the balanced equation for the reaction between aluminum sulfate and calcium chloride, which produces aluminum chloride and calcium sulfate.

Write the chemical formulas on their correct sides:

In this reaction, the aluminum sulfate looks the most complicated, so start there. Look at what happens with sulfate—since it remains sulfate on the right side of the reaction, treat it as a unit. You have three on the left side and only one on the right side, so place a coefficient of 3 in front of calcium sulfate. Now deal with the aluminum. You have three on the left and one on the right, so place a coefficient of 2 in front of aluminum chloride. Last, you must place a coefficient of 3 in front of calcium chloride.

Count the atoms on both sides of the reaction and you’ll see that you’re done.