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In this lesson, you’ll learn how to balance a chemical reaction equation using the conservation of matter law. You’ll also learn how to write both word and formula equations, what the subscripts after a letter mean and what the numbers in front of compounds mean.

Conservation of Matter

Conservation of matter is a basic law of physics that was discovered in the 19th century. Essentially what it means is that matter is never created or destroyed. It can change into energy, but it never disappears.

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Although the law is considered to be a law of physics, it also applies to chemistry. In a chemical equation, the law of conservation of matter must be met. This means that whatever amount of reactants you started with, this is the amount of product you end up with.

An example of a formula equation before it is balanced
4 + O2 = CO2 + H2 O. There is something wrong with this equation, though. This equation is not a balanced equation. To be correctly written, it must have the same amount of elements on both sides of the equation, and if you look, it doesn’t. It has four hydrogens on the left, but only two hydrogens on the right.

That means matter was destroyed somewhere, and we know that can’t happen.When writing your formula equations, remember that a subscript refers to the atom directly before it. So, in CH4, the 4 means four hydrogen atoms. Also, remember if there is a number in front of a compound, that number applies to all the atoms in that compound. The number in front of a compound or element is called the coefficient. When there isn’t a number there, it is assumed to be the number 1.

So, if I put a 4 in front of CH4, so it looked like 4CH4, that would mean I have four carbons (four times one) and 16 hydrogens (four times four).

An explanation of the coefficient and subscript values
Compound Element Number Explanation

How to Balance an Equation

To write a balanced equation, here are the steps:Step 1: If you are given a word equation such as water = hydrogen + oxygen, turn it into a formula equation. H2 O = H2 + O2.Step 2: Balance the different types of atoms one at a time.Step 2a: First balance the atoms that are combined and appear only once on each side of the equation.Step 2b: Balance other compounds that appear on both sides of the equation.

Step 2c: Balance the hydrogen (H) atoms and oxygen (O) atoms last.Step 3: Count the atoms on each side of the equation to make sure it is balanced.So, here is the way it would work: You are given water = hydrogen + oxygen.

Step 1: You rewrite it as H2 O = H2 + O2.Step 2 a: Balance the atoms that are combined and only appear once. Hmmm. This doesn’t apply too much in this example.

Step one is changing the word equation into its equivalent compounds.
2 O. This gives us 2H2 O = H2 + O2.Step 3: Recount the atoms. Are they all balanced? Now we have two oxygens on the left and two on the right.

But recount your hydrogens. Now you have four hydrogens on the left and only two on the right. So, what do you do? Go back to Step 2.Step 2 (again): You add a 2 in front of the H2 on the right. Now the equation looks like 2H2 O =2 H2 + O2.Step 3: Recount the atoms. Are they all balanced? Four hydrogen on the left and four on the right.

Two oxygen on the left and two oxygen on the right. Yep, they are balanced, and we are done!

Practice Equations

Let’s try a harder one: Cu + AgNO3 = Cu(NO3) 2 + AgStep 1: Already done for you.Step 2a: You can see that there is only one NO3 on the left side of the equation but two on the right, so place a two in front of the AgNO3 on the left. It now looks like Cu + 2AgNO3 = Cu(NO3) 2 + Ag.Step 2b: But, now you have two Ag (silver) atoms on the left and only one on the right, so put a two in front of the Ag on the right. It now looks like Cu + 2AgNO3 = Cu(NO3) 2 + 2Ag.

Step 2c: No oxygen or hydrogen standing alone, so this step is done.Step 3: So, let’s count things up. Left: 1Cu, 2Ag, 2NO3 . Right: 1Cu, 2Ag, 2NO3 .

Woohoo! The equation is balanced.

Lesson Summary

So, to review, all matter is conserved, including the matter in a chemical reaction. In order to show that matter is conserved, you must write a balanced chemical equation. Balancing equations is easy.

You just follow the steps and make sure that the atoms on the left side of the equation equal the same number on the right.Step 1: Rewrite the word equation into a formula equation.Step 2: Balance the different types of atoms one at a time.Step 3: Check your work.

Learning Outcomes

At the conclusion of this lesson, you will be able to:

  • Explain the law of conservation of matter
  • List the steps involved in balancing chemical equations
  • Use those steps to balance chemical equations

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