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Instruction 5-1

The Law of Conservation of Mass | Reactions and Energy | Acids, Bases, and Neutrals


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CCSTD Science Grade 8 5.a., b.

Chemical Equations

If you want to say something in the English language, you would use words in sentences.

If you want to say something about mathematics, you would use numbers in equations.

If you want to say something about chemical reactions, you would use atomic symbols in chemical equations.

For example, we know that hydrogen gas and oxygen gas can react to form water. A chemical equation for the production of water is;

H2 + O2 –› H2O

Instead of an equal sign, chemical equations use arrows. The starting materials, or reactants, are to the left of the arrow, and the ending materials, or products, are to the right of the arrow.  

Conservation of Mass

Matter cannot be created nor destroyed; there are no more atoms at the end of a chemical reaction than there were at the beginning.  This is called the Law of Conservation of Mass.

When you react 2 molecules of hydrogen gas (H2) with 1 molecule of oxygen gas (O2), you will get 2 molecules of water (H2O):

2 H2 + 1 O2 –› 2 H2O

Notice in the equation above that you started with 4 atoms of hydrogen and 2 atoms of oxygen, and you ended with 4 atoms of hydrogen and 2 atoms of oxygen.


The atoms were rearranged in the chemical reaction above, but they were not created nor destroyed.  No atoms were lost and no atoms were added, so the Law of Conservation of Mass was followed.

When heat is added to a solid and it is converted to a liquid, the number of solid molecules is equal to the number of liquid molecules.  An atom or molecule may change its phase of matter with the addition or subtraction of heat, but it did not disappear.

Balancing Chemical Equations

A ‘balanced’ chemical equation means that you have the same number of atoms on both sides of the chemical equation arrow.

When you balance a chemical equation, you are following the Law of Conservation of Mass because you are not creating, nor are you destroying, mass.

There are some simple rules to follow so that you can balance a chemical equation;

  1. make sure the chemical formulas for your reactants and products are correct
  2. choose an element that occurs only once in the reactants and in the products (if possible)
  3. change the coefficients (the numbers in front of the molecules) of the molecules that have the element you selected, so that that the amount of that element is the same on both sides of the arrow
  4. repeat step 2., choosing the next ‘easier’ element to balance, and repeat step 3. until the entire equation is balanced

For example, let’s look at the reaction where hydrogen gas and chlorine gas are converted to hydrochloric acid:

H2 + Cl2 –› HCl

The chemical formulas of all reactants and products are correct, so we’ll move on to step 2) and choose the ‘easiest’ element, which could be hydrogen (H) in the products.

We must put a 2 in front of the HCl of the products to balance the element H. (This balances the element of Cl at the same time!).

H2 + Cl2 –› 2 HCl

Our final chemical equation is indeed balanced because we have satisfied the Law of Conservation of Mass by having the same number of elements (a total of  2 atoms of H and Cl ) on both sides of the arrow.

cemical equation

Video Instruction
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for Students, Parents and Teachers

Now let's do Practice Exercise 5-1 (top).

Next Page: Reactions and Energy (top)