Write the complete balanced equation for the reaction between alf3 and cao.

This will be a double replacement reaction. 

First let's figure out what the products would be before we balance it.

AlF₃ + CaO --> Al₂O₃ + CaF₂

Al has a charge of 3+
O has a charge of 2-
so when finding what the product would be, you give one element the subscript of the other charge, and that is how we come up with Al₂O₃. This is what you do for ionic compounds. Remember, an ionic compound is made up of a metal and a nonmetal.

You find CaF₂ similarly. Ca has a charge of 2+ and F has a charge of 1-. 

Now let's balance our reaction.
AlF₃ + CaO --> Al₂O₃ + CaF₂
There is one calcium on the left side, and one calcium on the right side
There is one aluminum on the left side but 2 aluminum on the right side
There are 3 fluorine on the left side and 2 on the left side
There is one oxygen on the left side and 3 on the right side

This is obviously not balanced.
We can start off by adding a coefficient of 2 in front of AlF₃ so that way the aluminum will be balanced.
We will now have
2AlF₃ + CaO --> Al₂O₃ + CaF₂

The aluminum atoms are balanced, and this has now increased the fluorine atoms on the left side to 6 and we have 2 fluorine atoms on the right. We can add a coefficient of 3 on CaF₂ so that way fluorine will be balanced.

2AlF₃ + CaO --> Al₂O₃ + 3CaF₂
We now have 1 calcium atom on the left, and 3 calcium atoms on the right. If we add a coefficient of 3 to CaO, we will balance the calcium.

2AlF₃ + 3CaO --> Al₂O₃ + 3CaF₂
You will see now that all the atoms are balanced. Remember when balancing an equation, you can only change the  coeffiecients, you may not change the subscripts.

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