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.

Ireally don’t know this