Teaching balancing equations with a lecture is tough because no matter how well you explain it abstractly, only the students who have entered Piaget's formal/abstract reasoning stage can understand the explanation. Those students still in Piaget's concrete operational stage just can't understand what you're saying. And as more advanced science gets pushed down to younger and younger ages, that doesn't help at all.
So, basically instead of teaching it as an abstract lecture, what you need is to have an activity with concrete manipulatives so those still at the concrete operational state can understand it. When I lectured, about 50% of my grade 9 students understood it after 3 40 minute blocks of instruction. Then I tried worksheets with cutout of the molecules and comprehension rose to about 60% over the same time period - the directions about coefficients and subscripts was too complicated to follow. So I wrote my Chembalancer game to simplify the directions. In the game, they can only change the coefficients and the computer draws out the diagrams for each molecule.
With the game, it make it super clear exactly what they were supposed to do and they could see the drawings instantly as soon as they changed the coefficients and count up the number of atoms on each side of the equation. Everyone understood balancing chemical equations in just half an hour of instruction. Even students who had done poorly on earlier foundational concepts (ex. difficulty drawing the structure of a molecule like Na2SO4) someone seemed to get it and then could draw both the molecule and balance chemical equations after playing the game.
It's not your technique. It's the need for concrete manipulatives. Try the game. All you have to do it tell them to start playing and type in all one's for the coefficients and then play with the numbers as directed in the "How to play the game" link. They get it pretty quickly. If they make a mistake like 2O2 + 4H2 -> 4H2O, the game will tell them "There are the same number of atoms on each side, but your answer is not in lowest terms. Please put your answer in lowest terms and then click the Balanced button."
If after mastering the easier equations on Classic Chembalancer (only the last question is hard), you want them to practice balancing the hard equations, have them play Brain Boggle Chembalancer in which all the questions are hard.
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