I suggest that you take a look at
my notes, in particular pages 5-6, 12 (accuracy is on pages 2-3).
In addition I suggest that you read the following articles which were published in
Curriculum Support for teaching in Science 7-12. Which is a publication aimed at teachers, distributed to all DET teachers, by Curriculum Services Unit DET. They often have good articles that teachers never seem to pass on to the students.
If after reading these you still need clarification, just post again.