in response to Paperback
1) I would be wary about using mathematical symbols in the exam. There is a chance (albeit minute) that your examiner will misread the '<' symbol as a greater than symbol. It's far better to write out "less than" to avoid any confusion. It's not wasted time when you're ensuring that you get the marks.
2) In the graph you're talking about, when enzyme concentration equals substrate concentration, the system is in what is called "chemical equilibrium". This is pretty much just a fancy way of saying that the rate of reaction is constant. Consequently, the rate graph will plateau but not drop (as the reaction hasn't completely stopped, rather it is just occurring at a constant rate). I'm happy to elaborate on this further if you would like.
in response to PKingDrogba
I have to admit, I initially shared your confusion about the control of the experiment - it is not very well designed. I would argue that the control is the test tube to which nothing has been added but H202 as, essentially, this test tube (in which (fundamentally) no chemical reactions have taken place) allows comparison to the other test tubes.
The error due to the stoppers popping off is more of an instrument error. Although it's crude, I think the best way of minimising this error is to keep a finger on the stopper.
Your last question is vital. Dramatic changes in pH do cause denaturation!. When pH changes, ionic interactions in the tertiary structure of proteins may be disrupted. Consequently, the protein may unravel (denature). This is irreversible and changes the shape of the active site of enzymes. Consequently, function is impaired as the enzyme no longer has an optimal fit for its specific substrate.
Good luck for the year ahead and feel free to ask any more questions!