HSC 2007 Question 24 (b) (ii) on page 19..
heres the paper
http://www.boardofstudies.nsw.edu.au/hsc_exams/exam-papers-2007/pdf_doc/chemistry_07.pdf
i really cant think of a CHEMICAL reason lol
btw the graph is just a straight line.
becoz i bought success one as second hand so it doesnt contain...