in the experiment of reacting starch and amylase in different pH's, why would the trend be that?

Question

the more alkaline the solution the quicker the iodine decolourises?

Answer 1

The amylase in saliva has a pH optimum of 5.6 - 6.9. So at alkaline pH, the enzyme reaction should be slower than at neutral pH. The iodine color should be slower to decolonize.

Answer 2

Because each enzyme has an optimal pH at which it functions best. Amylase must be better working at a more basic pH. Hence, the more alkaline the solution, the faster the reaction (since, it is speeding up the enzyme). However this will only happen upto a point. If its very alkaline it might not get quicker any more and might actually start decreasing.