Question

what do you think will happen to the rate of reaction if one increases the concentration of substrate to 6ml? why does the speed of reaction level off?

Answer 1

According to your question I assume that you are asking about the reaction catalyzed by enzymes. If that is the case, then it is not that every time you increase your substrate concentration means you end up an increase in the rate of the reaction. Yes, initially you will see a rise in the rate but when the enzyme gets saturated nothing will happen to your reaction rate regardless adding more substrates into the reaction mixture.

This type of situation is achieved at a particular point named as maximum velocity of the enzyme. From the rate vs substrate concentration curve you know that when the enzyme approaches its maximum velocity, the rate curve levels off horizontally. So up to this point you can increase the rate by increasing your substrate concentration. Once the enzyme achieved its maximum velocity you cannot do much about your rate of the reaction.