Why is water added in two separate portions to the acetic anhydride/concentrated h2so4 mixture?

Question

Lab question about esters and preparation of aspirin 

Answer 1

To control the temperature: The reaction between acetic anhydride and concentrated sulfuric acid is highly exothermic. If all the water were added at once, the sudden release of heat could lead to a violent reaction, which could further lead to splashing over the reacting solution. By adding water in two separate portions, you control the rate of heat generation and prevent the reaction mixture from becoming too hot too quickly.

Stabilization of Intermediates: The reaction typically involves the formation of an acylated intermediate that is highly reactive and can lead to the formation of various undesirable side products. By adding water after the initial reaction, chemo selectively forms the desired acetyl derivative.