Most chemical reactions take place in the cytoplasm
I hope that's help !
The reason why you cant test the activity of Lipase in test tube 5 is because in test tube 5 the PH is low. This decrease in pH might make it difficult to detect the activity of lipase since the optimal ph of pancreatic lipase is around 8.0. generally high or low PH result to complete loss of activities of most enzymes.