Does acid or alkali stop bacteria from growing, and does the strength of the solution matter? You soak filter paper discs in hydrochloric acid or sodium hydroxide at four concentrations: 5%, 10%, 15%, and 20%. You place the soaked discs onto Petri dishes coated with E. coli bacteria. After three days, you measure the inhibition zone (the clear ring with no bacterial growth) around each disc. Higher concentrations create larger clear zones. Acid produces bigger inhibition zones than alkali at every concentration.