Bacteria Science Fair Project: How Does Bleach Kill Bacteria? (page 2)

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Author: Beth Touchette


You are likely to find no colonies on the plates with 10% and 1% bleach solution.  The lower concentrations of bleach are likely to have progressively more and more colonies.  The plate with no bleach is likely to have a very high number of colonies. 


The tube with water only served as your control: you need to see how many bacteria would grow without any bleach (if the control agar plate had no bacteria growing on it, you’d know that your experimental results were invalid). You waited a ½ hour after adding the bleach to each of the tubes to make sure that the bleach solution had time to work, as it takes time for the bleach to damage the bacteria’s proteins.  The 10% and 1% bleach solutions contained enough bleach to kill all the bacteria. 

You might be wondering why you were instructed use 10% solution to clean your area when your experiment demonstrated that a 1% solution would be sufficient. Unfortunately, many bleach solutions lose their effectiveness over time, so a stronger solution will kill 100% of bacteria longer.  You might advocate for using fresh 1% bleach solution regularly.

Going Further

You might repeat this experiment using samples of  1% bleach solution that have sat for 0 days, one day, two days, three days, and four days to see how quickly bleach solutions lose their effectiveness.

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