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Is It Clean or Just Unseen?: Dirty Water and the Naked Eye

Source Institutions

    American Society for Microbiology

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Is It Clean or Just Unseen?: Dirty Water and the Naked Eye

Learners perform serial dilutions of water with dye and of water mixed with soil. They take a sample from the most dilute solution of water and soil and spread it over an agar plate to incubate overnight. Learners can then count the bacteria colonies that grow from the diluted sample and calculate the number of bacteria in the original soil solution. Time required for activity is spread over multiple meetings.

Quick Guide

Preparation Time:
30 to 45 minutes

Learning Time:
1 to 2 hours

Estimated Materials Cost:
$1 - $5 per student

Age Range:
Ages 14 - 18

Resource Types:
Activity, Experiment/Lab Activity, Lesson/Lesson Plan


Materials List (per student)

  • Dye (1-2 drops)
  • Fresh soil in sandwich bag
  • Six 10 ml test tubes
  • Two 1 ml pipettes
  • Petri dish with nutrient agar
  • Sterilized paperclip spreader
  • Distilled water
  • Graduated cylinder, 10 ml
  • Student Handout (p8-11 of PDF)


  • Life Sciences
    • Cells
    • Diversity of Life
      • Viruses and Bacteria
  • Mathematics
    • Measurement
      • Units of Measurement
      • Size and Scale
    • Number and Operations
      • Decimals
  • Physical Sciences
    • Chemistry
      • Solutions
  • The Nature of Science
    • The Scientific Process
      • Conducting Investigations
      • Gathering Data

Informal Categories

  • Gardening


To use this activity, learners need to:

  • see
  • see color
  • touch

Learning styles supported:

  • Involves hands-on or lab activities


Components that are part of this resource:

Includes alignment to state and/or national standards

Includes assesments for student learning

Access Rights:

  • Free access


  • Weigel, Kylie; Gallo, Ph.D., Mark; American Society for Microbiology


  • All Rights Reserved, American Society for Microbiology, ©2012


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