Water Purification Test

Overview

Water is the most critical survival resource. A human can survive weeks without food but only about three days without water. Yet most water found in nature — rivers, lakes, puddles, streams — contains dirt, bacteria, parasites, and other things that will make you sick. Every survival tradition in human history has developed methods to make water safe: boiling, filtering through sand, collecting dew, using sunlight.

This experiment lets your child build three different water filters, test them side by side, and see with their own eyes which method cleans water most effectively. They will not drink any of the water — but they will understand the principles that every water purification system in the world is based on.

The Question

"If you found dirty water in the wild and needed to make it cleaner, what would you use to filter it?"

Background

Share this with your child before you begin:

"Every drop of water you drink at home has been cleaned. The water company takes water from a river or a well, filters it through sand and gravel — just like we are going to do today — and then adds a tiny bit of chlorine to kill germs. But if you are out in the woods and you need water, you do not have a water company. You have to do it yourself."

"Today we are going to make dirty water and then try three different ways to clean it. We will see which one works best. But here is the important thing to remember — even water that LOOKS clean might still have tiny germs in it that we cannot see. Filtering makes water clearer, but to make it truly safe, you also need to boil it or use purification tablets. We will talk about that at the end."

Hypothesis

Before building anything, have the child predict:

"We are going to try three filters: a coffee filter, cotton balls, and a sand-and-gravel filter. Which one do you think will make the water the cleanest?"

Write their prediction in the notebook. There is no wrong answer — this is a hypothesis, and hypotheses are meant to be tested.

Procedure

Setup (10 minutes)

1. Make the dirty water. In a large jar, mix clean water with a handful of garden soil, some small leaf bits, and a pinch of sand. Stir it well. Let it sit for 2 minutes so the heaviest particles settle slightly — but it should still look obviously dirty and cloudy. This is your "source water."

2. Label four jars. Use tape and marker:

   • Jar 1: "Source" (the dirty water)
   • Jar 2: "Coffee Filter"
   • Jar 3: "Cotton Ball Filter"
   • Jar 4: "Sand & Gravel Filter"

3. Build the filters.

Filter A — Coffee Filter: Place a coffee filter over the top of Jar 2 and secure it with a rubber band. Simple.

Filter B — Cotton Ball Filter: Take the funnel (or the top half of your cut plastic bottle, inverted). Stuff cotton balls into the narrow end, packing them loosely but filling at least 3 inches of the funnel. Place this over Jar 3.

Filter C — Sand & Gravel Filter: This is the real one. Take the cut plastic bottle, inverted (cap end down — poke a small hole in the cap or remove it). Layer from bottom to top:

• A coffee filter or cotton ball at the very bottom (keeps sand from falling through)
• 2 inches of clean sand
• 1 inch of small pebbles or gravel
• Another inch of larger pebbles on top

Place this over Jar 4.

Experiment (15 minutes)

Pour dirty water through each filter, one at a time. Use the same amount of water for each — about one cup.

Coffee filter: Pour slowly. Watch the water pass through. How fast does it flow? What gets caught on the filter?

Cotton ball filter: Pour slowly. Watch the water seep through the cotton. Is it faster or slower than the coffee filter?

Sand and gravel filter: Pour slowly over the top layer of pebbles. The water will take the longest to pass through. This is important — the slower the water moves, the more particles get trapped. Wait for it to fully drip through.

Record (10 minutes)

Line up all four jars. Look at them together.

For each jar, record in the notebook:

• Color: Is it brown, tan, yellowish, or clear?
• Clarity: Can you see through it? Hold a finger behind the jar — can you see your finger clearly?
• Particles: Are there visible bits floating in the water? Use the magnifying glass.
• Light test: Shine a flashlight through each jar. Which one lets the most light through?

Have the child draw each jar in the notebook with colored pencils, showing the water color and any visible particles.

Analysis

Ask these questions and discuss together:

1. "Which filter produced the clearest water?" (Almost always the sand and gravel filter.)
2. "Which was the fastest?" (Usually the coffee filter.)
3. "Is there a trade-off between speed and quality?"
4. "Why do you think the sand-and-gravel filter works best?" (Because it has multiple layers that catch different-sized particles. The gravel catches big stuff, the sand catches tiny stuff.)
5. "Look at the coffee filter after you used it. What do you see?" (The dirt that got caught. That is what the filter kept out of your water.)

The Explanation

"Here is what happened. Every filter works by trapping particles — bits of dirt, sand, leaves — as the water passes through. A coffee filter has tiny holes that catch some particles, but really small ones slip through. Cotton balls are denser, so they catch more. But the sand-and-gravel filter has millions of tiny spaces between the grains of sand, and each space catches something. By the time the water reaches the bottom, most of the visible dirt is gone."

"This is exactly how water treatment plants work — just on a much bigger scale. And it is exactly how people in the wilderness have cleaned water for thousands of years. You dig a hole near a stream, and the water that seeps into the hole has been filtered through the ground — through layers of sand and rock — just like our bottle filter."

"But remember: filtering removes dirt, but it does not kill germs. Bacteria and parasites are too small to see, even in clear-looking water. In a real survival situation, after filtering, you would also need to boil the water for at least one minute. Boiling kills the germs that filtering misses."

Extensions

• Build a bigger filter: Use a 2-liter bottle and add more layers: charcoal (activated charcoal from a pet store), fine sand, coarse sand, gravel. Charcoal is what makes commercial water filters work — it absorbs chemicals and improves taste.
• Test different source water: Try filtering water with food coloring added. Does the color come out? (No, unless you use charcoal — this demonstrates the difference between particles and dissolved substances.)
• Boiling experiment: With close supervision, filter the dirty water through the sand filter, then boil a small amount. Compare the boiled and unboiled filtered water. Discuss why boiling is the final step.
• Research assignment: Look up how your local water company cleans the water that comes out of your faucet. Visit their website — many have diagrams of their filtration process that look remarkably similar to your bottle filter.

Safety Notes

• Do not drink any of the filtered water. None of these methods produce water that is safe to drink. Make this clear before, during, and after the experiment. The lesson is about understanding the process, not producing drinking water.
• Cutting the plastic bottle: The parent should cut the bottle. Use scissors, not a knife. The cut edge may be sharp — sand it lightly or cover with tape.
• Dirty water handling: Wash hands after handling the source water, especially if you used garden soil. Garden soil can contain bacteria that are harmless on skin but should not go in mouths or eyes.
• Glass jars: If using glass mason jars, supervise younger children. Consider plastic cups as an alternative for 5-year-olds.
• Cleanup: Dispose of the dirty water outside or down a drain. The sand and gravel can be rinsed and reused or discarded. Coffee filters and cotton balls go in the trash.