Wash Out

Grade Level
Middle School and up

Activity source: Junior Engineering Technical Society (JETS)

Materials (per team of 2-3 students)

  • 3 paper cups
  • Coffee filter
  • 1/3 cup of sand
  • 1 Tbsp salt
  • Water
  • Magnifying glass or hand lens
  • Pencils

All that glitters may not be gold, but for centuries gold has captured the eyes of many throughout the world. Gold—as well as copper, coal, and iron—is extracted from the ground. Engineers find environmentally friendly ways to prepare and process these natural resources for products that go into our computers or our buildings. One process is leaching.


Leaching is the action of dissolving out parts of a mixture by percolating liquid. An analogy could be the hot water in a coffee maker flowing through the ground coffee beans so that the resulting solution contains a substance that was previously trapped in the beans. Similarly, some metals are easily dissolved in acid or water. In this activity, the students will observe the leaching process by dissolving salt out of sand and then precipitating the salt as solid crystals.

Activity 1
  1. Introduce the class to the meaning of the word "leaching."
  2. Explain to the students that in this activity they will be "modeling" the leaching process using a sand and salt mixture to represent crushed ore in a paper cup leach pad and using water as the percolating liquid.
  3. Using a pencil, punch holes in the bottom of one paper cup.
  4. Line that cup with the coffee filter and tear away any excess filter that sticks out of the cup.
  5. Mix the sand and salt and put this "ore" mixture in the lined cup.
  6. Fill a second cup with water.
  7. Hold the cup of standard salt over the remaining empty cup and slowly pour the water into the standard salt mixture, catching the drainage in the bottom cup.
  8. Reverse the positions of the empty cup and the cup of water and pour the water through the standard salt cup again.
  9. Set the resulting solution aside in a warm place for at least a week to partially evaporate the water. Observe the crystals forming in the cup.
  10. Set aside one cup of plain water for the class as a "control" for comparison.