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Grade 6 Beginner About 10 min Version 1 Awaiting review Premium

Sedimentation and decantation: separating sand from water

When you stir sand into water, the water turns cloudy. Sand does not dissolve, and its grains are heavier than water, so if you leave the mixture still, they slowly sink and collect at the bottom. This settling is called sedimentation. Pouring off the clear liquid above the sediment, without disturbing it, is called decantation.

What you will learn

  • Describe sedimentation and decantation.
  • Explain why the mixture must be left to settle before decanting.
  • Recognise that very fine particles such as clay settle too slowly for this method to work well.

Before you start

  • Know that sand does not dissolve in water.

Materials

  • Water (60 mL in Beakers A and C)
  • Sand
  • Clay

Equipment

  • 3 beakers (100 mL)
  • Stirring rod
  • Stopwatch

Procedure

  1. Add sand to the water in Beaker A and stir. Observe the cloudy water.
  2. Leave Beaker A still and use the timer until the sand has settled.
  3. Decant the clear water from Beaker A into Beaker B without pouring the sand.
  4. Optional: stir clay into Beaker C and see how long it stays cloudy.

How this simulation is simplified

  • Settling times are shortened. Real sand settles in about a minute, and fine clay can take hours or days.
  • Decanting leaves all the settled sediment behind. In reality a little water stays with the sediment.