Sedimentation and decantation: separating sand from water
Stir sand into water, let it settle, then pour off the clear water without disturbing the sand.
Core concept
In short: Let mud settle, then pour off the clear water on top.
Heavy insoluble particles settle, so the liquid above can be poured off
Topic: Separation of mixtures
When you stir sand into water, the water turns cloudy. Sand does not dissolve: To mix into a liquid so completely that you can no longer see the pieces. All key words, and its grains are heavier than water, so if you leave the mixture: Two or more substances mixed together, which can be separated again. All key words still, they slowly sink and collect at the bottom. This settling is called sedimentation. Pouring off the clear liquid above the sediment: Heavy bits that sink and settle at the bottom of a liquid. All key words, 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
- Add sand to the water in Beaker A and stir. Observe the cloudy water.
- Leave Beaker A still and use the timer until the sand has settled.
- Decant the clear water from Beaker A into Beaker B without pouring the sand.
- 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.