Exploring Mixtures and their Separation · Lesson 10 of 13
Suspensions
“When tiny particles refuse to settle politely, spinning and clumping enter the story.”
• Describe the defining properties of a suspension. • Explain sedimentation, decantation and filtration. • Explain centrifugation as density-based separation. • Describe the paperfuge model. • Explain coagulation and the role of a coagulant.
Stir mud into water and the liquid becomes cloudy. Large grains may settle quickly, but fine particles can remain suspended for much longer and may pass through a coarse cloth. Clearing such water may require more than one separation step.
A suspension is a heterogeneous mixture in which large undissolved particles remain spread through a medium for some time and settle on standing.
Suspension particles are larger than 1000 nm in diameter. They may be visible to the naked eye, scatter light, settle under gravity and can generally be separated by filtration. Sand in water, sawdust in water and tea leaves in water are familiar examples.
When muddy water stands, heavy particles settle at the bottom by sedimentation. The clearer upper liquid can be poured away by decantation. Filtration removes many remaining particles, but very fine suspended matter can keep the water cloudy.
Sedimentation is the settling of heavier suspended particles under gravity when a mixture is left undisturbed.
Decantation is the careful pouring of a clearer upper liquid away from settled material.
Centrifugation
Centrifugation is a separation method in which rapid spinning causes denser components to move farther outward and collect separately from less dense components.
In a spinning centrifuge, tubes swing outward toward a horizontal position. Denser particles move farther outward and collect toward the outer end of each tube, while lighter liquid remains above them. The method separates components much faster than ordinary settling.
Problem
How can centrifugation help when fine mud settles very slowly?
- 1.Place equal quantities of the muddy mixture in balanced centrifuge tubes.
- 2.Spin the tubes rapidly in a centrifuge.
- 3.Denser mud particles move toward the outer ends and form a compact deposit.
- 4.The clearer liquid remains above the deposit.
- 5.Carefully decant the upper liquid without disturbing the settled mud.
Bridging Science and Society
A paperfuge is a simple hand-powered spinning device inspired by a whirling toy. A small sample attached to a cardboard disc can be rotated rapidly by pulling twisted strings. It can separate denser blood components from lighter ones without electricity and may support basic testing in places where powered equipment is unavailable.
Activity: Let Us Make a Model
Cut a strong cardboard disc, make two central holes and thread a thick string through them to form handles on both sides. Twist the string and pull rhythmically to spin the disc. Use only safe demonstration materials; real blood samples require trained handling and must not be used in a classroom model.
Coagulation
Coagulation is the process in which fine suspended particles are made to clump together into larger masses that can settle more readily.
A coagulant is a substance added to cause fine suspended particles to form larger clumps.
Powdered alum can act as a coagulant in muddy water. Fine particles that once remained dispersed join into larger clumps. Gravity then produces sedimentation, after which decantation or filtration removes the clumps more effectively.
Problem
Arrange coagulation, sedimentation, decantation and filtration into a sensible order.
- 1.Add a suitable coagulant so fine particles begin to clump: coagulation.
- 2.Leave the mixture undisturbed so the larger clumps settle: sedimentation.
- 3.Pour away the clearer upper water carefully: decantation.
- 4.Pass the water through a suitable filter to remove remaining particles: filtration.
- 5.Each step prepares the mixture for the next step.
Paneer formation is another example of coagulation. Lemon juice or vinegar acts as a coagulant, causing milk proteins to clump into solid curds that can be separated from the liquid.
Centrifuge tubes must be balanced with equal masses opposite one another. Improper balancing can cause strong vibration and equipment damage. Powered centrifuges require trained supervision.
Quiz
Which description best matches Suspension?
Which description best matches Sedimentation?
Which term matches this description: A suspension is a heterogeneous mixture in which large undissolved particles remain spread through a medium for some time and settle on standing.
Which term matches this description: Sedimentation is the settling of heavier suspended particles under gravity when a mixture is left undisturbed.
Which statement is a key takeaway from this lesson?
Practice Problems
- Why can muddy water remain cloudy after ordinary settling?
- Explain how centrifugation speeds up separation.
- State the role of alum in muddy water.
- Describe how paneer formation illustrates coagulation.
Key Takeaways
• Suspension particles are large and settle on standing. • Sedimentation, decantation and filtration can work in sequence. • Centrifugation accelerates density-based separation. • A paperfuge applies the same spinning principle without electricity. • Coagulation makes fine particles form settleable clumps.