You are here
๐ฌ Think about it
- You have surely seen muddy water in the rainy season.
- Take a sample and stir it as much as you like — the soil particles still float about, clearly visible.
- That is completely unlike salt or sugar in water, which simply vanish. So what is this kind of mixture called?
What is a suspension?
- In a suspension , the solid particles do not dissolve in the liquid — they stay suspended throughout the medium.
- The particles that do not dissolve are much larger than those in a solution, which is why we can see them with the naked eye.
- Other everyday examples — sawdust in water, and tea leaves in water.
Definition
A suspension is a heterogeneous mixture in which solid particles do not dissolve but remain suspended throughout the bulk of the medium.
How can we separate mud from water?
Why simple methods are not enough
Leave the muddy water undisturbed
the heavier mud particles settle, but the water still looks cloudy
↓
Filter it through cotton cloth or filter paper
the larger particles are removed, but the cloudiness remains
↓
Filtration alone is not always enough for very fine particles
- So if the muddy water is still not clear after standing, how do we clean it completely?
- In such cases we use two techniques — centrifugation and coagulation .
A. Centrifugation
What is centrifugation?
- Try this first — cross arms with a partner, hold each other's hands and spin round together quickly. Do you feel as though something is pulling you outwards ?
- That spinning game is called phugadi in Marathi and kikli in Punjabi.
- Exactly the same force acts when we centrifuge a mixture.
- In centrifugation , the mixture is put in a tube and spun at very high speed in a centrifuge machine.
- As the tubes spin they become horizontal , and the centrifugal force — the outward force on a body moving in a circle — pushes the heavier particles outwards.
- The heavy particles collect at the bottom of the tube and the lighter liquid stays on top.
Definition
Centrifugation is the process of separating the components of a mixture by spinning it at high speed, so that the centrifugal force drives the denser particles outwards to the bottom of the tube.
Centrifugation is widely used in laboratories to separate the components of
blood
— red blood cells from plasma — and in many chemical industries.
๐
Activity 5.8 — Let us make a model
In this Activity, we will build a working centrifuge out of cardboard and thread, and watch heavy particles move outwards when it spins.
Materials needed
A cardboard disc, thick thread, tape handles, and a few tiny tubes of muddy water. This low-cost device is called the
Paperfuge
, and it works on the idea of a common spinning toy.
Procedure
1. Cut a cardboard disc and make two small holes near its centre.
2. Pass the thread through both holes and tie a tape handle at each end.
3. Tape the tiny tubes of muddy water onto the disc.
4. Wind up the thread, then pull the handles apart and let them come together, again and again.
5. The disc spins one way, unwinds and spins back — extremely fast.
2. Pass the thread through both holes and tie a tape handle at each end.
3. Tape the tiny tubes of muddy water onto the disc.
4. Wind up the thread, then pull the handles apart and let them come together, again and again.
5. The disc spins one way, unwinds and spins back — extremely fast.
Observation
The suspended particles collect at the bottom of the tubes and the water above them turns clear — with no electricity at all.
Explanation
Spinning generates the same
centrifugal force
as a laboratory centrifuge. It pushes the heavier suspended particles outwards, and they settle at the far end of each tube.
Put tiny tubes of a blood sample on the same Paperfuge and it separates the heavy blood cells from the light plasma, just as a big laboratory centrifuge does.
โ Summary
- A cardboard disc on a thread spins very fast
- Centrifugal force drives heavy particles outwards
- It works on muddy water and on blood samples
B. Coagulation
What is coagulation?
- When the tiny suspended particles in muddy water will not settle and filtering still leaves the water cloudy, we mix in a little powdered alum , or fitkari .
- Alum is a white crystalline chemical commonly used to clean water.
- As soon as it enters the water, alum makes the very fine, light soil particles stick to one another and build up into large clumps .
- This joining of particles into clumps is called coagulation , and alum acts as a coagulant .
- Once the clumps are large and heavy, gravity pulls them quickly to the bottom — this is sedimentation .
- The clear water above is then removed easily by decantation or filtration .
Definition
Coagulation is the process in which fine suspended particles join together to form larger clumps, so that they can settle and be filtered out. The substance that causes it is called a coagulant .
Where do we use coagulation in daily life?
Making paneer at home
Add lemon juice or vinegar to hot milk
the acid acts as a coagulant
↓
The fine liquid proteins in the milk stick together
they turn into solid clumps
↓
Strain the mixture through a cotton cloth
the solid stays in the cloth
↓
Solid paneer on one side, the watery liquid on the other
Example
The commonest real-life example is
making paneer
at home. Milk proteins are coagulated by an acid exactly as clay particles are coagulated by alum.
โ
Test Yourself
-
Why can the tiny particles in muddy water not be removed by filtration alone?
View Answer
They are far too fine, so they pass straight through the pores of the filter paper. They must first be coagulated into larger clumps. -
Which force does a centrifuge use, and what does it do?
View Answer
The centrifugal force — the outward force on a body in circular motion. It drives the heavier particles to the bottom of the spinning tube. -
Name the coagulant used for muddy water and the one used for milk.
View Answer
Alum (fitkari) for muddy water, and lemon juice or vinegar for milk.
๐ Summary - What we have learnt
- A suspension is a heterogeneous mixture in which solid particles stay suspended without dissolving, and are visible to the naked eye.
- Centrifugation spins a mixture so the centrifugal force drives denser particles to the bottom — it separates blood into cells and plasma.
- The Paperfuge does the same job with cardboard and thread, without electricity.
- Coagulation makes fine particles clump together so they can settle and be filtered; alum does this for water, and acid does it for milk.
