Why Do We Separate Substances?
Most things around us — milk, salt water, mud, air — are mixtures of different substances. To use the parts we want (or remove what we don’t), we separate them. This chapter shows how everyday techniques solve real problems: getting clean drinking water, extracting salt from seawater, refining cooking oil.
For Class 6 students, this is the first big chapter in chemistry’s “real world” applications. Every method we cover here is used in homes, industries, and science labs daily.
We will walk through each separation method, why it works, and where you’ll see it in action.
Key Terms & Definitions
Mixture: Two or more substances mixed without chemical bonding. Each component keeps its own properties.
Pure substance: A material made of only one type of particle (e.g., distilled water, table salt).
Separation: Pulling apart the components of a mixture using physical methods.
Component: Each individual substance in a mixture.
The Common Separation Methods
Hand-picking
Used when components are different in size, shape, or colour AND big enough to grab. Example: removing small stones from rice or pulses before cooking.
Best for: small amounts, large clearly different particles.
Threshing
Beating dried grain stalks to separate grains from stalks. Used by farmers when harvesting wheat, rice, or pulses.
The grains fall out; the stalks remain.
Winnowing
After threshing, the mixture of grain and chaff (lighter husk) is dropped from a height while wind blows. The lighter chaff is carried away by wind; the heavier grain falls straight down.
Best for: separating heavy and light components when wind is available.
Sieving
Passing the mixture through a sieve (mesh with small holes). Particles smaller than the mesh fall through; larger ones stay back.
Used in: separating flour from bran, sand from gravel, baby food preparation.
Sedimentation and Decantation
For mixtures of insoluble solids and liquids:
- Sedimentation: Let the heavier solid settle at the bottom. Wait for the mixture to become still.
- Decantation: Carefully pour off the clear liquid above without disturbing the settled solid.
Used in: cleaning muddy water at home (settle the mud, pour off clear water).
Filtration
Pours the mixture through filter paper or cloth. Liquids pass through; solids are caught on the filter.
Better than decantation when settling is incomplete or particles are very small.
Used in: tea making (filter tea leaves), water purifiers, lab filtration.
Evaporation
For dissolved solids in liquids: heat the mixture until the liquid evaporates, leaving the solid behind.
Used in: getting salt from seawater (in saltpans), making sugar from sugarcane juice.
Condensation
Cooling vapour back into liquid. The reverse of evaporation. Used to recover the liquid that has evaporated.
Used in: distillation (separating liquids by boiling and condensing), getting fresh water from seawater.
Magnetic Separation
Using a magnet to pull out iron or iron-containing materials. Used in: separating iron filings from sand, recycling.
How to Choose a Method
For a given mixture, ask:
- Are the components solid, liquid, or both?
- Are they soluble in each other?
- Can size, weight, or magnetism be exploited?
| Mixture Type | Best Method |
|---|---|
| Stones in rice | Hand-picking |
| Heavy grain + light chaff | Winnowing |
| Mud in water | Sedimentation + Decantation, or Filtration |
| Salt in water | Evaporation |
| Iron filings in sand | Magnetic separation |
| Two liquids | Distillation (Class 9 onwards) |
Solved Examples
Easy — Choose the Method
How would you separate sand from a mixture of sand and water?
The sand is insoluble in water. Let it settle (sedimentation), then pour off the water (decantation). Or use filtration directly.
Medium — Multi-Step Separation
You have a mixture of iron filings, sand, and salt. How would you separate all three?
Step 1: Use a magnet to pull out the iron filings. Step 2: Add water to the remaining mixture. Salt dissolves; sand doesn’t. Step 3: Filter out the sand. Step 4: Evaporate the saltwater to recover the salt.
Hard — Real-World Setup
A company makes drinking water from salty groundwater. Which methods do they use?
They first filter out solid impurities (filtration). Then they remove dissolved salts using either evaporation followed by condensation (distillation) or membrane filtration. The result is clean drinking water.
Tips for Class 6 Students
Tip 1: Always check if the substance is soluble or insoluble in the liquid. This decides whether to use evaporation or filtration.
Tip 2: For mixtures with multiple types (like iron filings + sand + salt), think about each component separately. Apply the right method for each.
Tip 3: Practice drawing simple diagrams. CBSE often awards 1 mark for a clear, labelled setup of filtration or evaporation.
Common Mistakes to Avoid
Mistake 1: Confusing sedimentation with filtration. Sedimentation is just waiting for solids to settle. Filtration uses a filter and is faster.
Mistake 2: Thinking salt can be separated from water by filtration. Salt is dissolved — it passes through the filter. We must evaporate the water.
Mistake 3: Using a magnet to separate everything. Magnets only attract iron and iron-containing materials, not all metals.
Mistake 4: Confusing mixtures with pure substances. Salt water is a mixture; the components keep their properties (water can still be boiled, salt can still be tasted).
Mistake 5: Forgetting that some methods need multiple steps. Recovering all parts of a complex mixture often requires three or four techniques in sequence.
Practice Questions
Q1. Which method would you use to separate butter from curd?
Churning. The butter, being lighter, rises to the top and can be skimmed off.
Q2. Why doesn’t sieving work for separating sand from salt?
Sand and salt particles are similar in size, so a sieve cannot tell them apart. Use water (which dissolves salt but not sand) and then filter.
Q3. Name the method used to separate cream from milk.
Centrifugation (or churning). Spinning the milk separates the lighter cream from the rest.
Q4. A glass contains a mixture of water and ice. Are they separable by filtration?
Not effectively — ice will melt during filtration. Better methods: cooling to keep ice solid, then picking out, or letting ice melt completely and removing nothing.
Q5. What is the simplest way to remove insoluble dust from drinking water at home?
Filtration through a cloth or filter paper. For dissolved impurities, additional steps (boiling, distillation, or filter cartridge) are needed.
Q6. Salt is obtained from seawater in coastal areas. Describe the process.
Seawater is collected in shallow flat-bottomed pits called saltpans. Sun heat evaporates the water, leaving solid salt behind. The salt is collected and refined.
Q7. Why is winnowing not useful in a closed room?
Winnowing relies on wind to blow away the lighter component. Without wind, both heavy and light parts fall straight down together.
Q8. Are dissolved gases in water visible? How would you remove them?
No, they are not visible (the water looks clear). Boiling drives out dissolved gases, which then escape into the air as bubbles.
FAQs
Why is filtration faster than sedimentation? Filtration uses a porous material to catch solid particles immediately. Sedimentation requires waiting for solids to settle, which can take minutes to hours.
Can we use evaporation to separate two liquids? Only if they have very different boiling points. We boil the more volatile liquid first, then condense it (distillation). Class 9 chapter covers this in detail.
Why do we wash rice before cooking? Washing removes dust and starch on the surface — a form of separation by dissolution and decantation.
What is the difference between solute and solvent? The solute is what dissolves (e.g., salt). The solvent is what dissolves it (e.g., water). Together they form a solution.
Is air a mixture? Yes — air is a mixture of nitrogen, oxygen, carbon dioxide, water vapour, and tiny dust particles. Each component keeps its own chemistry.
Can sand be separated from sugar without water? Difficult. The cleanest way is to dissolve sugar in water, filter out sand, and then evaporate to recover sugar.
Why is distilled water special? Distilled water has no dissolved minerals or impurities — it is pure . Useful in batteries, labs, and medical settings.
What method separates oil from water? A separating funnel. Oil floats on water (less dense). Open the bottom tap to drain the heavier water; close before the oil starts coming out.