This MCQ module is based on: Mixtures and Separation Techniques
TOPIC 29 OF 50
Mixtures and Separation Techniques
🎓 Class 8
Science
CBSE
Theory
Ch 8 — Reproduction in Animals
⏱ ~27 min
🌐 Language: [gtranslate]
🧠 AI-Powered MCQ Assessment
▲
Myaischool Lt Science Assessment
▲
[myaischool_lt_science_assessment grade_level="class_8" science_domain="biology" difficulty="basic"]
8.5 Mixtures
Most real materials around us are not pure substances. The milk in your glass, the air in your lungs, the steel in your scissors, the sherbet at a wedding — all are mixtures.
Mixture: A material containing two or more substances blended together without any chemical bonding. Each component keeps its own properties and can usually be recovered by physical methods.
Three Key Differences from Compounds
| Feature | Compound | Mixture |
|---|---|---|
| Combination | Chemical | Physical |
| Composition | Fixed ratio | Any ratio |
| Separation | Chemical methods only | Physical methods |
| Properties | Entirely new | Show properties of components |
| Example | Water (H2O) | Salt solution, air |
8.5.1 Homogeneous vs Heterogeneous Mixtures
Homogeneous mixture: Looks uniform throughout — no visible boundary between components. Examples: salt dissolved in water, vinegar, brass, air.
Heterogeneous mixture: Components are visibly different — you can see patches, layers or particles. Examples: oil and water, sand and iron filings, muddy water, a fruit salad.
Heterogeneous mixture: Components are visibly different — you can see patches, layers or particles. Examples: oil and water, sand and iron filings, muddy water, a fruit salad.
Identify These Examples
- Seawater — homogeneous (a clear solution of salts in water).
- Sand + salt — heterogeneous; the grains are plainly different.
- Milk — looks uniform but is actually a colloid: tiny fat droplets spread in water. Technically still heterogeneous at the microscopic level.
- Blood — heterogeneous; red cells, white cells and platelets float in plasma.
- Air — homogeneous gaseous mixture of N2, O2, Ar, CO2 and traces.
- Brass — homogeneous solid solution (alloy) of copper and zinc.
8.6 Separating the Components of a Mixture
Because the components of a mixture are not chemically locked, we can pull them apart using physical tricks. The method we choose depends on how the components differ — in size, in solubility, in boiling point, in density, in magnetism, or in the way they stick to surfaces.
Filtration — Solid + Liquid
If a solid is insoluble in the liquid (like chalk powder in water, or mud in water), pour the mixture through filter paper. The liquid (filtrate) passes through; the solid (residue) is trapped.
Evaporation — Soluble Solid from Liquid
If the solid has dissolved (e.g., salt in water), filtration won't work. Instead, heat the solution in a china dish; the water turns to vapour and escapes, leaving the salt behind. This is how coastal villages harvest sea-salt in flat pans under the sun.
Distillation — Recover the Liquid Itself
If you want pure water from seawater (not just the salt), use distillation: boil the solution, then let the steam cool and condense into a separate flask.
Decantation — Let the Heavy Stuff Settle
Rice is washed by stirring with water, waiting a minute for the rice grains to sink, and then gently pouring off the cloudy water on top. This pouring-off is decantation. It works whenever a denser solid (or liquid) settles below a lighter liquid.
Sieving — Different Sizes
Bran and flour are separated when wheat flour is sifted: finer flour particles fall through the mesh while the coarser bran stays on top. Builders use larger sieves to separate small stones from sand.
Magnetic Separation — Pull Out the Iron
When one component is magnetic (iron, steel, nickel), a magnet plucks it out of the rest. This method is used on conveyer belts at scrap yards to recover iron from mixed trash.
Chromatography — Splitting the Colours
A single black ink dot placed on filter paper with water creeping up will spread into several colour bands — blue, red, yellow — because the dyes travel at different speeds. This technique, called chromatography, is also used to detect pigments in plant leaves and drugs in urine tests.
Centrifugation — Spinning Out the Heavy
In a blood test, a small tube of blood is spun at high speed. The denser red cells are thrown to the bottom, leaving the pale yellow plasma at the top. Dairies use centrifuges to separate cream from milk for the same reason.
Clean Drinking Water — Using Several Methods Together
🔬 Activity 8.6 — Separate a Mixture of ThreeL4 Analyse
🤔 Predict first: You are given a mixture of sand + iron filings + common salt. Design a plan to obtain all three components separately.
You need: mixture, bar magnet, beaker, water, stirring rod, filter paper + funnel, china dish, spirit lamp.
- Spread the mixture on a paper. Run the magnet above it — iron filings cling to it. Collect. (Iron separated.)
- Add the remaining sand + salt to water. Stir — salt dissolves, sand does not.
- Filter. Residue on paper = sand (wash, dry, collect).
- Heat the filtrate (salt solution) gently. Water evaporates, leaving behind white salt crystals.
Three methods chained together: magnetic separation → filtration → evaporation. We were able to do this because each pair of components differed in some physical property (magnetism / solubility / state after heating). Notice that no chemical change occurred — the salt is still the same NaCl we started with. This is why we say mixtures are separated by physical means.
🎯 Best Separation Method L3 Apply
For each mixture, click the most suitable separation method.
1. Chalk powder in water.
Filtration Distillation Magnetic separation2. Salt dissolved in water (you want the water back pure).
Filtration Distillation Evaporation (loses the water)3. Iron filings mixed with sulphur powder.
Magnetic separation Sieving Chromatography4. Colours in a fountain pen ink.
Filtration Chromatography Decantation5. Flour and bran.
Sieving Distillation Magnetic separation6. Cream from milk.
Centrifugation Filtration Evaporation📋 Competency-Based Questions
A water tanker supplies a village from a muddy river during the monsoon. Before drinking, the water has to be treated: the mud must settle, fine particles must be trapped, and finally any germs must be killed. Kiran watches the whole process at the village treatment plant.
Q1. L1 Remember Name any two physical methods used to separate components of a mixture.
Answer: Any two from: filtration, evaporation, distillation, decantation, sieving, magnetic separation, chromatography, centrifugation.
Q2. L2 Understand Which method removes fine suspended mud that does not settle down quickly?
Answer: B. Water-works use multi-layered sand/charcoal filters that trap fine particles too small for simple settling.
Q3. L3 Apply Kiran finds that seawater tastes salty. She wants pure drinking water from it at home. Which single method should she use and why?
Answer: Distillation. She should boil the seawater, cool the steam in a condenser, and collect the pure water that drips out. Evaporation alone would lose the water; filtration would not help because salt is dissolved, not suspended.
Q4. L4 Analyse Classify each: (i) brass, (ii) muddy water, (iii) air, (iv) sand + iron filings, as homogeneous or heterogeneous mixtures.
Answer: (i) Brass — homogeneous (uniform alloy of Cu + Zn). (ii) Muddy water — heterogeneous (particles visible). (iii) Air — homogeneous (uniform gas mixture). (iv) Sand + iron filings — heterogeneous (two solids, visibly different).
Q5. L5 Evaluate A friend says: "A sieve and a filter paper do the same job." Do you agree?
Answer: Only partly. Both separate by size, but a sieve has much larger pores — it separates one solid from another solid (like bran from flour). Filter paper has microscopic pores — it separates tiny solid particles from a liquid (like mud from water). The principle is similar (size filter), but they are used for very different mixtures.
🔗 Assertion–Reason Questions
Assertion (A): A mixture of sugar and water can be separated by filtration.
Reason (R): Filtration separates an insoluble solid from a liquid.
Answer: D. Sugar is soluble in water, so filtration cannot separate it (sugar passes through the filter paper). A is false. R is a correct statement on its own.
Assertion (A): Air is a homogeneous mixture.
Reason (R): Its components (N2, O2, Ar, CO2) are thoroughly mixed at the molecular level and give a uniform composition in a room.
Answer: A. Being uniformly mixed is precisely what makes air homogeneous.
Assertion (A): Chromatography can separate the different dyes present in a single drop of black ink.
Reason (R): Different dyes move at different speeds along a wet paper strip.
Answer: A. Differing travel speeds is exactly the principle that lets the ink split into bands.
AI Tutor
Science Class 8 — Curiosity
Ready