CLASS 9 SCIENCE · CHAPTER 5 · STUDY NOTES
Exploring Mixtures and Their Separation
Understand exploring mixtures and their separation through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
EXPLORE
Uniform appearance needs a closer look
Distinguish homogeneous and heterogeneous mixtures.Uniform versus visibly separate
- Homogeneous mixture
- Composition is uniform at the scale considered.
- Heterogeneous mixture
- Components are not uniformly mixed at the relevant scale.
A dissolved salt solution has uniform composition. Sand in water and separated oil–water layers are heterogeneous.
A colloid such as milk can look uniform to the eye but contains a dispersed phase. Appearance alone is not enough to identify a true solution.
Take a moment. What do you think?
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Question
Does a mixture that looks uniform always have to be a true solution?
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EXPLORE
Name the dissolved material and the medium
Concentration compares amount with total solution or solvent.Same solute, more solvent
- Solute
- The substance dissolved in a solution.
- Solvent
- The dissolving medium.
- Concentration
- Amount of solute in a stated amount of solution or solvent.
In salt water, salt is the solute and water the solvent. The concentration changes when you add more solute or solvent.
Concentration labels must say what masses or volumes are compared. “10%” is incomplete without the basis, such as mass by mass. A correctly labelled medicine or rehydration product has a specific formulation, not an arbitrary sweet or salty mixture.
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Question
If more water is added to the same dissolved solute, what happens to its concentration?
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EXPLORE
Use total solution mass in the denominator
Add solute and solvent masses first.The total includes both parts
- Mass by mass percentage
- 100 × solute mass/solution mass, using the same mass unit.
An original example uses 12 g solute with 48 g water. Total solution mass is 60 g, so concentration is 12/60×100 = 20% m/m.
12/48 compares with solvent mass and gives a different ratio. A 20% m/m label means 20 g solute in each 100 g of solution, not 100 g water.
Original mass-percentage calculation
12/(12+48) × 100 = 20% m/mKeep mass units consistent and include the solute in the total.
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Question
Why is the denominator 60 g rather than 48 g?
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EXPLORE
Final volume matters
Keep m/v and v/v distinct.Read the final solution level
- Mass by volume percentage
- Grams of solute per 100 mL of final solution.
- Volume by volume percentage
- 100 × solute volume/final solution volume, with matching volume units.
4 g solute made up to 200 mL solution gives 2% m/v. 15 mL liquid solute made up to 150 mL solution gives 10% v/v. These are original arithmetic examples.
Use the final solution volume. Mixing liquid volumes does not always make a volume exactly equal to their sum. Mass percentage and volume percentage cannot be swapped.
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Question
What does 2% m/v mean?
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EXPLORE
There is a temperature-dependent limit
Separate how much dissolves from how fast it dissolves.Two hypothetical solubility curves
Solubility (g per 100 g water)
Temperature (°C)
- Solid A · Strong temperature dependence.
- Solid B · Weak temperature dependence.
Assumptions: Original teaching data for hypothetical solids, not measured values for named chemicals. Compare each curve at the same temperature.
Read the plotted values
Solid A
| Temperature (°C) | Solubility (g per 100 g water) |
|---|---|
| 20 | 20 |
| 40 | 35 |
| 60 | 60 |
| 80 | 85 |
Solid B
| Temperature (°C) | Solubility (g per 100 g water) |
|---|---|
| 20 | 28 |
| 40 | 29 |
| 60 | 30 |
| 80 | 31 |
Original teaching data for hypothetical solids, not measured values for named chemicals. Compare each curve at the same temperature.
- Solubility
- Maximum amount of a solute dissolved in a stated amount of solvent at stated conditions.
- Saturated solution
- At its dissolution limit for those conditions.
Stirring helps a solute dissolve faster but does not remove its equilibrium solubility limit. Many solid solutes become more soluble as temperature rises; not all behave alike.
Gas solubility often decreases as temperature rises, and pressure also matters. A solubility curve needs clear units, such as g solute per 100 g water.
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Question
Does vigorous stirring make unlimited salt dissolve in a fixed amount of water?
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EXPLORE
Cooling can leave excess solute as crystals
The solid amount follows the changing solubility.Some dissolved particles join an ordered solid
- Crystallisation
- Formation of an ordered solid from a solution or another appropriate phase.
Using the hypothetical A curve, a saturated solution in 100 g water holds 60 g at 60°C but only 35 g at 40°C. If water is not lost, 25 g can crystallise during cooling.
A suitable solvent and conditions help leave impurities behind. Slow formation often gives larger crystals; rapid formation can give smaller ones. Crystal purification is not simply heating everything to dryness.
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Question
How much hypothetical A can separate on cooling 60°C to 40°C?
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EXPLORE
Recover the solvent by condensing its vapour
Match the technique to the material you want.Follow vapour to the cooled condenser
- Model
- Conceptual apparatus, not a lab construction plan. Vapour leaves the mixture, cools in the condenser and becomes collected liquid. The cooling water remains separate from the vapour.
- Orange path
- Vapour travels along the inner tube and condenses into collected liquid; the blue jacket represents separate cooling water.
- Evaporation
- Liquid changes into vapour from its surface.
- Distillation
- Vaporise a component and condense the vapour for collection.
- Distillate
- Collected condensed liquid.
Evaporation can leave a dissolved nonvolatile solid behind but usually loses the solvent. Distillation can recover the volatile liquid through cooling and condensation.
Simple distillation is useful when volatility differs sufficiently. The book uses about 25°C boiling-point difference as a classroom guideline, not a universal law. Close-boiling mixtures may need fractional distillation, with repeated vaporisation and condensation.
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Question
Which method can collect water from a salt solution as a separate liquid?
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Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 8, 9, 10, 22.