📚 SCIENCE EXAMINATION
Chapter 2: Is Matter Around Us Pure?
Class: IX | Total Marks: 80 | Time: 3 Hours
Roll No: ___________ Name: ___________________ Date: __________
✏️ QUESTIONS
SECTION A: MCQs (1 Mark × 20) = 20 Marks
Q1. What is the scientific definition of a pure substance?
1
a) A substance with no adulteration
b) A substance where all particles are chemically identical
c) Any substance from nature
d) A substance that is white in color
Q2. Which of the following is a homogeneous mixture?
1
a) Milk b) Soil c) Salt water d) Oil and water
Q3. The diameter of particles in a true solution is:
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a) Greater than 100 nm b) Less than 1 nm c) Between 1 nm and 100 nm d) Greater than 1000 nm
Q4. Which mixture can be separated by filtration?
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a) Solution b) Colloid c) Suspension d) All of the above
Q5. The Tyndall effect is observed in:
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a) True solutions b) Colloids c) Both solutions and colloids d) Neither
Q6. In a solution, the component present in larger quantity is:
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a) Solute b) Solvent c) Both equal d) Cannot be determined
Q7. Which is an example of a gas in liquid solution?
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a) Sugar in water b) Soda water c) Brass d) Air
Q8. A saturated solution at a given temperature:
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a) Can dissolve more solute without heating b) Has dissolved maximum solute
c) Contains no solute d) Is always white in color
Q9. Which can be separated by centrifugation?
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a) True solutions b) Colloidal solutions c) Suspensions d) All of the above
Q10. Air is an example of which type of solution?
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a) Solid in liquid b) Gas in gas c) Liquid in gas d) Solid in solid
Q11. A colloid appears homogeneous but is actually:
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a) Homogeneous b) Heterogeneous c) Pure substance d) An element
Q12. Which property distinguishes a mixture from a compound?
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a) Mixtures have variable composition b) Compounds have fixed composition
c) Both a and b d) Mixtures are always pure
Q13. Example of a colloid is:
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a) Pure water b) Salt solution c) Milk d) Oil and water
Q14. The solubility of most substances increases with:
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a) Decrease in temperature b) Increase in temperature
c) Change in pressure only d) Remains unchanged
Q15. Which mixture shows the Tyndall effect?
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a) Salt water b) Sugar solution c) Fog d) All of the above
Q16. Alloys are examples of which type of solution?
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a) Liquid in liquid b) Solid in liquid c) Solid in solid d) Gas in solid
Q17. The main components of air by percentage are:
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a) Oxygen 78%, Nitrogen 21% b) Nitrogen 78%, Oxygen 21%
c) Carbon dioxide 78%, Oxygen 21% d) Nitrogen 50%, Oxygen 50%
Q18. A mixture can be separated by physical methods because:
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a) No chemical bond is formed b) Components retain their properties c) Both a and b d) It is always a solution
Q19. Which has a fixed composition?
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a) Mixture b) Solution c) Compound d) Suspension
Q20. The concentration formula (Mass by mass percentage) is:
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a) (Mass of solvent / Mass of solution) × 100
b) (Mass of solute / Mass of solution) × 100
c) (Mass of solute / Mass of solvent) × 100
d) (Volume of solute / Volume of solvent) × 100
SECTION B: Short Answer (3 Marks × 10) = 30 Marks
Q21. Differentiate between a pure substance and a mixture with two examples each. 3
Q22. What is the difference between solute and solvent? Give one example each. 3
Q23. Explain why milk is considered a mixture and not a pure substance. 3
Q24. What is the Tyndall effect? How can it distinguish between solution and colloid? 3
Q25. What happens when a suspension is left undisturbed? Explain with an example. 3
Q26. What is the difference between saturated and unsaturated solution? How to convert? 3
Q27. Give three properties that distinguish a colloid from a suspension. 3
Q28. Write steps to identify whether a mixture is solution, suspension, or colloid. 3
Q29. What is an alloy? Give two examples and mention their uses. 3
Q30. A solution contains 50 g salt in 450 g water. Calculate concentration (mass by mass %). 3
SECTION C: Long Answer (5 Marks × 6) = 30 Marks
Q31. Explain classification of mixtures into homogeneous and heterogeneous with 3+ examples each and key characteristics. 5
Q32. What are important properties of true solutions? Explain why cannot be filtered but can be separated by evaporation. 5
Q33. Describe Tyndall effect with detailed explanation and three real-world examples. 5
Q34. Compare and contrast solutions, suspensions, colloids with detailed comparison table. 5
Q35. Explain difference between mixture and compound with examples. Why physical vs chemical separation? 5
Q36. Discuss solubility concept. How does temperature affect solubility of solids, liquids, and gases? Explain with examples. 5
✅ ANSWER KEY WITH EXPLANATIONS
SECTION A: MCQ Answers
Q1: (b) A substance where all particles are chemically identical
Explanation: Pure substances have all particles chemically identical. Even labeled "pure," milk contains water, fat, proteins, etc., making it a mixture.
Q2: (c) Salt water
Explanation: Salt water is homogeneous with uniform composition. Milk is colloid, soil and oil-water are heterogeneous.
Q3: (b) Less than 1 nm
Explanation: True solution particles are < 1 nm, making them invisible and unable to scatter light.
Q4: (c) Suspension
Explanation: Suspensions have large visible particles that filter paper can catch. Solutions particles are too small, colloids also cannot be filtered.
Q5: (b) Colloids
Explanation: Tyndall effect occurs in colloids where medium-sized particles scatter light. Solutions particles too small, no scattering.
Q6: (b) Solvent
Explanation: Solvent is present in larger quantity and dissolves solute. Example: In salt water, water is solvent, salt is solute.
Q7: (b) Soda water
Explanation: Soda water is CO₂ (gas) dissolved in water (liquid) - perfect gas in liquid solution example.
Q8: (b) Has dissolved maximum solute
Explanation: Saturated solution has maximum solute at that temperature. Adding more won't dissolve; heating can dissolve more.
Q9: (b) Colloidal solutions
Explanation: Centrifugation separates colloids by spinning fast - particles pushed to outside and settle. For solutions: particles too small; suspensions: simple filtration works.
Q10: (b) Gas in gas
Explanation: Air is homogeneous mixture of gases: N₂ (78%), O₂ (21%), others (1%).
Q11: (b) Heterogeneous
Explanation: Colloids appear uniform but are heterogeneous microscopically. Milk has tiny fat droplets in water.
Q12: (c) Both a and b
Explanation: Key property: Mixtures have variable composition (any ratio), compounds have fixed composition (always same ratio).
Q13: (c) Milk
Explanation: Milk is classic colloid - tiny fat droplets scatter light (Tyndall), too small to see individually, cannot be filtered.
Q14: (b) Increase in temperature
Explanation: For most solids, solubility increases with temperature. Gases show opposite (decrease). Heat provides energy for dissolving.
Q15: (c) Fog
Explanation: Fog is colloid (water droplets in air) and shows Tyndall effect. Salt/sugar solutions don't scatter light.
Q16: (c) Solid in solid
Explanation: Alloys blend two metals (solids). Example: Brass (Cu 70% + Zn 30%). Even though solid, they form solutions.
Q17: (b) Nitrogen 78%, Oxygen 21%
Explanation: Air composition: N₂ 78%, O₂ 21%, Others 1% (Ar, CO₂). We breathe O₂ but N₂ is main component.
Q18: (c) Both a and b
Explanation: Mixtures separate by physical methods because: no chemical bonds formed, components retain properties. Salt recoverable from salt water by heating.
Q19: (c) Compound
Explanation: Compounds have fixed composition (H₂O always 2:1 ratio). Mixtures have variable composition.
Q20: (b) (Mass of solute / Mass of solution) × 100
Explanation: Mass by mass percentage = (Mass of solute / Mass of solution) × 100, where Mass of solution = Mass of solute + Mass of solvent.
SECTION B: Short Answer Solutions
Q21: Pure Substance vs Mixture
Pure Substances: Contain one particle type, fixed composition
Examples: Sodium chloride, Distilled water, Pure iron, Oxygen gas
Mixtures: Contain 2+ substances, variable composition
Examples: Salt water, Milk, Air, Soil
Key Difference: Pure substances have fixed composition, mixtures have variable composition.
Q22: Solute vs Solvent
Solute: Component in smaller quantity that gets dissolved
Solvent: Component in larger quantity that dissolves solute
| Example | Solute | Solvent |
|---|---|---|
| Salt water | Salt (NaCl) | Water |
| Sugar solution | Sugar | Water |
| Soda water | CO₂ (gas) | Water |
Q23: Why Milk is Mixture
Milk is a mixture because it contains multiple components: water, fats, proteins, lactose, minerals in variable ratios. Components are physically mixed, not chemically bonded. All parts retain individual properties. Therefore, milk doesn't meet the definition of pure substance which requires all particles to be chemically identical.
Q24: Tyndall Effect
Tyndall Effect: Scattering of light by colloidal particles when light beam passes through. The light path becomes visible and glows.
How to distinguish:
- Solution: Light passes straight - path NOT visible (particles too small)
- Colloid: Light gets scattered - path IS visible and bright (medium-sized particles)
Quick Test: Shine light beam through mixture. If path glows and is clearly visible → Colloid. If not visible → Solution.
Q25: Suspension Settling
What happens: Solid particles gradually settle to bottom due to gravity. Mixture becomes unstable and separates into clear liquid (top) and settled particles (bottom).
Example - Chalk in Water:
- Initially: Cloudy white everywhere
- After 5-10 minutes: Chalk particles settle at bottom
- Finally: Clear water on top, chalk powder at bottom
This is why: We shake medicine and paint bottles before use!
Q26: Saturated vs Unsaturated Solution
Saturated Solution: Contains maximum solute at given temperature. No more dissolves.
Unsaturated Solution: Contains less solute than maximum. More can dissolve.
How to convert Unsaturated → Saturated:
- Add more solute to solution
- Heat the solution (increases solubility)
- Evaporate some solvent
Q27: Colloid vs Suspension Properties
Three Key Differences:
| Property | Suspension | Colloid |
|---|---|---|
| Particle Size | > 100 nm (very large) | 1-100 nm (medium) |
| Visibility | Easily visible | Too small individually |
| Stability | Unstable - settles | Stable - doesn't settle |
Q28: Identifying Mixtures
Step 1: Visual Observation
- If uniform and clear → Solution or Colloid (go to Step 2)
- If cloudy with visible particles → Suspension
Step 2: Tyndall Effect Test
- Direct light beam through mixture
- If path visible and glowing → Colloid
- If path invisible → Solution
Q29: Alloys
What is an Alloy? Solid solution made by mixing 2+ metals in molten state then cooling. Solid-in-solid solution.
| Alloy | Composition | Uses |
|---|---|---|
| Brass | Cu 70% + Zn 30% | Door handles, Musical instruments |
| Bronze | Cu + Tin | Sculptures, Bells |
| Steel | Fe + C | Buildings, Tools, Vehicles |
Q30: Concentration Calculation
Given: Salt = 50 g, Water = 450 g
Step 1: Total mass = 50 + 450 = 500 g
Step 2: Concentration = (50/500) × 100 = 10%
The solution is 10% by mass
SECTION C: Long Answer Solutions (Summaries)
Q31: Classification of Mixtures
Homogeneous Mixtures: Uniform composition, appears same everywhere
- Salt water, Sugar solution, Soda water, Air, Brass
Heterogeneous Mixtures: Non-uniform, distinct components visible
- Oil & water, Sand & gravel, Soil, Milk (colloid), Chalk in water (suspension)
Key Characteristics: Homogeneous = uniform throughout, Heterogeneous = components distinguishable
Q32: Properties of True Solutions
Properties:
- Homogeneous mixture with uniform composition
- Particles < 1 nm - cannot see with naked eyes
- Do NOT scatter light - no Tyndall effect
- Stable - particles never settle
- Cannot be filtered (particles too small)
- Can be separated by evaporation (solvent evaporates, solute remains)
Why not filterable: Particles smaller than filter paper pores
Why can evaporate: Heat drives off solvent, leaving dissolved solute behind
Why can evaporate: Heat drives off solvent, leaving dissolved solute behind
Q33: Tyndall Effect - Detailed
Definition: Scattering of light by colloidal particles. Path of light becomes visible and glows.
Why Happens: Colloidal particles (1-100 nm) comparable to light wavelength. They scatter light in all directions, making path visible.
Real-World Examples:
- Forest Sunlight: Sunlight through dense forest with mist shows bright "god rays"
- Dark Room Light: Light beam through small window shows path due to dust/smoke
- Fog/Headlights: Fog (water droplets) scatters headlights, reducing visibility
Q34: Solutions vs Suspensions vs Colloids
| Feature | Solution | Suspension | Colloid |
|---|---|---|---|
| Appearance | Uniform | Cloudy | Uniform (but hetero) |
| Particle Size | < 1 nm | > 100 nm | 1-100 nm |
| Visibility | Invisible | Visible | Not individually |
| Light Scatter | No | Yes | Yes (Tyndall) |
| Stability | Stable | Unstable | Stable |
| Filtration | Cannot | Can | Cannot |
| Examples | Salt water, air | Chalk in water | Milk, fog, smoke |
Q35: Mixture vs Compound
| Feature | Mixture | Compound |
|---|---|---|
| Formation | Components mix - NO reaction | Elements REACT chemically |
| Composition | VARIABLE - any ratio | FIXED - always same ratio |
| Properties | Shows original properties | TOTALLY DIFFERENT properties |
| Separation | Easy - PHYSICAL methods | Difficult - CHEMICAL methods |
Example: Salt water (mixture) vs NaCl (compound) - Salt water is variable ratio, salts are always Na:Cl ratio
Q36: Solubility and Temperature
Solubility: Maximum grams of solute dissolving in 100g solvent at given temperature.
SOLIDS (like salt, sugar): Solubility INCREASES with temperature
- Reason: Heat provides energy, particles dissolve faster
- Practical: Hot water dissolves more salt than cold
GASES (like CO₂, O₂): Solubility DECREASES with temperature
- Reason: Heat causes gas molecules to escape faster
- Practical: Warm soda goes flat - CO₂ escapes
Real Examples: Hot tea dissolves sugar faster | Boiled water tastes different (less O₂) | Fish tank needs more aeration when warm
