This MCQ module is based on: Concentration, Suspensions and Colloids
TOPIC 33 OF 50
Concentration, Suspensions and Colloids
🎓 Class 8
Science
CBSE
Theory
Ch 9 — Reaching the Age of Adolescence
⏱ ~25 min
🌐 Language: [gtranslate]
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9.7 How Strong? — Concentration of a Solution
Two glasses of lemonade look alike — but one may be gentle and the other fiercely tangy. They differ in concentration.
Concentration: The quantity of solute present in a definite amount of solution (or solvent). A dilute solution has little solute; a concentrated solution has a lot.
(a) Dilute vs Concentrated
If you dissolve half a spoon of sugar in a glass of water, it is a dilute solution. Add ten spoons and it becomes concentrated. These words are relative — they compare two solutions rather than give a fixed measurement.
(b) Mass Percentage
To put a number on concentration, chemists use a formula:
\( \text{Mass \%} = \dfrac{\text{Mass of solute}}{\text{Mass of solution}} \times 100 \)
Example: If 10 g of sugar is dissolved in 90 g of water, the mass of the solution is 10 + 90 = 100 g. Mass % of sugar = (10 / 100) × 100 = 10%.
(c) A quick mention of mole percentage
In higher classes you will meet mole percent, which counts the number of particles of solute and solvent rather than their mass. For now, remember that it is another way of expressing concentration by particle count instead of by mass.
9.8 Why Water is Called the "Universal Solvent"
Look around: tea, soft drinks, medicines, our blood, sea water, rain water — they are all water-based solutions. Water dissolves more substances than any other liquid on Earth. This is because water molecules are polar — one end slightly positive, the other slightly negative — so they grip both positive and negative parts of many solutes and pull them apart.
Because water is such an efficient solvent, rivers carry dissolved minerals to the sea, blood carries dissolved nutrients and oxygen through our body, and plants drink up dissolved nitrates from soil. Life as we know it would simply not exist without water's dissolving power.
9.9 Solution, Suspension and Colloid
Not every mixture is a true solution. Compare three glasses:
True Solution
Salt in water. Clear, transparent, particles smaller than 1 nm. Never settles; passes through filter paper.
Colloid
Milk, fog, smoke. Slightly cloudy. Particles (1 nm–1000 nm) stay suspended for a long time; pass through filter paper but scatter light (Tyndall effect).
Suspension
Muddy water, chalk in water. Turbid and opaque. Particles are visible, settle down on standing, can be filtered out.
| Property | True Solution | Colloid | Suspension |
|---|---|---|---|
| Particle size | < 1 nm | 1–1000 nm | > 1000 nm (visible) |
| Appearance | Clear, transparent | Translucent, slightly cloudy | Turbid, opaque |
| Settling | Never | Very slow or never | Settles on standing |
| Filter paper | Passes through | Passes through | Retained on paper |
| Tyndall effect | No | Yes (scatters light) | Yes, but particles visible |
| Examples | Salt water, sugar water, air | Milk, fog, smoke, toothpaste, blood | Muddy water, chalk-water, paint |
The Tyndall Effect
Shine a torch through a glass of salt water — the beam is invisible inside the liquid. Shine the same torch through a glass of milky water — the beam glows as a bright line! Colloidal particles are large enough to scatter light. This scattering is called the Tyndall effect, named after John Tyndall. You see the same effect when sunbeams stream through trees on a foggy morning, or through dusty air in a dark room.
🧪 Activity 9.3 — The Tyndall Torch Test
You need: 3 clean glasses, water, salt, a little milk, a pinch of soil, a small torch, a dark room.
- Glass A: dissolve a spoon of salt in water and stir till clear.
- Glass B: add a few drops of milk to water and stir.
- Glass C: add a pinch of garden soil to water and stir well; let it stand 1 minute.
- Darken the room. Shine a torch through each glass horizontally and look at the beam from the side.
Predict: In which glass will the torch beam be clearly visible as a line of light inside the liquid?
What you should see:
- Glass A (salt water) — no visible beam. Light passes through without scattering. This is a true solution.
- Glass B (milk + water) — a clear bright line of light inside the liquid. The milk particles scatter light — this is a colloid.
- Glass C (muddy water) — the beam is partly blocked; you can also see soil particles and a layer settling at the bottom. This is a suspension.
Why? True-solution particles are too tiny (~atomic size) to scatter visible light. Colloid particles are just big enough to bounce light in all directions, making the beam glow. Suspension particles are even larger — they settle and block light.
🧠 Competency-Based Questions
Scenario: Meera's kitchen experiments — she prepares sugar syrup, shakes salad-dressing (oil + vinegar), and mixes muddy water from the garden. She also looks at fog passing by the street lamp on a winter morning.
Q1. L1 Remember Write the formula for mass percentage of a solute.
Answer: Mass % = (mass of solute / mass of solution) × 100, where mass of solution = mass of solute + mass of solvent.
Q2. L2 Understand Calculate the mass % of a solution made by dissolving 20 g of salt in 80 g of water.
Answer: A. Mass of solution = 20 + 80 = 100 g. Mass % = (20/100) × 100 = 20%.
Q3. L3 Apply Meera sees fog glowing around a street lamp at dawn. Which property of which type of mixture does this demonstrate?
Answer: Fog is a colloid (tiny water droplets suspended in air). The glowing halo around the lamp is the Tyndall effect — colloidal droplets scattering light in all directions.
Q4. L4 Analyse Why is water called the universal solvent, even though it cannot dissolve oil or wax?
Answer: "Universal" here is relative — water dissolves more different substances than any other single liquid known, because its polar nature lets it break apart many ionic and polar solids (salt, sugar, acids, vitamins). It still cannot dissolve non-polar substances like oils and wax — but no other solvent comes close to its range.
Q5. L5 Evaluate Meera says milk is a "true solution" because it looks uniform. Do you agree? Defend your answer using two pieces of evidence.
Answer: Disagree. Evidence 1 — milk is translucent, not transparent; we cannot see clearly through it. Evidence 2 — a torch beam shining through milk glows (Tyndall effect). A true solution would be transparent and would not scatter light. Milk is actually a colloid (tiny fat droplets suspended in water).
🔗 Assertion–Reason Questions
Assertion (A): Water is called the universal solvent.
Reason (R): Water is a polar molecule that can pull apart many ionic and polar solutes.
Answer: A. Polarity is precisely the reason water dissolves so many substances.
Assertion (A): Muddy water is a colloid.
Reason (R): The particles in muddy water are large enough to be seen and will settle down on standing.
Answer: D. A is false — muddy water is a suspension, not a colloid. R is true and in fact supports the suspension classification.
Assertion (A): A torch beam is visible when passed through milk but invisible when passed through salt water.
Reason (R): Milk is a colloid whose particles scatter light (Tyndall effect), while salt water is a true solution whose particles are too small to scatter light.
Answer: A. This is exactly how the Tyndall effect distinguishes colloids from true solutions.
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