This MCQ module is based on: Solutions, Solutes and Solvents
TOPIC 31 OF 50
Solutions, Solutes and Solvents
🎓 Class 8
Science
CBSE
Theory
Ch 9 — Reaching the Age of Adolescence
⏱ ~26 min
🌐 Language: [gtranslate]
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Probe and Ponder
Imagine a warm afternoon on the coast of Gujarat. Workers wade into shallow, sparkling ponds carved out of the sea — the famous salt pans. Sea water is fed in, the sun and wind do their work for weeks, and finally only dazzling white crystals are left behind. Where did the water go? Where was the salt hiding all along? And if you tasted the water straight from the pan before the sun dried it, it would seem perfectly clear — no sand, no lumps — just a clean, salty liquid.
- When you stir sugar into a glass of water and it disappears, where does it go?
- Why does sea water look just like plain water even though it contains so much salt?
- Is air — the mixture you are breathing this very second — also a kind of solution?
- Why do oil and water refuse to mix, while lemon juice and water mix easily?
In this chapter we will get to know solutions — everyday mixtures that are so smoothly blended that you cannot spot their parts with naked eyes.
9.1 What is a Solution?
Take a clean glass, half-fill it with water, and sprinkle in a spoon of common salt. Stir. In a few seconds the salt disappears — yet if you sip the water, it tastes unmistakably salty. The salt has not vanished. Its tiny particles have slipped in between the water particles, spreading so evenly that light passes through without any cloudiness.
Solution: A homogeneous mixture of two or more substances, in which every drop or puff has exactly the same composition. The parts cannot be separated by simply filtering or by letting them stand.
A solution has two (or more) roles to play. The substance that dissolves — usually the smaller amount — is called the solute. The substance that dissolves it — usually the larger amount — is called the solvent.
Solute + Solvent → Solution
Salt (solute) + Water (solvent) → Salt water (solution)
Sugar (solute) + Water (solvent) → Sugar syrup (solution)
Oxygen (solute) + Nitrogen (solvent) → Air (solution)
Salt (solute) + Water (solvent) → Salt water (solution)
Sugar (solute) + Water (solvent) → Sugar syrup (solution)
Oxygen (solute) + Nitrogen (solvent) → Air (solution)
Is air a solution too?
Absolutely! The air around you is a homogeneous mixture of gases — about 78% nitrogen, 21% oxygen, and small amounts of argon, carbon dioxide and water vapour. Because nitrogen is the largest share, it acts as the solvent, while oxygen and the other gases are the solutes.
9.2 Types of Solutions
We naturally think of a solution as a solid dissolved in a liquid — like salt in water. But solutions can form in any combination of solid, liquid or gas. The table below lists the main types you will meet.
| Solute | Solvent | Type of Solution | Everyday Example |
|---|---|---|---|
| Gas | Gas | Gas in gas | Air (O2 + other gases in N2) |
| Gas | Liquid | Gas in liquid | Fizzy cola (CO2 in water); O2 dissolved in river water |
| Liquid | Liquid | Liquid in liquid | Vinegar (acetic acid in water); lemonade |
| Solid | Liquid | Solid in liquid | Salt water; sugar syrup; iodine tincture |
| Solid | Solid | Solid in solid (alloy) | Brass (Zn in Cu); stainless steel (C, Cr in Fe) |
| Gas | Solid | Gas in solid | Hydrogen dissolved in palladium metal |
Cola fizz
Carbon dioxide gas squeezed into sweet water — a gas-in-liquid solution that bubbles out the moment you open the bottle.
Air in your lungs
A gas-in-gas solution. Every breath delivers an oxygen dose from the nitrogen "solvent" of the atmosphere.
Brass bell
A solid-in-solid solution (alloy): zinc atoms scattered evenly among copper atoms give the bell its golden ring.
9.3 Miscible and Immiscible Liquids
When two liquids meet, either they mingle smoothly or they keep their distance. Chemists describe this with two words.
Miscible liquids: Liquids that dissolve into each other in every proportion, forming a single clear layer. Examples — water & alcohol, water & vinegar.
Immiscible liquids: Liquids that refuse to mix and stay in separate layers, usually with the lighter one floating on top. Examples — oil & water, kerosene & water.
Immiscible liquids: Liquids that refuse to mix and stay in separate layers, usually with the lighter one floating on top. Examples — oil & water, kerosene & water.
🧪 Activity 9.1 — Which Liquids Mix?
You need: 5 test tubes, water, kitchen oil, vinegar, milk, kerosene (optional — adult help), a spoon.
- Fill each test tube half-way with water.
- Add 2 spoons of oil to tube 1, vinegar to tube 2, milk to tube 3, kerosene to tube 4, and sugar to tube 5.
- Shake each tube well and let it stand for 2 minutes.
- Observe whether you see one layer or two layers in each tube.
Predict: Which tubes will show a single clear layer and which will separate into two layers?
What you should see:
- Tube 1 (oil + water) → two layers, oil floats. Immiscible.
- Tube 2 (vinegar + water) → single clear layer. Miscible.
- Tube 3 (milk + water) → milky, cloudy throughout. This is actually a colloid, not a true solution.
- Tube 4 (kerosene + water) → two layers. Immiscible.
- Tube 5 (sugar + water) → sugar dissolves, single clear layer. A true solution.
Why? Liquids mix when their particles attract each other strongly enough to slip between each other's spaces. Oil and water particles do not attract well — they refuse to mingle.
🧠 Competency-Based Questions
Scenario: Riya prepares four glasses: (i) salt in water, (ii) oil in water, (iii) sugar in tea, (iv) sand in water. She stirs each for a minute and inspects them against a bright window.
Q1. L1 Remember Define "solute" and "solvent" in one line each.
Answer: The solute is the substance that dissolves (usually the smaller amount). The solvent is the substance that does the dissolving (usually the larger amount).
Q2. L2 Understand Which of Riya's glasses will form a true solution?
Answer: B. Salt in water and sugar in tea both dissolve completely to form clear, homogeneous mixtures. Oil floats on water (immiscible) and sand settles at the bottom (suspension).
Q3. L3 Apply In a brass ornament (70% Cu, 30% Zn), identify the solute and solvent.
Answer: Copper is present in greater amount (70%), so Cu is the solvent. Zinc (30%) is the solute. Brass is therefore a solid-in-solid solution, also called an alloy.
Q4. L4 Analyse Sea water looks perfectly clear, yet 3.5 g of salt is present in every 100 g of it. Why cannot we see the salt?
Answer: In a true solution the solute particles are so tiny (of the size of individual ions and molecules) that they slip between the water particles and cannot scatter light. Because nothing scatters or blocks the light passing through, the mixture appears transparent.
Q5. L5 Evaluate A student claims, "Air is just a gas, not a solution." Do you agree? Justify.
Answer: Disagree. Air is a homogeneous mixture of nitrogen, oxygen, argon, carbon dioxide and water vapour. Since nitrogen is the largest share it acts as solvent and the rest as solutes — making air a classic gas-in-gas solution.
🔗 Assertion–Reason Questions
Assertion (A): A solution is a homogeneous mixture.
Reason (R): The solute particles in a solution are evenly distributed throughout the solvent.
Answer: A. Even distribution is exactly what makes a mixture "homogeneous".
Assertion (A): Oil and water are miscible liquids.
Reason (R): Oil is less dense than water and floats on top.
Answer: D. Oil and water are immiscible (A is false). R is correct — oil does float because it is less dense.
Assertion (A): Brass is an example of a solid-in-solid solution.
Reason (R): Brass is a chemical compound of copper and zinc.
Answer: C. Brass is an alloy (solid solution), not a compound — the ratio of Cu to Zn can be varied, which a compound cannot allow.
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