'The Amazing World of Solutes, Solvents, and Solutions' — Practice

Question 1

What is the universal solvent and why?

Accepted Answer

✦ Answer: WATER. It dissolves more substances than any other liquid because of its polar molecular structure (positive H end, negative O end). This is why it can dissolve ionic compounds like salt.

Question 2

Find concentration if 25 g salt is dissolved in 75 g water.

Accepted Answer

✦ Answer: Solution mass = 25 + 75 = 100 g. Concentration = (25/100) × 100 = 25% (m/m).

Question 3

Explain why cold drinks fizz when opened.

Accepted Answer

Step 1 — CO₂ is dissolved in the drink.
  Soft drinks have a lot of dissolved carbon dioxide gas, which gives them the fizz.

Step 2 — High pressure in bottle.
  Manufacturers seal drinks under HIGH pressure of CO₂. Higher pressure → MORE gas dissolved in liquid.

Step 3 — Opening releases pressure.
  When you open the cap, pressure drops to ATMOSPHERIC pressure (much lower).

Step 4 — CO₂ comes out as bubbles.
  Lower pressure can hold less dissolved gas. Excess CO₂ escapes as bubbles — the FIZZ.

Step 5 — Why shake creates more fizz?
  Shaking creates bubble-formation sites, helping CO₂ escape faster. That's why warm + shaken soda erupts violently.

Step 6 — Why cold drinks fizz better?
  Gases dissolve more in COLD liquids. Cold drinks retain CO₂ longer; warm drinks lose it faster.

✦ Answer: Cold drinks contain CO₂ dissolved under HIGH pressure. When opened, pressure drops to atmospheric, and excess CO₂ escapes as bubbles (fizz). Cold liquids hold more gas — so cold drinks stay fizzy longer than warm ones.

Question 4

Why does fish suffocate in heated/polluted water? Explain in terms of gas solubility.

Accepted Answer

Step 1 — Fish need dissolved oxygen.
  Fish breathe through gills, extracting OXYGEN that is DISSOLVED in water. They don't breathe air directly.

Step 2 — Dissolved oxygen depends on temperature.
  Solubility of gases DECREASES with temperature.
  • Cold water (10°C): contains ~11 mg O₂ per litre
  • Warm water (30°C): contains only ~7 mg O₂ per litre
  Less dissolved O₂ = less for fish to breathe.

Step 3 — Heat pollution.
  Industrial wastewater is often hot when released into rivers. This 'thermal pollution' raises water temperature, kicking out dissolved oxygen. Fish suffocate.

Step 4 — Organic pollution.
  Sewage and decaying organic matter cause bacteria to bloom. These bacteria consume dissolved oxygen. Less left for fish.

Step 5 — Combined effect.
  Hot, polluted water has VERY LOW dissolved O₂. Fish gasp at surface, then die. Mass fish kills are common in polluted Indian rivers (Ganga, Yamuna in some stretches).

Step 6 — Solutions.
  • Aerate water (artificial oxygenation) in fish farms
  • Reduce thermal and organic pollution
  • Plant submerged aquatic plants (photosynthesise → O₂)
  • Maintain cooler water in fish ponds

Step 7 — Indian context.
  River monitoring shows DO (dissolved oxygen) levels. Rivers like Yamuna have DO as low as 0-1 mg/L in some stretches — too low for fish to survive. Conservation requires reducing pollution.

✦ Answer: Fish breathe dissolved oxygen from water through gills. Gas solubility DECREASES with temperature, so warmer water has less dissolved O₂. Thermal pollution (hot industrial discharge) and organic pollution (sewage consumed by bacteria) reduce dissolved oxygen further. Fish suffocate — common in polluted Indian rivers. Solutions: aeration, reducing pollution, planting aquatic vegetation.

Practice — 'The Amazing World of Solutes, Solvents, and Solutions'

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