9.7 Going Deeper: Enrichment & Exam Preparation

Solutions & Density Around You

Everyday Chemistry & Physics Your tea and ORS are solutions; the fizz in a cold drink is gas dissolved under pressure (and why a warm drink goes flat faster — gases leave warm water!). Oil floating on water, ice cubes bobbing, a boat carrying its load, the husk rising when you wash rice — all are density at work. Two ideas, endless everyday examples.

Quick Reference

Idea In one line
Solution Uniform mixture (solute + solvent)
Solute / solvent The dissolved substance / the dissolving liquid
Saturated No more solute will dissolve (at that temperature)
Solubility Maximum solute that dissolves in a fixed solvent
Solids ↑ temp Solubility increases
Gases ↑ temp Solubility decreases (cold water holds more O₂)
Density mass ÷ volume (g/cm³, kg/m³)
Float / sink Less dense than water floats; denser sinks
Heating Volume up → density down
Ice Less dense than water (expands on freezing) → floats

Memory Tricks & One-Page Revision

Revision Card

  • Solute + Solvent → Solution. Solid in liquid: solid = solute. Two liquids: smaller amount = solute.
  • Unsaturated = room for more; Saturated = full (at that temperature).
  • Solids like it hot, gases like it cold. (Solid solubility ↑ with temp; gas solubility ↓.)
  • Density = Mass ÷ Volume. Water ≈ 1 g/cm³.
  • Float if density < 1 g/cm³; sink if > 1.
  • Heat → bigger volume → lower density (hot-air balloon, hot air rises).
  • Pressure squeezes gases most, liquids little, solids negligibly.
  • Ice floats because water expands on freezing (least dense as a solid; densest at 4 °C).

Spot the Mistake

Find the Flaw

  1. “Oxygen dissolves better in hot water.” — False: gas solubility falls with heat; cold water holds more oxygen.
  2. “An unsaturated solution holds more solute than a saturated one.” — False: a saturated solution holds the maximum; an unsaturated one can still take more.
  3. “Flattening a lump of clay lowers its density.” — False: changing shape changes neither mass nor volume, so density is unchanged.

Test Your Reflexes

A quick-fire drill: an object’s density is given. Will it Float or Sink in water (1 g/cm³)? Build a streak!

Float or sink in water?
Less dense than water (1 g/cm³) floats; denser sinks.
Score 0 · Streak 0

Exam Corner: CBSE-Style Practice

Mixed Practice (objective, short, long, HOTS)

Objective type (1 mark each)

  1. In a solution of sugar in water, the sugar is the ______ and the water is the ______.
  2. Density = ______ ÷ ______.
  3. As temperature rises, the solubility of a gas in water generally ______.

Short answer (2 marks each)

  1. Differentiate between a saturated and an unsaturated solution.
  2. Why does cold water support more aquatic life than warm water?

Long answer (3 marks each)

  1. An object has mass 400 g and volume 40 cm³. Find its density, and state whether it floats or sinks in water (1 g/cm³).
  2. Explain why ice floats on water, and why this is important for aquatic animals.

HOTS (Higher Order Thinking)

  1. An iron block has mass 600 g and density 7.9 g/cm³. Find its volume. (Hint: Volume = Mass ÷ Density.)
  2. Reema moulds 120 g of clay into a cube of 60 cm³, then flattens it into a sheet. What happens to its density, and why?

Assertion–Reason (Choose: (a) both true, R explains A; (b) both true, R doesn’t explain A; (c) A true, R false; (d) A false, R true.)

  1. Assertion (A): Oil floats on water.   Reason (R): Oil is less dense than water.
  1. solute … solvent.
  2. mass ÷ volume.
  3. decreases.
  4. A saturated solution holds the maximum solute possible at that temperature (no more dissolves); an unsaturated solution can still dissolve more.
  5. Gas solubility falls as water warms, so cold water holds more dissolved oxygen — supporting more fish and aquatic life. Warm water holds less oxygen.
  6. Density = 400 ÷ 40 = 10 g/cm³. Since 10 > 1 g/cm³, it sinks.
  7. Water expands on freezing, so ice is less dense than liquid water and floats. A floating ice layer insulates the water below, keeping it liquid so fish can survive winter.
  8. Volume = Mass ÷ Density = 600 ÷ 7.9 ≈ 75.9 cm³.
  9. No change. Flattening alters neither the mass (120 g) nor the volume (60 cm³), so the density (120 ÷ 60 = 2 g/cm³) stays the same — density doesn’t depend on shape.
  10. (a) — Both true and R explains A: being less dense than water is exactly why oil floats.

Glossary

Key Terms

  • Solution: a uniform mixture of solute and solvent.
  • Solute / solvent: the substance that dissolves / the one it dissolves in.
  • Saturated / unsaturated: holding the maximum solute / able to hold more.
  • Solubility: the maximum solute that dissolves in a fixed solvent at a temperature.
  • Concentration: the amount of solute in a fixed quantity of solution (dilute or concentrated).
  • Density: mass per unit volume (mass ÷ volume).
  • Relative density: density of a substance ÷ density of water (a unit-less number).
  • Meniscus: the curved surface of a liquid in a measuring cylinder.