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Chapter 9 — The Amazing World of Solutes, Solvents & Solutions
Table of Contents
- 9.1 What Are Solute, Solvent & Solution?
- 9.2 How Much Solute Can a Solvent Dissolve?
- 9.2.1 Temperature & Solubility of Solids
- 9.3 Solubility of Gases
- 9.4 Why Do Objects Float or Sink?
- 9.5 What is Density?
- 9.5.1 Measuring Mass & Volume
- 9.5.2–3 Temperature & Pressure on Density
- 📌 Quick Summary & All Formulas
9.1 What Are Solute, Solvent & Solution?
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Fig 9.3 — Solute evenly distributed in Solvent
Salt in water, sugar in tea, ORS — all are solutions !
- Two or more substances mixed to form a uniform mixture
- Components are evenly distributed — can't see them separately
- Non-uniform: Sand + Water, Sawdust + Water, Chalk + Water
- Air is also a solution — a uniform mixture of gases!
📖 The 3 Key Terms —
solid / less qty liquidSolute
＋
liquid / more qtySolvent
→
uniform mixtureSolution
- Solute — substance that dissolves
- Solvent — substance that dissolves the solute
- Two liquids mixing → smaller quantity = Solute, larger = Solvent
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Fig 9.4 — Gulab Jamun Chashni (sugar syrup solution)
In Gulab Jamun Chashni — even though sugar is MORE than water, water is still the Solvent and sugar is the Solute!
ORS — salt + sugar + water. Every sip tastes same because all components are evenly distributed throughout the solution!
Key Definition
Solution = a uniform mixture where the solute dissolves completely in the solvent. Components are not visible separately. e.g. Salt water, sugar water, air (mixture of gases).
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9.2 How Much Solute Can a Solvent Dissolve?
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Fig 9.5 — Unsaturated Solution
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Fig 9.6 — Saturated Solution (undissolved salt at bottom)
Unsaturated Solution
More solute CAN still dissolve at given temperature
Saturated Solution
No more dissolves — excess settles at the bottom!
- Concentration = amount of solute in a fixed quantity of solution
- Dilute Solution — less solute dissolved
- Concentrated Solution — more solute dissolved
- Dilute & Concentrated are relative terms — compare to each other
Key Definition
Solubility = the maximum amount of solute that dissolves in a fixed quantity (100 mL) of solvent at a particular temperature. A saturated solution at one temp becomes unsaturated if temperature is raised!
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9.2.1 Temperature & Solubility of Solids
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Fig 9.7 — Dissolving baking soda by heating
🔬 Baking Soda + Water Experiment:
- At 20°C → some baking soda left undissolved
- Heat to 50°C → undissolved solid dissolves ✅
- Heat to 70°C → even more dissolves ✅✅
- Conclusion: More heat → more dissolving!
20°C
Least dissolves
🌡️→
Least dissolves
50°C
More dissolves
🌡️→
More dissolves
70°C
Even more! 🔥
Even more! 🔥
Key Rule
For most solid solutes: Solubility INCREASES as temperature increases. A saturated solution at a lower temperature becomes unsaturated at a higher temperature!
Asima Chatterjee — developed anti-epileptic & anti-malarial drugs using solvents & solutions to extract compounds from medicinal plants. First woman to win Shanti Swarup Bhatnagar Award in Chemical Science. Awarded Padma Bhushan.
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9.3 Solubility of Gases
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Fig 9.8 — Aquatic life survives on dissolved O₂
- Many gases like oxygen dissolve in water
- This dissolved O₂ sustains all aquatic life 🐟🐠
- Gas + Water = uniform mixture
- Gases dissolve evenly → it is a solution!
Gas Solubility — OPPOSITE to Solids!
For gases: Solubility DECREASES as temperature increases. Cold water holds more O₂ → supports aquatic life. Warm water → less O₂ dissolved → danger for fish!
| Solute Type | Temp ↑ | Solubility | Example |
|---|---|---|---|
| Solid in Liquid | Higher 🔥 | 📈 Increases | Salt, Sugar in Water |
| Gas in Liquid | Higher 🔥 | 📉 Decreases | O₂ in Water |
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9.4 Why Do Objects Float or Sink?
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Fig 9.9 — Some objects float, others sink
- Objects less dense than liquid → FLOAT 🛶
- Objects more dense than liquid → SINK 🪨
- Oil floats on water → oil is less dense than water
- Rice husk floats, rice grains sink → different densities!
Unpeeled orange floats — air pockets in peel lower its overall density. Peeled orange sinks — no air, denser than water! Density of substance is NOT the only factor — shape & trapped air also matter!
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9.5 What is Density?
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Fig 9.10a — Dense forest = high density
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Fig 9.10b — Less dense forest = low density
Density = Mass ÷ Volume
D = M / V | mass per unit volume of a substance
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Fig 9.11 — 1 litre oil = only 910g → Density < water!
- SI unit: kg/m³ | Common: g/mL or g/cm³
- Density is independent of shape & size
- Depends on temperature & (for gases) pressure
- 1 mL of water ≈ 1 g at room temperature
- 1 litre oil = 910 g → oil density < water density → oil floats!
🌍 Relative Density
Relative Density = D(substance) ÷ D(water)
No units | e.g. Aluminium = 2.7 g/cm³ → Rel. Density = 2.7
- Rel. Density < 1 → substance floats in water
- Rel. Density > 1 → substance sinks in water
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Fig 9.20 — Earth's layers, density increases toward centre
🌍 Earth's Layers — Density increases ↓
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9.5.1 Measuring Mass & Volume
Mass
Quantity of matter. Units: g / kg
Tool: Digital Weighing Balance
Tool: Digital Weighing Balance
Volume
Space occupied. Units: mL, cm³, L
Tool: Measuring Cylinder
Tool: Measuring Cylinder
⚖️ How to Measure Mass — Steps:
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Step 1: Switch ON → bring to zero
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Step 2: Place watch glass → tare/reset to zero
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Step 3: Place object → read mass (16.400 g)
🧪 How to Measure Volume of Liquids:
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Fig 9.15 — Measuring cylinders of different sizes
- Use a narrow transparent Measuring Cylinder
- Pour liquid slowly to the required mark
- Water forms a curved surface = Meniscus
- Read at bottom of meniscus (colourless liquids)
- Read at top of meniscus (coloured liquids)
- Eyes must be level with meniscus!
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Fig 9.16 — 100 mL measuring cylinder
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Fig 9.17 — Pouring water into cylinder
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Fig 9.18 — Reading the meniscus at eye level
📦 Volume of Regular Solids (Cuboid):
Volume = Length × Width × Height
V = l × w × h (in cm³)
🪨 Volume of Irregular Solids — Displacement Method:
- Fill cylinder with water → note initial level (A)
- Lower the solid into water using a thread
- Note the new water level → final level (B)
- Volume of object = B − A (mL = cm³ for solids)
Volume of Object = B − A
Final water level − Initial water level
Why are measuring cylinders narrow & tall? A narrow cylinder makes even a tiny volume change produce a bigger, more visible rise in liquid level → gives more accurate readings!
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9.5.2–3 Temperature & Pressure Effects on Density
Temperature ↑
Particles spread → Volume ↑ → Mass same → Density decreases
Temperature ↓
Particles closer → Volume ↓ → Mass same → Density increases
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Fig 9.22 — Hot air balloons rise due to lower density of hot air
Hot Air Balloon: Heated air expands → less dense than cool surrounding air → balloon rises! Same principle as hot air rising up from the ground.
🗜️ Effect of Pressure on Density:
| State | Pressure Effect | Reason |
|---|---|---|
| 🌬️ Gases | Large effect | Compressible → Vol ↓ → Density ↑ |
| 💧 Liquids | Small effect | Nearly incompressible |
| 🪨 Solids | Negligible | Particles already very close |
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Fig 9.23 — Ice floats on water (density of ice < liquid water)
Special Case — Ice floats on Water!
Water's density is highest at 4°C. When cooled to 0°C, water freezes → particles rearrange → takes more space (expansion) → same mass, more volume → density decreases → ice floats! 🧊
Ice layer on top keeps water underneath warm → fish & aquatic life survive in winter. ❄️🐟
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Fig 9.21 — Bamboo raft floats due to low density of bamboo
In ancient times, people used bamboo & wooden logs to travel rivers & seas. Bamboo is light, hollow & floats easily → less dense than water. People tied bamboo poles to make rafts for fishing, trading & crossing water bodies.
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Quick Summary & All Formulas
- Uniform mixture = Solution
- Solid in solution = Solute | Liquid = Solvent
- Saturated = no more solute dissolves at that temperature
- Unsaturated = more solute CAN still be dissolved
- Solid solubility ↑ with temperature ↑
- Gas solubility ↓ with temperature ↑
- Density = Mass / Volume
- Density ↓ when temperature ↑ (generally)
- Ice floats — water's density is highest at 4°C
- Pressure mainly affects density of gases; solids → negligible effect
All Formulas at a Glance
DensityD = Mass / Volume
Vol. of CuboidV = l × w × h
Displacement MethodVol = B − A (mL)
Relative DensityD(sub) ÷ D(water)