Science · Class 8 · Chapter 9

The Amazing World of Solutes, Solvents, and Solutions

• Solute / Solvent — What dissolves / what it dissolves in.
• Solution — A uniform mixture of solute and solvent.
• Saturated / Unsaturated — Can't / can still dissolve more solute.
• Concentration — Amount of solute in a fixed quantity of solution.
• Solubility — Maximum solute dissolvable at a given temperature.
• Density — Mass per unit volume (Mass ÷ Volume).
• Relative density — Ratio of a substance's density to water's.
• Meniscus — The curved surface of a liquid in a container.

Chapter 9 · Concept 1 of 11

Solute, Solvent, and Solution

• Solute — The substance that dissolves.
• Solvent — The substance it dissolves in.
• Solution — The uniform mixture that results.

Chapter 9 · Concept 2 of 11

Saturated and Unsaturated Solutions

• Unsaturated — Can still dissolve more solute at that temperature.
• Saturated — No more solute can dissolve — any extra settles at the bottom.

Chapter 9 · Concept 3 of 11

Concentration and Solubility

• Concentration — The amount of solute in a fixed quantity of solution. Dilute means less solute; concentrated means more.
• Solubility — The maximum amount of solute that can dissolve in a fixed amount of solvent, at a given temperature.

Chapter 9 · Concept 4 of 11

Solubility Rises with Temperature

• Heating water lets it dissolve more baking soda — a saturated solution can become unsaturated simply by warming it up.
• True for most solids — As temperature rises, the solubility of most solid solutes in a liquid increases too.

Chapter 9 · Concept 5 of 11

Solubility of Gases

• Oxygen dissolves in water in small amounts — enough to sustain all aquatic life.
• Cold water holds more gas — Unlike solids, a gas's solubility in a liquid generally decreases as temperature rises — cold water holds more dissolved oxygen than warm water.

Chapter 9 · Concept 6 of 11

Why Do Objects Float or Sink?

• It's about density, not just weight — A wooden stick and an iron rod of the same size differ hugely in "heaviness" — husk and oil float, rice and stone sink. The real answer lies in density.

Chapter 9 · Concept 7 of 11

What Is Density?

• Density = Mass ÷ Volume — the mass packed into a unit of volume.
• Worked example — A sample with mass 16.4 g and volume 5 cm³ has a density of 16.4 ÷ 5 = 3.28 g/cm³. Density doesn't depend on shape or size — only on temperature and pressure.

Chapter 9 · Concept 8 of 11

Measuring Mass and Volume

• Regular solids — Mass is found using a balance. Volume is calculated directly: length × width × height.
• Irregular solids — Volume is found by water displacement — the rise in water level in a measuring cylinder after the object is submerged.

Chapter 9 · Concept 9 of 11

Relative Density

• Relative density = density of a substance ÷ density of water at that temperature.
• No units needed — It's just a ratio — for example, aluminium's relative density is about 2.7.

Chapter 9 · Concept 10 of 11

Temperature Changes Density

• Heating a substance spreads its particles apart — its volume grows, while its mass stays the same, so its density drops.
• Hot air balloons — Hot air is less dense than the cool air around it, so it rises — the same principle that lifts a hot air balloon.

Chapter 9 · Concept 11 of 11

Pressure, Density, and a Frozen Surprise

• Pressure mostly affects gases — Squeezing a gas packs its particles closer, raising its density a lot. Liquids barely compress, and solids hardly at all.
• Ice is less dense than water — As water freezes, its particles rearrange to take up more space — so ice floats, forming an insulating layer that protects life below in cold climates.

Quick Recap

Check Your Understanding

1. 1Oil poured into water floats on top. What does this tell you? (i) Oil is denser than water (ii) Water is denser than oil (iii) They have the same density (iv) Oil dissolves in water.
2. 2A stone sculpture weighs 225 g and has a volume of 90 cm³. Find its density, and predict whether it floats or sinks.
3. 3A 2-litre bottle has 500 mL of water poured into it. How much more water can it hold?