The Amazing World of Solutes, Solvents, and Solutions
Every sip of homemade ORS tastes exactly the same, no matter how much you drink. Why is that — and why do sugar and salt dissolve in water, but not in oil?
Key idea
Solute, Solvent, and Solution
1
Solute
The substance that dissolves.
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Solvent
The substance it dissolves in.
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Solution
The uniform mixture that results.
Key idea
Saturated and Unsaturated Solutions
1
Unsaturated
Can still dissolve more solute at that temperature.
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Saturated
No more solute can dissolve — any extra settles at the bottom.
Key idea
Concentration and Solubility
1
Concentration
The amount of solute in a fixed quantity of solution. Dilute means less solute; concentrated means more.
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Solubility
The maximum amount of solute that can dissolve in a fixed amount of solvent, at a given temperature.
Key idea
Solubility Rises with Temperature
Heating water lets it dissolve more baking sodaa saturated solution can become unsaturated simply by warming it up.
True for most solidsAs temperature rises, the solubility of most solid solutes in a liquid increases too.
Key idea
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.
Key idea
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.
Key idea
What Is Density?
1
Density = Mass ÷ Volume
the mass packed into a unit of volume.
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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.
Key idea
Measuring Mass and Volume
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Regular solids
Mass is found using a balance. Volume is calculated directly: length × width × height.
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Irregular solids
Volume is found by water displacement — the rise in water level in a measuring cylinder after the object is submerged.
Key idea
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.
Key idea
Temperature Changes Density
Heating a substance spreads its particles apartits volume grows, while its mass stays the same, so its density drops.
Hot air balloonsHot air is less dense than the cool air around it, so it rises — the same principle that lifts a hot air balloon.
Key idea
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.
Chapter · Key terms to remember
Key Terms
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.
Chapter · Key terms to remember
More Key Terms
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).
Chapter · Key terms to remember
More Key Terms
Relative density
Ratio of a substance's density to water's.
Meniscus
The curved surface of a liquid in a container.
Questions for your notebook
Write these down, then discuss
1
Oil 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
A stone sculpture weighs 225 g and has a volume of 90 cm³. Find its density, and predict whether it floats or sinks.
3
A 2-litre bottle has 500 mL of water poured into it. How much more water can it hold?
Project as-is — students copy the questions, then the class discusses answers together.