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Lesson 5 of 6 · Textbook 3.3

Solutions, dissolving and solubility

How fast something dissolves and how much can dissolve are two different questions with two different sets of answers. Mixing them up is the classic mistake in this chapter.

Solute, solvent, solution

Three words A solution is a mixture in which one substance dissolves completely in another. The substance that dissolves is the solute; the substance it dissolves in is the solvent.

Sugar in water: sugar is the solute, water is the solvent, and the result is sugar solution. Mineral water and soda water are solutions too. Most solvents are liquids, and water dissolves so many things it is called the universal solvent.

In a solution the solute particles cannot be seen — they are spread out among the solvent particles — and if you shine a torch through it, light passes through fully.

Dilute and concentrated

For a fixed volume of solvent: a small amount of dissolved solute gives a dilute solution; a large amount gives a concentrated one. Same solute, same solvent — different proportion. (Which, incidentally, proves a solution is a mixture: a compound could not vary like that.)

Rate of dissolving — how fast

A substance that dissolves completely in a solvent is soluble in it; one that does not is insoluble. The rate of dissolving is how quickly a soluble solute dissolves. Three things speed it up:

Rate of dissolving

5 g of sugar in 100 cm³ of water
Predicted time to dissolve—
StatusReady.
Notice what does not change: it is 5 g of sugar every time, and all 5 g dissolves every time. The controls change how fast, never how much.
FactorEffect on rate of dissolving
Size of solute particlesThe smaller the particles, the faster the solute dissolves — powder beats a cube.
Temperature of solventThe higher the temperature, the faster the solute dissolves — hot water beats cold.
Rate of stirringThe faster the stirring, the faster the solute dissolves.

Solubility — how much

Definition — the one G2 skips and G3 must know Solubility is the largest amount of solute that can dissolve in a fixed volume of a particular solvent at a specific temperature.

Solubility is different from rate of dissolving. Stirring makes sugar dissolve faster, but it does not let you dissolve more sugar than the water can hold. Keep adding spoonfuls and see what happens.

Solubility

Spoonfuls dissolved0
Spoonfuls undissolved0

Numbers are illustrative, not measured — but the pattern is the textbook's: sugar is more soluble in water than salt, chalk is insoluble in water, and salt dissolves in water but not in oil.

Two things decide solubility:

The two lists, side by side

Rate of dissolving (how fast) depends on: particle size · temperature of solvent · rate of stirring.

Solubility (how much) depends on: type of solute · type of solvent.

Stirring is not on the solubility list. Temperature is not on the solubility list in your textbook — the definition fixes it (“at a specific temperature”). If a question asks what affects solubility and offers “stirring”, that is the wrong answer every time.

Check yourself