S.1 Solutions & Solubility

UNIT SYLLABUS

MYP

Dissolving is chemistry's quiet workhorse: a solute disperses particle-by-particle through a solvent until it forms a uniform solution. Understanding the vocabulary — solute, solvent, saturated, concentration — lets you handle everything from making a drink to preparing laboratory reagents. Solubility varies with temperature (most solids dissolve better hot; gases dissolve better cold, which is why warm fizzy drinks go flat), and solubility curves turn that behaviour into readable graphs. Concentration calculations connect the chemistry to real quantities.

Guiding Questions

? Where does the sugar go when it dissolves — and why can water only hold so much?
? Why does a warm soft drink lose its fizz faster than a cold one?

✓ What the IB expects you to master

Define solute, solvent, solution, and saturated solution.
Explain dissolving with particle theory: solvent particles surround and separate solute particles.
Distinguish dilute from concentrated solutions, and calculate concentration in g/dm³.
Describe how temperature affects the solubility of solids (usually increases) and gases (decreases).
Read and interpret solubility curves, including predicting crystallisation on cooling.
Explain why the mass is conserved when a solute dissolves.
Recognise water as an exceptional solvent and its implications for life and pollution.

1 Key Formulas

Concentration

c = \frac{\text{mass of solute (g)}}{\text{volume of solution (dm}^3)}

2 Exam Preparation & Topic Explanations

Solubility curves and concentration sums (Criteria A & C)

Solubility-curve questions follow a routine: read the solubility at each temperature, scale to the actual mass of water in the question, and subtract for crystallisation amounts. Show every step.

For concentration, keep one rule in view: volumes in dm³ (divide cm³ by 1000). g/dm³ is the MYP standard unit.

Pro Exam Strategy

Saturated means "holding the maximum at that temperature" — always attach the temperature.
Solids: usually more soluble when hot. Gases: always less soluble when hot. State the difference explicitly.
If a question gives 250 cm³, convert to 0.25 dm³ before anything else.
Global-context link (Criterion D): thermal pollution, ocean oxygen levels, and fizzy drinks all hinge on gas solubility.

3 MCQ Practice

Q1. 25 g of salt is dissolved in water to make 0.50 dm³ of solution. The concentration is:

12.5 g/dm³
25 g/dm³
50 g/dm³
75 g/dm³

Q2. A solution is saturated at 60 °C. As it cools to 20 °C, crystals appear because:

The water evaporates
The solubility decreases with temperature, so excess solute leaves the solution
The solute reacts with the water
The solution becomes less dense

Q3. The mass of a sealed flask of water does not change when sugar dissolves in it because:

The sugar becomes weightless in solution
The sugar particles still exist, dispersed among the water particles
Dissolving is a chemical reaction that conserves mass
The water absorbs the mass of the sugar

4 Short Answer Questions

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