2.4.7 Solubility

This topic defines solubility — how much solute can dissolve in a given amount of solvent. It is one of the most important concepts for understanding solutions.

Definition

Solubility is the maximum amount of solute which dissolves in a specified amount of solvent at a specific temperature.

Simple explanation: Solubility tells you how much of a substance can dissolve in a certain amount of solvent at a certain temperature.

Main Idea

Solubility is not a fixed number for all conditions. It depends on:

• The solvent used — different solvents dissolve different amounts of solute.
• Temperature — changing the temperature usually changes solubility.
• Pressure — mainly affects the solubility of gases.

Simple explanation: How much sugar you can dissolve in water depends on how much water you have, what temperature it is, and what you're trying to dissolve.

Core Concept

Solubility is expressed as:

Maximum grams of solute per 100 g of solvent at a specific temperature

Example from the book: "36.5 g/100 g H₂O at 20 °C" means 36.5 grams of NaCl can dissolve in 100 grams of water at 20 °C.

Detailed Explanation

What Solubility Tells Us

• If the amount of solute added is less than the solubility → the solution is unsaturated.
• If the amount of solute added is equal to the solubility → the solution is saturated.
• If the amount of solute added is more than the solubility → the extra solute settles at the bottom.

Factors Affecting Solubility

1. Nature of the solvent "Like dissolves like" — polar solvents (like water) dissolve polar solutes; non-polar solvents (like oil) dissolve non-polar solutes.

• For most solids in liquids, solubility increases with temperature.
• For some solids, solubility decreases with temperature (e.g., Ca(OH)₂).
• For gases in liquids, solubility usually decreases with temperature.

• Pressure has little effect on solids and liquids.
• Pressure has a large effect on gases — higher pressure increases gas solubility (this is why soft drinks are bottled under pressure).

Intermediate Level

How Solubility Is Expressed

Solubility is usually given as:

grams of solute per 100 g of solvent at a stated temperature

Example from Table 2.2 (page 23):

Key observation: Most salts show increased solubility with temperature, but Ca(OH)₂ shows decreased solubility.

Solubility and Saturation

Solubility sets the boundary between unsaturated and saturated:

• Below solubility → unsaturated solution
• At solubility → saturated solution
• Above solubility → supersaturated (unstable) or excess solute settles

Why Pressure Affects Gases

• Soft drinks are carbonated under high pressure.
• When you open the bottle, pressure drops, gas escapes (bubbles form).

Important Points

• - Solubility: maximum amount of solute that dissolves in a specified amount of solvent at a specific temperature.
• - Depends on:
• - Nature of solvent (like dissolves like)
• - Temperature
• - Pressure (mainly for gases)
• - Usually expressed as grams per 100 g of solvent at a stated temperature.
• - For most solids, solubility increases with temperature.
• - For some solids (like Ca(OH)₂), solubility decreases with temperature.
• - For gases, solubility decreases with temperature and increases with pressure.
• - Solubility defines the boundary between unsaturated, saturated, and supersaturated solutions.

Remember

• - Solubility = maximum solute that dissolves in a specified solvent at a specific temperature.
• - Depends on: solvent, temperature, pressure.
• - Expressed as grams per 100 g of solvent.
• - Most solids: solubility increases with temperature.
• - Ca(OH)₂: solubility decreases with temperature.
• - Gases: solubility decreases with temperature; increases with pressure.
• - Solubility defines the boundary between unsaturated and saturated.

Common Mistakes

• 1. Thinking solubility is fixed. It changes with temperature, solvent, and pressure.
• 2. Confusing solubility with concentration. Solubility is the maximum; concentration is how much is actually dissolved.
• 3. Thinking all solids become more soluble when heated. Some (like Ca(OH)₂) become less soluble.
• 4. Ignoring pressure for gases. Pressure strongly affects gas solubility.
• 5. Thinking solubility is the same in all solvents. Solubility depends on the solvent.

Quick Revision

• Solubility: maximum amount of solute that dissolves in a specified amount of solvent at a specific temperature.
• Depends on solvent, temperature, and pressure.
• Expressed as grams per 100 g of solvent at a stated temperature.
• Most solids → solubility increases with temperature.
• Ca(OH)₂ → solubility decreases with temperature.
• Gases → solubility decreases with temperature, increases with pressure.
• Solubility sets the boundary between unsaturated and saturated solutions.

60-Second Revision

• Solubility = maximum solute per specified solvent at a specific temperature.
• Depends on: solvent, temperature, pressure.
• Most solids: solubility increases with temperature.
• Ca(OH)₂: solubility decreases with temperature.
• Gases: solubility increases with pressure.
• Warning: solubility is not fixed — it changes with conditions.