Solubility and Solubility Curves Definition, factors affecting solubility, solubility calculations, interpretation of solubility curves.

CHEMISTRY S S 2: Solubility and Solubility Curves

Core Concepts

What is Solubility? Solubility is the maximum amount of a solute that can dissolve in a specific amount of solvent at a particular temperature to form a saturated solution. A saturated solution is one where no more solute can dissolve at that temperature. If more solute can still dissolve, it's an unsaturated solution. If a solution contains more solute than a saturated solution at that temperature (usually achieved by cooling a hot saturated solution carefully), it's a supersaturated solution, which is unstable.

Solubility is often expressed in grams of solute per 100 grams of solvent (g/100g solvent) or sometimes in moles per dm³ (mol/dm³). For example, if 36g of NaCl dissolves in 100g of water at 25°C, its solubility is 36g/100g water.

Factors Affecting Solubility

1. Nature of Solute and Solvent: The principle "like dissolves like" is key here. Polar solvents (like water) dissolve polar or ionic solutes (like salt, sugar). Non-polar solvents (like benzene, oil) dissolve non-polar solutes (like grease, wax). This is why oil and water don't mix – one is non-polar, the other is polar.

2. Temperature:

   • For Solids: The solubility of most solids in liquids generally increases as temperature increases. This is because higher temperatures provide more energy for the solvent molecules to break apart the solute particles and for the solute particles to spread throughout the solvent.
   • For Gases: The solubility of gases in liquids generally decreases as temperature increases. Think of a carbonated drink going flat faster in warm weather. Higher temperatures give gas molecules more kinetic energy, allowing them to escape from the liquid more easily.

3. Pressure:

   • For Solids and Liquids: Pressure has very little effect on the solubility of solids and liquids in liquid solvents.
   • For Gases: The solubility of gases in liquids increases significantly with increasing pressure. This is described by Henry's Law. This is why carbonated drinks are bottled under high pressure to keep the carbon dioxide dissolved.

Solubility Calculations

Solubility calculations typically involve using the definition of solubility.

Formula: Solubility (g/100g solvent) = (Mass of solute / Mass of solvent) × 100g

Example: If 20g of a salt dissolves in 50g of water at 30°C to form a saturated solution, what is its solubility?

• Mass of solute = 20g
• Mass of solvent = 50g
• Solubility = (20g / 50g) × 100g = 0.4 × 100g = 40g/100g water.

Solubility Curves

A solubility curve is a graph that shows how the solubility of a substance changes with temperature.

• The x-axis usually represents temperature (°C).
• The y-axis represents solubility (g/100g solvent).
• Each point on a solubility curve represents a saturated solution at that specific temperature.

Interpretation of Solubility Curves

1. Effect of Temperature:

   • A curve that slopes upwards (like KNO₃, NaNO₃) indicates that the solubility of the substance increases as temperature rises.
   • A curve that slopes downwards (like Ce₂(SO₄)₃) indicates that the solubility decreases as temperature rises.
   • A relatively flat curve (like NaCl) indicates that temperature has little effect on its solubility.

2. Comparing Solubilities: You can compare the solubilities of different substances at the same temperature by looking at their positions on the y-axis at that temperature. The higher the curve, the more soluble the substance.

3. Crystallization: If a hot saturated solution of a substance (with an upward-sloping curve) is cooled, the solubility decreases, and the excess solute will crystallize out of the solution. The amount that crystallizes can be calculated by finding the difference in solubility at the initial and final temperatures.

   Example: If a saturated solution of KNO₃ at 60°C is cooled to 20°C, you can find the solubility at 60°C and 20°C from the graph, then subtract to find the amount that crystallizes per 100g of water.

Real-World Example

The fizziness in soft drinks (like Coca-Cola or Pepsi) is due to dissolved carbon dioxide gas. The drink is bottled under high pressure and kept cold. When you open the bottle, the pressure is released, and the gas becomes less soluble, forming bubbles. If you leave an open bottle in a warm place, it will go "flat" faster because the higher temperature further reduces the solubility of the CO₂ gas.

Practice/Reflection

Consider a solubility curve for potassium chlorate (KClO₃). If 100g of water contains 20g of KClO₃ at 50°C and it's an unsaturated solution, what does this tell you about the solubility of KClO₃ at 50°C? What would happen if you slowly added more KClO₃ to this solution at the same temperature?