Solubility - Chemistry

1. Fundamental Concepts

Solubility Definition: The maximum mass/amount of a solute that can dissolve in a fixed quantity of solvent (e.g., 100 g water) at a specific temperature and pressure. Expressed in g solute/100 g solvent or mol/L.

Core Dependence: Solubility is not a fixed property — it changes with temperature, pressure, and the nature of solute-solvent interactions.

2. Key Concepts

Intermolecular Forces (Primary Factor):

Rule: Like dissolves like — solubility is high when solute and solvent have similar intermolecular forces.

Polar solutes (e.g., NaCl, sugar) dissolve in polar solvents (e.g., water) via dipole-dipole interactions or hydration (for ionic compounds).

Nonpolar solutes (e.g., oil, O₂) dissolve in nonpolar solvents (e.g., hexane) via London dispersion forces.

Temperature Effect:

Solid solutes: Solubility increases with rising temperature (e.g., KNO₃ solubility rises sharply with temperature; more energy breaks solute-solute bonds).

Gas solutes: Solubility decreases with rising temperature (e.g., CO₂ escapes from warm soda; higher temperature reduces gas-solvent intermolecular attractions).

Pressure Effect (Only for Gases):

Henry’s Law: Gas solubility is directly proportional to its partial pressure above the solution (\(C=kP\)).

Example: Soda is pressurized with CO₂ to increase solubility; opening the can lowers pressure, reducing CO₂ solubility and releasing bubbles.

3. Examples

Easy

Question: Is AgCl soluble in water? Why?

Answer: No. Ag⁺ is a chloride ion exception to solubility rules — AgCl cannot dissociate enough to reach a measurable solubility limit in water.

Medium

Question: Why does more sugar dissolve in hot water than cold water?

Answer: Sugar is a solid solute. Higher temperature provides more kinetic energy to overcome intermolecular forces between sugar molecules, allowing more sugar to be hydrated by water molecules — thus increasing sugar’s solubility in hot water.

Hard

Question: A saturated solution of NaCl is prepared at 25°C (solubility = 36 g/100 g water). If the solution is heated to 100°C (solubility = 39.8 g/100 g water) and 5 g of NaCl is added, will the added NaCl dissolve? Explain how solubility changes drive this outcome.

Answer: Yes, the added NaCl will dissolve. NaCl solubility increases slightly with temperature — at 100°C, the solubility limit (39.8 g/100 g water) is higher than the original dissolved amount (36 g/100 g water). The temperature rise raises the solubility threshold, making the solution unsaturated and allowing extra NaCl to dissolve.

4. Problem-Solving Techniques

Predict Solubility via Intermolecular Forces:

Step 1: Identify polarity of solute and solvent.

Step 2: Apply "like dissolves like" — polar-polar or nonpolar-nonpolar pairs have high solubility; polar-nonpolar pairs have low solubility.

Use Solubility Rules for Ionic Compounds:

Step 1: Break the ionic compound into cation and anion.

Step 2: Check if either ion is in the "always soluble" list; if not, check for insoluble ion pairs.

Step 3: Predict solubility (e.g., Na₂CO₃ = soluble; CaCO₃ = insoluble).

Analyze Temperature/Pressure Impact on Solubility:

For solids: If solubility increases with T, heat saturated solutions to dissolve more solute.

For gases: To increase solubility, lower temperature or raise pressure (e.g., store soda in a cool place).

Calculate Solubility Limits:

Example: If solubility of KCl is 40 g/100 g water at 25°C, maximum KCl dissolved in 200 g water = (40 g/100 g water) × 200 g water = 80 g.