Solubility Notes

Introduction to Solubility

Basic Definitions

• Solute: The stuff that gets dissolved.

• Solvent: The stuff that does the dissolving.

• Solution: What you get when the solute mixes into the solvent.

Examples of Solutions in Different States of Matter (Table 8.1)

• Solid in Solid:

  • Steel: Has a bit of carbon mixed in with iron.

• Solid in Liquid:

  • Seawater: Has salt and other stuff mixed in water.

• Solid in Gas:

  • Mothballs: They evaporate and mix with the air, which keeps moths away.

Solubility

• Definition: How much of a solute can dissolve in a solvent at a certain temperature. Usually measured in grams per milliliter (g/mL).

  • Slightly soluble: Not much dissolves.

  • Soluble: A good amount dissolves.

  • Insoluble: Almost nothing dissolves.

Types of Solutions

• Unsaturated: You can still add more solute and it will dissolve.

• Saturated: You can't add any more solute; it won't dissolve.

• Supersaturated: Has more solute dissolved than it should at that temperature; might form crystals.

Crystallization

• Definition: When solids form out of a supersaturated solution as it cools down.

• You need a tiny crystal to start the process.

Solubility and Dissolving

• Solubility: How much stuff dissolves at a certain temperature (like 36 g of salt in 100 mL of water).

• Dissolving: When the solute breaks apart and mixes with the solvent.

Solubility Rules for Ionic Compounds in Water (Table 8.3)

• General Rule: If a compound dissolves well (at least 1 g/100 mL of water), we say it's soluble.

• Usually Soluble:

  • Alkali metal ions (Li+, Na+, etc.)

  • Hydrogen ion (H+)

  • Ammonium ion (NH₄+)

  • Nitrate (NO₃-)

  • Ethanoate (acetate) (CH₃COO-)

• Exceptions for Chlorides, Bromides, and Iodides:

  • Don't dissolve well with Ag+, Pb2+, etc.

• Exceptions for Fluorides:

  • Don't dissolve well with Mg2+, Ca2+, etc.

• Exceptions for Sulfates:

  • Don't dissolve well with Ca2+, Sr2+, etc.

• Sulfides:

  • Dissolve well with Alkali ions and H+, NH₄+, Be2+, Mg2+, Ca2+, Sr2+, Ba2+

  • Don't dissolve well with others

• Hydroxides:

  • Dissolve well with Alkali ions and H+, NH₄+, Sr²+, Ba²+, Tl+

  • Don't dissolve well with others

• Phosphates, Carbonates, and Sulfites:

  • Dissolve well with Alkali ions and H+, NH₄+

  • Don't dissolve well with others

"Like Dissolves Like" Principle

• Polar stuff dissolves in polar stuff because they attract each other.

• Nonpolar stuff dissolves in nonpolar stuff because they don't have charges.

• Soap can dissolve in both because it has polar and nonpolar parts.

Dissolving by Hydration

• Water is attracted to ions and breaks them apart.

• Water surrounds the ions and spreads them out.

• Not all ionic compounds dissolve in water.

Factors Affecting Solubility of Ionic Compounds

1. Ionic Radius: Bigger ions usually dissolve better.

2. Charge: More charge usually makes it harder to dissolve.

Ion Orientation in Water

• The positive side of water faces negative ions.

• The negative side of water faces positive ions.

Salt vs. Sugar Dissolved in Water

• Salts (Ionic Compounds): Break into ions when dissolved.

• Sugar (Molecular Compounds): Stay as molecules when dissolved.

Molecular Compounds and Solubility

• Polar molecules stick together; if water pulls them apart, they dissolve.

• Nonpolar molecules don't dissolve in water.

• Bigger nonpolar parts make it harder to dissolve (e.g., ethanol vs. methanol).

Factors Affecting the Rate of Dissolving

1. Surface Area: Smaller pieces dissolve faster.

2. Agitation: Stirring helps it dissolve faster.

3. Temperature:

   • Hotter temperatures help things dissolve faster.

Temperature and Pressure Effects on Solubility

• Solids: Hotter (↑T) means more dissolves (↑S).

• Liquids: Temperature and pressure don't matter much.

• Gases:

  • Hotter (↑T) means less dissolves (↓S).

  • More pressure (↑P) means more dissolves (↑S).

Solubility Curves

• Graphs that show how much solute dissolves in 100 mL of water at different temperatures.

• The steeper the line, the more the temperature affects solubility.

Interpreting Solubility Curves

• Unsaturated Region: Below the line.

• Saturated Region: On the line.

• Supersaturated Region: Above the line.

Questions from Solubility Curve Graph

• Which salt dissolves the least at 0°C?

• At what temperature does $$K