solubility

Insoluble versus Soluble Salts

Understanding the solubility of salts is essential for IGCSE candidates. Salts can be categorized based on their solubility, which requires memorization of a grid or rules. Key rules for solubility include:

  • Soluble Salts: All compounds containing ammonium, sodium, or potassium are soluble. All nitrates are also soluble.

  • Chlorides: Most chlorides are soluble, with exceptions being silver chloride and lead(II) chloride, which are insoluble.

  • Sulfates: Generally soluble but exceptions include lead(II) sulfate, barium sulfate, and calcium sulfate, which are insoluble.

  • Carbonates: All carbonates are typically insoluble except for ammonium, sodium, and potassium carbonates.

  • Hydroxides: Most are insoluble, but ammonium, sodium, and potassium hydroxides are exceptions and are soluble.

Methods for Making Salts

Three methods are used to synthesize salts, depending on whether the salt is soluble or insoluble:

1. Insoluble Salts Method (Precipitation Method)

To create an insoluble salt:

  • Procedure: Mix two aqueous solutions to produce an insoluble salt, which will precipitate out of the solution.

  • Filtration: Use filter paper to separate the insoluble salt from the solution, then wash and dry the salt.

  • Summary of Steps: React, filter, wash, dry.

2. Soluble Salts Method (Simple Filtration and Evaporation)

For salts not containing ammonium, sodium, or potassium:

  • Procedure: React solutions as before, but since the salt is soluble, filter out excess solids to leave a salt solution.

  • Evaporation: Evaporate excess liquid using heat to obtain crystallized salt.

  • Summary of Steps: React, filter, evaporate, cool, dry.

3. Titration Method for Soluble Salts with Ammonium, Sodium, or Potassium

Due to the high reactivity of these metals, a specific method is required:

  • Procedure: Use a conical flask with a known volume of acid and a burette for alkali. Add an indicator to determine neutralization (e.g., phenolphthalein changes color).

  • Neutralization Point: Record the volume of alkali needed for complete reaction. Repeat the reaction without the indicator to avoid interference.

  • Summary of Steps: React, use indicator, repeat without indicator, evaporate, cool, dry.

In all cases, knowing these methods and the rules regarding solubility will facilitate the understanding and preparation of salts for exam purposes.