Overview
This lecture covers the definition and properties of salts in chemistry, rules for salt solubility, and the three main methods to prepare salts, with a focus on the double decomposition (precipitation) method.
Definition of Salts
- In chemistry, a salt is an ionic compound formed when the H+ ion of an acid is replaced by a metal or ammonium ion.
- Table salt (sodium chloride) is just one kind of salt; many others exist with different ions.
- Salts are formed by the reaction of acids with bases, metals, or ammonium ions.
Solubility Rules for Salts
- Nitrates, sodium, potassium, and ammonium salts are always soluble in water.
- Most sulfates are soluble except those of lead (PbSO₄), barium (BaSO₄), and calcium (CaSO₄), which are insoluble.
- Most chlorides are soluble, except those of silver (AgCl), lead (PbCl₂), and mercury (Hg₂Cl₂).
- Most carbonates and hydroxides are insoluble except those of sodium, potassium, and ammonium.
Methods for Preparing Salts
- There are three main methods to make salts: double decomposition (precipitation), titration, and direct acid-base (neutralization) reaction.
- Choosing the method depends on whether the target salt is soluble or insoluble, and whether it contains group 1 ions (spa: sodium, potassium, ammonium).
- Use double decomposition (precipitation) to prepare insoluble salts.
- Use titration for soluble salts containing sodium, potassium, or ammonium.
- Use neutralization for other soluble salts.
Double Decomposition (Precipitation) Method
- Mix two soluble salts so their ions exchange partners, producing an insoluble salt (precipitate) and a soluble byproduct.
- The insoluble salt forms as a solid at the bottom of the container.
- Filter the mixture to separate the solid (desired salt) from the liquid.
- Wash the solid with distilled water to purify it, then dry it between filter papers (compression method).
Worked Examples
- Example 1: To make lead(II) sulfate (PbSO₄), mix lead(II) nitrate and sodium sulfate, which react to precipitate PbSO₄.
- Example 2: To make magnesium carbonate, mix magnesium nitrate and potassium carbonate, which react to give insoluble MgCO₃.
Salt Preparation Strategy
- Step 1: Decide on the correct preparation method using the solubility rules and a flow chart.
- Step 2: Choose the appropriate reactants (soluble salts that yield the desired ions).
- Step 3: Write and balance the chemical equation for the reaction.
- Step 4: Draw diagrams or sequence drawings to visualize and plan each step.
- Step 5: Convert diagrams into clear written steps for procedures.
Key Terms & Definitions
- Salt — An ionic compound formed when the H+ of an acid is replaced by a metal or ammonium ion.
- Solubility — The ability of a substance to dissolve in water.
- Precipitate — An insoluble solid formed from the reaction of two solutions.
- Double decomposition — A method where two soluble salts react to form an insoluble salt and a soluble byproduct.
- SPA — Abbreviation for sodium, potassium, and ammonium; indicates high solubility in their salts.
Action Items / Next Steps
- Practice identifying suitable salt preparation methods for various salts.
- Complete exercises on the double decomposition method.
- Next lesson: Learn preparation details for titration and neutralization methods.