Chapter 12- Salts

• Salts are ionic compounds formed when a metallic ion or an ammonium ion replaces one or more hydrogen ions of an acid.
• Salts that have water are called hydrated salts while salts that don’t have water are called anhydrous salts.
• Salts can be characterised as either soluble or insoluble in water. This also determines the method of preparing a particular salt. Solubility can be determined by the following solubility rules.

SOLUBILITY RULES

1. All group I, ammonium and nitrate salts are soluble.
2. All chlorides are soluble except silver chloride and lead (II) chloride
3. All sulfates are soluble except barium sulfate, lead (II) sulfate and calcium sulfate
4. All carbonates are insoluble except group I and ammonium carbonates

METHODS OF PREPARING SALTS

• Reaction with acids cannot be used with explosively reactive metals (Group I) or unreactive metals (e.g. copper and silver). However, warm acid can sometimes be used for unreactive metals.

METHOD 1: REACTION OF ACID WITH INSOLUBLE METAL/CARBONATE/BASE

1. Fill a beaker with dilute acid and add insoluble metal/carbonate/base in excess. Wait until the reaction completes by ensuring that no more effervescence is observed.
2. Filter to remove excess metal/carbonate/base and collect the filtrate.
3. Heat the filtrate to concentrate the salt solution until crystals form (use a glass rod as a saturation test).
4. Let the crystals to cool.
5. Wash with organic solvent and dry between pieces of filter paper.

METHOD 2: PRECIPITATION

1. Add two solutions of soluble salts with the required anion and cation into a beaker until no precipitate forms.
2. Filter to collect the precipitate.
3. Wash the precipitate with distilled water and dry between sheets of filter paper.

METHOD 3: TITRATION

1. Fill up a burette with dilute acid.
2. Pipette an alkali to a conical flask with a few drops of pH indicator such as phenolphthalein.
3. Add dilute acid to the flask gradually, swirling after each addition until the end-point is reached (the indicator changes colour).
4. Note down the volumes of the acid and alkali.
5. Repeat using the recorded values in a volumetric flask without adding the indicator.
6. Heat gently in an evaporating dish until crystallisation point.
7. Wash with an organic solvent and dry between pieces of filter paper.

QUALITATIVE ANALYSIS

• The process of identifying cations and anions is called qualitative analysis.
• Tests for cations:

• Tests for anions:

• Tests for gases:

• Tests for water: Add anhydrous copper (II) sulfate. It changes from white to blue.
• Or test using cobalt (II) chloride paper. It turns from white to pink.