Salts

• a salt consists of a cation (positive ion) and an anion (negative ion).
• Ionic compounds, known as salts, are formed in acid reactions.
• Hydrated salts are salts that contain water of crystallisation.
• The amount of water crystallised in hydrated salt is indicated after the dot“.” in its chemical formula.

  
• Salts that do not contain water of crystallisation are known as anhydrous salts.
• Anhydrous salts are usually powder and can be obtained by heating hydrated salts.
• When hydrated salts are heated, they lose their water of crystallisation.

  • For example, for hydrated copper(II) sulfate:

  CuSO4.5H2O(s) → CuSO4(s) + 5H2O(g)
• Only sodium, potassium and ammonium (S.P.A) and Group 1 salts are always soluble.
• All nitrates are soluble
• All carbonates insoluble except S.P.A & Group 1
• Silver and lead (II) chloride/ iodide insoluble (SLC)
• Lead (II), calcium, barium sulfate insoluble
• All hydroxides except Group 1, calcium (sparingly soluble), barium are insoluble
• Most ionic compounds are soluble in water. Hence, they dissociate in water to give ions in the aqueous solutions.
• Spectator ions are substances that remain unchanged in their physical state or charge and do not take part in the reaction.
• 
  

  Things to take note of:

  • Insoluble solids, covalent liquids (such as water), aqueous covalent substances and gases do not form ions in water. Hence, they should not be split into ions when

  writing an ionic equation.

  • These substances should be written in full. For example, water should be written as H2O(l).
  • All ionic equations must have state symbols.
• Before deciding on how to prepare a salt, two factors must be considered:

  • the solubility of the salt in water
  • the solubility of the starting materials in water
• Precipitation is used to prepare insoluble salts
• Reaction of acid with insoluble B.C.M is used to prepare non-S.P.A soluble salts
• Reaction of acid with insoluble B.C.M

  1. must be added in excess so that all the acid is used up.

  • Apart from the soluble salt, the remaining products from acid reactions are either water or gas, which will not contaminate the salt solution.
  • If the acid is not used up, the salt produced will be contaminated with acid.

  2. must be insoluble in water

  • Excess starting materials can be removed from the salt solution by filtration.
• Preparing zinc sulfate crystals

  1. Fill half a beaker with sulfuric acid

  2. Add excess zinc powder and stir continuously until no more zinc will dissolve or when there is no more effervescence

  3. Filter to remove excess zinc powder

  4. Collect the filtrate which is zinc surface solution

  5. Heat the filtrate till a hot saturated solution is obtained

  6. Test for saturation by dipping a clean, dry and cold glass rod into the filtrate

  7. When the filtrate is saturated, leave it to cool and crystallise

  8. Filter to collect the residue

  9. Wash the crystals with a little cold distilled water to remove impurities

  10. Dry the crystals between a few sheets of filter paper
• To obtain S.P.A or Group 1 salts titration is used
• Why can’t reaction between acid and BCM be used for preparation of S.P.A or Group 1 salts.

  The addition of excess starting material cannot be separated from the salt and hence, for preparation of S.P.A or Group 1 salts, exact volume of reactants are needed.
• The experimental procedure for preparing S.P.A or Group 1 salts involves two parts:

  • Titration to determine the exact volumes of reactants required
  • Actual preparation of pure salt
• Apparatus & chemicals needed for titration:

  • Burette
  • Pipette + pipette filler
  • Conical flasks
  • Appropriate indicator
• volume of acid needed

  1. Fill up a burette with dilute nitric acid. Note the initial burette reading (V1 cm3).

  2. Pipette 25.0 cm3 of dilute sodium hydroxide solution into a conical flask.

  3. Add 2– 3 drops of methyl orange (indicator) to the sodium hydroxide solution. The solution turns yellow.

  4. While swirling the conical flask, add dilute nitric acid from the burette slowly until the solution just turns orange permanently.

  This is the end-point.

  5. Record the final burette reading as V2 cm3.

  Hence, the volume of acid required for complete neutralisation

  = (V2– V1) cm3

  .
• From the earlier steps, the volume of dilute nitric acid to completely neutralise was determined. To prepare pure sodium nitrate, the reaction is repeated without the addition of an indicator as the indicator would cause

  the salt to be impure.

  Step 6: Pipette 25.0cm3 of sodium hydroxide solution into a conical flask.

  Step 7: Add (V2– V1) cm3 of dilute nitric acid from the burette.

  Step 8: Heat the solution to evaporate the water till a hot saturated solution is obtained.
• Step 9: Allow the saturated solution to cool and crystallise.

  Step 10: Filter to collect the residue.

  Step 11: Wash crystals with a little cold water to remove impurities.

  Step 12: Dry the crystals between sheets of filter paper.
• Precipitation involves mixing two aqueous solutions (one containing the cation and the other containing the anion) to form an insoluble solid that separates out from the reaction mixture.
• Pure and dry sample of the salt can be obtained easily through filtration during precipitation