salts

Question 1

what is a salt?

Answer 1

A substance formed when a metallic ion or an ammonium ion (NH4+) replaces one or more hydrogen ions of an acid.

It is an ionic compound that consists of a cation other than H+ and an anion other than OH- or O2-

Question 2

solubility table

Answer 2

• All Group I and ammonium salts are soluble
• All nitrates are soluble
• All sulfates are soluble except PbSO4, BaSO4 (Note that CaSO4 and Ag2SO4 are
  sparingly soluble)
• All chlorides/bromides/iodides are soluble except silver and lead(II) chloride/iodide/bromide
• All carbonates are insoluble except Group I and ammonium carbonates e.g. Na2CO3, K2CO3, (NH4)2CO3
• All metal oxides are insoluble except Group I oxides e.g. Na2O, K2O (Note that oxides of Group II elements
  (Ca, Sr, and Ba) increases in solubility from calcium down caO is sparingly soluble. Solubility of SrO and BaO increases in water.)
• All hydroxides are insoluble except Group I hydroxides e.g. NaOH, KOH (Note that hydroxides of Group II elements (Ca, Sr, and Ba) increases in solubility from calcium down. Ca(OH)2 is sparingly soluble and the solubility of Sr(OH)2 and Ba(OH)2 increases in water.)

Question 3

preparation of insoluble salts

Answer 3

• precipitation: a reaction involving mixing two aqueous solutions to form an insoluble solid that separates out from the reaction mixture. The insoluble solid is the precipitate.

Procedure of Precipitation
- 2 solutions are selected. One of the reagents contributes the cation, X
n+, while another contributes the anion Ym-
- Mix the 2 solutions together. One of the reagents will be added in excess and
would be left as the unreacted reagent.
- The insoluble salt is formed as a precipitate and can then be separated from the
aqueous solution by filtration.
- The salt is then washed with distilled water and wiped dry with filter paper to
remove contaminant chemicals that are on the surface of the crystals.

Question 4

preparation of soluble salts

Answer 4

-Reaction of acid with an insoluble substance (solid – acid reaction)

The acid is added to an insoluble solid reactant. Possible types of
solid include insoluble carbonate, base (hydroxides or oxides) and metal. The insoluble solid reactant must be added in excess.
Three possible reactions:
1. Acid + reactive metals –> salt + hydrogen
2. Acid + insoluble base –> salt + water
3. Acid + insoluble metal carbonate –> salt + water + carbon dioxide
Note: The first reaction should not be used with highly reactive metals (e.g. Group I metals), and will not work with unreactive metals (e.g. copper, silver and gold).

Steps taken for Solid – Acid reaction:
1. Add excess insoluble carbonate/base/metals into dilute acid in a beaker.
- Warm the reactants. The solid react with acid to form salt and water. Since the solid is added in excess, all the acid will be used up and the excess solid remain as
insoluble solid.
(insoluble reactant added in excess to ensure that the reaction is complete and all the soluble reactant is used up)
2. The resultant mixture can then be filtered to remove the excess, unreacted solids. The filtrate is the solution of the soluble salt.
3. Heat the filtrate gently until it is saturated.
4. Allow the saturated solution to cool for the crystals to form.
5. Filter the mixture to collect the crystals.
6. Wash the soluble salt crystals with a small amount of cold distilled water.
Dry the crystals between pieces of filter paper.
evaporation to dryness is acceptable for salts that do not decompose on heating
or without water of crystallization

Titration method
Titration is used to produce ammonium salts and group I salts.
Possible reactions:
acid + alkali –> salt + water
acid + soluble carbonate–> salt + carbon dioxide + water