Acids, bases and salts
SALT

Salts are ionic compounds formed when the hydrogen ion \( (H^+) \) of an acid is replaced partially or completely by a metal ion or an ammonium ion \( (NH_4^+) \). Examples are; Sodium chloride, Calcium Carbonate, e.t.c

TYPES OF SALTS

Salts can be classified into several types depending on how they are formed or their composition.

1. Normal salts: Formed when all the replaceable hydrogen ions of an acid are completely replaced by a metal or ammonium ion. They are slightly neutral to litmus

$$ HCl + NaOH \rightarrow NaCl + H_2O $$

2. Acidic salts: Formed when only part of the hydrogen ions of a polybasic acid are replaced by a metal ion. These salts can further react with bases to form normal salts. They also behave like acids and can turn blue litmus red. $$\small{ NaOH + H_2SO_4 \rightarrow NaHSO_4 + H_2O} $$ (Sodium hydrogen sulphate is an acid salt)

Basic salts: These are salts formed when a base is not completely neutralized by an acid. They contain hydroxide ions. $$\small{ Pb(OH)_2 + HNO_3 \rightarrow Pb(OH)NO_3 + H_2O} $$

4. Double salts: These are salts formed when two different salts crystallize together in a fixed ratio of ions. $$ \tiny{FeSO_4 + (NH_4)_2SO_4 + 6H_2O \rightarrow FeSO_4·(NH_4)_2SO_4·6H_2O} $$ $$\text{Mohr’s salt} $$

5. Complex salts: These salts contain a complex ion (metal ion bonded to ligands). $$ [Cu(NH_3)_4]SO_4 $$ (Tetraamminecopper(II) sulphate)

6. Double decomposition salts: These are formed when two solutions of salts react and exchange their ions to produce new salts.

$$ AgNO_3 + NaCl \rightarrow AgCl + NaNO_3 $$
PREPARATION OF SALTS

Salts are prepared in the laboratory depending on whether they are soluble or insoluble in water.

  1. Preparation of Soluble Salts
  1. By Neutralization of Acid with Alkali

    2. This method is used for soluble salts of sodium, potassium, and ammonium (since they are all soluble and their carbonates are unstable on heating).

    $$\small{ H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O} $$
  2. By Neutralization with Insoluble Bases

    3. This method involves reacting an acid with an insoluble base. It is used when the base is insoluble (like copper(II) oxide, zinc oxide, etc.).

    $$ H_2SO_4 + CuO \rightarrow CuSO_4 + H_2O $$
  3. By Reaction with metals

    4. This method involves a metal reacting directly with an acid to form a salt and hydrogen gas.

    $$ Zn + 2HCl \rightarrow ZnCl_2 + H_2 $$
  4. By Reaction with trioxocarbonates

    5. When acids react with soluble carbonates, a soluble salt is formed with the evolution of carbon (IV) oxide

    $$\small{ HCl + Na_2CO_3 \rightarrow NaCl + H_2O + CO_2} $$
  5. By crystallization

    6. Crystallization is a common laboratory method used to prepare soluble salts that decompose on strong heating. It involves forming the salt in solution, concentrating the solution by evaporation, and allowing it to cool to form pure crystals.

    Example: Preparation of copper(II) sulfate crystals (CuSO₄·5H₂O) from copper(II) oxide and dilute sulfuric acid.

    $$ \small{CuO_{(s)} + H₂SO₄_{(aq)} → CuSO₄_{(aq)} + H₂O_{(l)}} $$
  1. Preparation of Insoluble Salts
  1. By Double Decomposition (Precipitation Reaction)

    2. Insoluble salts are formed when two soluble salts react to exchange ions and form a precipitate.

    $$\small{ BaCl_2 + Na_2SO_4 \rightarrow 2NaCl + BaSO_4 \downarrow} $$

    (Barium sulphate is insoluble and appears as a white precipitate)

PROPERTIES OF SALTS
SOLUBILITY OF SOME SALTS
Salt Type Soluble Salts Insoluble Salts
All salts of Sodium, Potassium and Ammonium
Carbonates Sodium carbonate (Na₂CO₃), Potassium carbonate (K₂CO₃), Ammonium carbonate ((NH₄)₂CO₃) Calcium carbonate (CaCO₃), Zinc carbonate (ZnCO₃), Lead(II) carbonate (PbCO₃)
Nitrates All nitrates are soluble —
Chlorides Most are soluble (NaCl, KCl) Lead(II) chloride (PbCl₂), Silver chloride (AgCl)
Sulphates Most are soluble (Na₂SO₄, K₂SO₄) Lead(II) sulphate (PbSO₄), Barium sulphate (BaSO₄), Calcium sulphate (CaSO₄)
Phosphates Phosphates of Sodium, Potassium and Ammonium Others
Uses of Salts

Salts are widely used in domestic, industrial, agricultural, and medical applications. Their unique chemical properties make them useful in diverse fields such as food preservation, fertilizer production, medicine, and chemical manufacturing.

Salt Use
Sodium chloride (NaCl) Used as table salt for seasoning and food preservation.

In the commercial manufacture of soap for precipitating out soap and separating it out from glycerol and impurities
Sodium carbonate (Na₂CO₃) Used in the manufacture of glass, soap, and as a water softener.
Sodium hydrogen carbonate (NaHCO₃) Used as baking soda, in fire extinguishers, and as an antacid.
Ammonium nitrate (NH₄NO₃) Used as a fertilizer and in explosives.
Calcium chloride (CaCl₂) Used for de-icing roads and as a drying agent (desiccant).
Calcium carbonate (CaCO₃) Used in making cement, lime, and as an antacid.
Copper(II) sulphate (CuSO₄) Used as a fungicide, algaecide, and in electroplating.
Lead(II) ethanoate (Pb(CH₃COO)₂) Used in dyeing and printing textiles (though toxic).
Silver nitrate (AgNO₃) Used in photography, and as an antiseptic.
Zinc chloride (ZnCl₂) Used in textile processing and as a wood preservative.
Magnesium sulphate (MgSO₄) Used as a laxative and to relieve muscle cramps (Epsom salt).
Potassium nitrate (KNO₃) Used in making gunpowder and fertilizers.
Ammonium chloride (NH₄Cl) Used in dry cells and as an expectorant in cough medicine.
Iron(II) sulphate (FeSO₄) Used in iron supplements and as a fertilizer.
Calcium sulphate (CaSO₄) Used to make plaster of Paris and in building materials.

Note: Some salts (e.g., lead(II) ethanoate) are toxic and must be handled with great care.

Common Lab Tests for Salts

Salts can be identified in the laboratory through various physical and chemical tests such as solubility, flame tests, reactions with acids, and precipitation reactions. These tests help to determine the cations and anions present in the salt.

Test Procedure Observation Inference / Example
1. Solubility Test Dissolve a small amount of the salt in water. Some salts dissolve easily (soluble), others do not (insoluble).
  • NaCl, KNO₃ — soluble
  • CaCO₃, PbSO₄ — insoluble
2. Flame Test Introduce a small portion of the salt into a non-luminous flame using a clean wire loop. Different metallic ions produce different flame colours.
  • Na⁺ — Yellow flame
  • K⁺ — Lilac flame
  • Ca²⁺ — Brick red flame
  • Ba²⁺ — Apple green flame
  • Cu²⁺ — Blue-green flame
3. Reaction with Dilute Acids Add a few drops of dilute HCl or H₂SO₄ to the salt. Effervescence may occur if a gas is evolved.
  • Carbonates produce CO₂:
    $$\text{CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑}$$
  • Sulphides produce H₂S gas:
    $$\text{FeS + 2HCl → FeCl₂ + H₂S↑}$$
4. Test for Chloride Ions Add AgNO₃ solution to a solution of the salt, then add dilute NH₃. White precipitate forms, soluble in ammonia. Presence of Cl⁻:
$$ Ag^{+} + Cl^{-}→ AgCl$$
5. Test for Sulphate Ions Add BaCl₂ solution to the salt solution, then add dilute HCl. White precipitate forms, insoluble in acid. Presence of SO₄²⁻:
$$Ba^{2+} + SO₄^{2-} → BaSO₄$$
6. Test for Carbonate Ions Add dilute HCl and pass the gas through limewater. Effervescence occurs and limewater turns milky. CO₂ evolved:
$$\text{CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑}$$
7. Test for Nitrate Ions Add FeSO₄ solution and carefully pour concentrated H₂SO₄ down the side of the test tube. Brown ring forms at the interface. Presence of NO₃⁻ (Brown Ring Test).
8. Test for Ammonium Ions Add NaOH solution and warm gently. Pungent gas evolved turns moist red litmus paper blue. NH₄⁺ present:
$$\text{NH₄Cl + NaOH → NH₃ + H₂O + NaCl}$$
9. Test for Iron(III) Ions Add NaOH or NH₄OH solution dropwise to salt solution. Reddish-brown precipitate forms, insoluble in excess alkali. Fe³⁺ present:
$$Fe^{3+} + 3OH^{-} → Fe(OH)₃ $$
10. Test for Copper(II) Ions Add NaOH or NH₄OH solution to the salt solution. Blue precipitate forms; dissolves in excess NH₄OH to give deep blue solution. Cu²⁺ present:
$$ Cu^{2+} + 2OH^{-} → Cu(OH)₂$$

Note: These tests are used in qualitative analysis to identify unknown salts based on their ionic composition.

Summary