If you add drop by drop solution of sodium hydroxide to the metallic salts, a precipitate of metal hydroxide forms. The formation and colour of the precipitate identify the metal ion. 

Some of the precipitated metallic hydroxides form soluble complexes when dissolved in an excess solution of sodium hydroxide.

SaltsSalt solution + Alkali (Sodium Hydroxide) → Metal Hydroxide (precipitate) + Salt formed in solution.The solubility of the precipitate in an excess amount of alkaliCalciumCa(NO3)2 + 2NaOH → Ca(OH)2 + 2NaNO3Calcium Nitrate +Sodium Hydroxide→ Calcium Hydroxide(White ppt) + Sodium Nitrate

Sparingly solubleIrona) Ferrous SaltsFeSO₄ +2NaOH → Fe(OH)₂ + Na₂SO₄

Ferrous Sulphate + Sodium Hydroxide→ Ferrous Hydroxide (green gelatinous ppt) & Sodium Sulphate

b) Ferric Salts

FeCl₃+ 3NaOH → Fe(OH)₃ +3NaCl

Ferric chloride + Sodium Hydroxide→ Ferrous Hydroxide (reddish-brown ppt) & Sodium chloride

InsolubleInsoluble

CopperCuSO₄ +2NaOH →Cu(OH)₂ + Na₂SO₄Copper Sulphate + Caustic Soda→ Copper Hydroxide (pale blue ppt) + Sodium sulphate

InsolubleZincZnSO₄ +2NaOH →Zn(OH)₂ + Na₂SO₄Zinc Sulphate + Caustic Soda→ Zinc Hydroxide (white, gelatinous ppt) + Sodium sulphate

InsolubleAmmonium(NH₄)₂SO₄ +2NaOH → Na₂SO₄ +2H₂O +2NH₃Ammonium sulphate + Sodium Hydroxide →Sodium sulphate + Ammonia gas

Lead Pb(NO₃)₂ +2NaOH → Pb(OH)₂ +2NaNO₃Lead Nitrate + + Sodium Hydroxide → Lead Hydroxide (white ppt) + Sodium Nitrate

Soluble

When added to the metallic salts, Ammonium hydroxide solution precipitates Hydroxides, identified by their distinct colors. The precipitated metallic hydroxides are soluble when treated with excess amounts of ammonium hydroxide.

SaltsSalt solution +Alkali (Ammonium Hydroxide) → Metal Hydroxide (precipitate) + Salt formed in solution.The solubility of the precipitate in an excess amount of alkaliCalciumNo precipitation of Ca(OH)2 even when an excess amount of NH₄OH. Because the concentration of OH ions is low formed from the ionization of NH₄OH.Irona) Ferrous SaltsFeSO₄ +2NH₄OH → Fe(OH)₂ + (NH₄)₂SO₄

Ferrous Sulphate + Ammonium Hydroxide→ Ferrous Hydroxide (Green ppt) & Ammonium Sulphate

b) Ferric Salts

FeCl₃+ 3NH₄OH → Fe(OH)₃ +3NH₄Cl

Fe₂(SO₄)₃ + 6NH₄OH →2Fe(OH₃) +3(NH₄)₂SO₄

(Reddish brown ppt)

InsolubleInsoluble

CopperCuSO₄ +2NH₄OH →Cu(OH)₂ (Pale blue ppt)+ (NH₄)₂SO₄The below reaction is a characteristic property of Cu ²⁺ ion when reacted with an excess of NH₄OH ppt. It is used for detection in qualitative analysis.  

Cu(OH)₂ + (NH₄)₂SO₄+2NH₄OH→ [Cu(NH)₃]₄SO₄(deep blue solution) + 4H₂0

SolubleZincZnSO₄ +2NH₄OH →Zn(OH)₂ + (NH₄)₂SO₄ZnCl₂ +2NH₄OH →Zn(OH)₂ + 2NH₄Cl

(White gelatinous ppt)

SolubleLeadPb(NO₃)₂ +2NH₄OH → Pb(OH)₂(Chalky white ppt) + 2NH₄NO₃Insoluble

Alkalis like sodium and potassium hydroxide react with certain metals like zinc, aluminum, and lead, giving corresponding soluble salt and liberating hydrogen.

Examples:

• Zinc
• When the metal like zinc reacts with the alkalis, sodium, and potassium hydroxide, the corresponding colorless sodium and potassium zincate solution is formed.
• Zn + 2NaOH → Na₂ZnO₂ٖ(Sodium Zincate) + H₂
• Zn+ 2KOH → K₂ZnO₂ٖ(Potassium Zincate) + H₂
• Aluminium
• Aluminum reacts with only boiling and dilute form of alkalis to produce colorless solutions of sodium aluminate.
• 2Al +2NaOH +2H₂0 → 2NaAlO₂ٖ (Sodium aluminate) +H₂
• 2Al +2KOH +2H₂0 → 2KAlO₂ٖ (Potassium aluminate) +H₂
• Lead
• Pb +2NaOH → Na₂PbO₂ٖ (Sodium plumbite) +H₂
• Pb +2KOH → KPbO₂ٖ (Potassium plumbite) +H₂

In general, metal oxides are basic. They dissolve in water to produce alkalis(Hydroxides). Both metal oxides and hydroxides react with the acids but not with the bases. 

Example:

Na₂O + H₂O → 2NaOH

(Sodium oxide)  (Sodium Hydroxide)

Na₂O + 2HCl  → 2NaCl + H₂O

NaOH +HCl → Nacl + H₂O

Few metallic oxides and hydroxides exhibit both acidic and basic character. If they show both characters, they are said to be amphoteric.

Example:

Zinc oxide and zinc hydroxide react with acid and alkalis(NaOH and KOH) to form salt and water.

With Acid:

• ZnO + 2HCl → ZnCl₂+ H₂O

          Zinc Oxide + Hydrochloric acid → Zinc Chloride +water

• Zn(OH)₂ + 2HCl → ZnCl₂+ 2H₂O

         Zinc Hydroxide + Hydrochloric acid → Zinc Chloride +water

With Base Sodium Hydroxide:

• Zinc Oxide/Zinc Hydroxide
• ZnO+ 2NaOH → Na₂ZnO₂ٖ + H₂O

         Zinc oxide + Sodium Hydroxide → Sodium Zincate +water

• Zn(OH)₂+ 2NaOH → Na₂ZnO₂ٖ +2H₂O

       Zinc Hydroxide + Sodium hydroxide → Sodium Zincate +water

• Aluminium Oxide/Aluminium Hydroxide
• Al₂O₃ + 2NaOH → 2NaAlO₂(Sodium aluminate) + H₂O
• Al(OH)₃ + NaOH → NaAlO₂(Sodium aluminate) +2H₂O

With Base Potassium Hydroxide:

1. a) Zinc Oxide/Zinc Hydroxide

• ZnO+ 2KOH → K₂ZnO₂ٖ + H₂O

          ZincOxide + potassium Hydroxide → Potassium Zincate(colorless, soluble) +water

• Zn(OH)₂+ 2KOH → K₂ZnO₂ٖ +2H₂O

b)Lead Oxide/Lead Hydroxide

• PbO + 2KOH → K₂PbO₂ٖ (potassium plumbite) + H₂O
• Pb(OH)₂ + 2KOH → K₂PbO₂ٖ +2H₂O

Alkalis are necessary laboratory reagents and essential in our daily lives as cleaning agents. Ammonium hydroxide and sodium hydroxide are the most used alkalis and give characteristics test with many metals, metal oxides, salt solutions, and thus cation can be identified.

1. Which salt solution would you choose to distinguish between NaOH and NH₄OH solutions?
2. Calcium salt solution is used to differentiate between NaOH and NH₄OH solutions. NaOH reacts with calcium salt and forms a precipitate, and in where NH₄OH does not precipitate upon reaction.
3. What is observed when hot concentrated caustic soda solution is added to zinc and Aluminium?
4. When NaOH is added to the Zinc, it forms sodium zincate and hydrogen, and in the case of aluminum, it produces sodium aluminate, Hydrogen, and sodium oxide.
5. What is the action of alkalis on NH₄ salt?
6. When NaOH or KOH are added to the ammonium salt, ammonia gas is evolved.
7. How do you distinguish acids from alkalis?
8. Tests AcidsAlkalisLitmus testBlue turns red.Red turns blue.Methyl orangeOrange turns pink.Orange turns yellow.On adding sodium carbonateCarbon dioxide is evolved.Carbon dioxide is not evolved.On adding ammonium carbonateNo ammonia gas has evolved.Ammonia gas has evolved.
9. Mention some uses of NaOH and KOH?
10. A. KOH acts as an electrolyte solution and NaOH as a water purifier.
11. B. They are used in manufacturing soaps.
12. C. To make biodiesel
13. D. They are used as a thickening agent and stabilizer in processed foods.