Action of Sodium Hydroxide on Metallic Salt Solutions: In chemical laboratories, if we get an unknown substance from our lab assistant or teacher and are asked to identify that substance, what do we do? Yes, we go for certain qualitative analyses. In order to identify the unknown substance in the laboratory, we carry out certain physical and chemical tests; this type of analysis is known as qualitative analysis. Physical tests include identifying an unknown substance based on certain physical properties like colour, solubility, or texture.

At the same time, chemical tests include the reaction of the given substance with other elements or compounds. Sodium hydroxide is a base, and when it is added to the metallic salt solution, metal hydroxide gets precipitated. The colour and solubility of the precipitate in the presence of an excess of sodium hydroxide gives us a clear identification of the metal ion present in that metallic salt solution. In this article, we will learn about different chemical reactions involved when sodium hydroxide reacts with metallic salt solutions.

Preparation of Sodium Hydroxide

One of the most famous methods for the industrial preparation of sodium hydroxide is the chloralkali process. In this process, electrolysis of brine solution, i.e., aqueous sodium chloride solution, occurs. It is a widespread technology used to produce chlorine and sodium hydroxide.

The overall chemical reaction involved in the electrolysis of brine is:

\(2{\rm{NaCl}} + 2{{\rm{H}}_2}{\rm{O}} \to 2{\rm{NaOH}} + {\rm{C}}{{\rm{l}}_2} + {{\rm{H}}_2}\)

Different Examples for Action of Sodium Hydroxide on Metallic Salt Solutions

Generally, the reaction of sodium hydroxide solution with metallic salt solution gives metal hydroxide as a precipitate with the salt solution. The general chemical equation is represented as:

\({\rm{Metalic}}\,{\rm{Salt}} + {\rm{Sodium}}\,{\rm{Hydroxide}} \to {\rm{Metal}}\,{\rm{hydroxide}} + {\rm{Salt}}\,{\rm{formed}}\,{\rm{in}}\,{\rm{the}}\,{\rm{solution}}\)

1. The reaction of Calcium nitrate with sodium hydroxide
   Calcium nitrate is a colourless metallic salt solution that reacts with sodium hydroxide when it is added dropwise to the solution of calcium nitrate to form a white precipitate of calcium hydroxide along with sodium nitrate, which is colourless. When an excess of sodium hydroxide is added to this precipitate, calcium hydroxide becomes sparingly soluble. The following chemical equation denotes the chemical reaction involved in this process:
   \({\rm{Ca}}{\left( {{\rm{N}}{{\rm{O}}_3}} \right)_2} + 2{\rm{NaOH}} \to {\rm{Ca}}{({\rm{OH}})_2} \downarrow + 2{\rm{NaN}}{{\rm{O}}_3}\)
2. The reaction of Ferrous Sulphate with sodium hydroxide
   Ferrous Sulphate is a pale-green metallic salt solution that reacts with sodium hydroxide when it is added dropwise to the solution of ferrous sulphate to form a dirty-green gelatinous precipitate of ferrous hydroxide along with sodium sulphate, which is colourless. When an excess of sodium hydroxide is added to this precipitate, ferrous hydroxide is not soluble at all. The following chemical equation denotes the chemical reaction involved in this process:
   \({\rm{FeS}}{{\rm{O}}_4} + 2{\rm{NaOH}} \to {\rm{Fe}}{({\rm{OH}})_2} \downarrow + {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}\)
3. The reaction of Ferric Chloride with sodium hydroxide
   When sodium hydroxide solution is added dropwise to the solution of ferric chloride, it reacts with sodium hydroxide to form a reddish-brown precipitate of ferric hydroxide along with sodium chloride, which is colourless. When an excess of sodium hydroxide is added to this precipitate, ferrous hydroxide is not soluble at all. The following chemical equation denotes the chemical reaction involved in this process:
   \({\rm{FeC}}{{\rm{l}}_3} + 3{\rm{NaOH}} \to {\rm{Fe}}{({\rm{OH}})_3} \downarrow + 3{\rm{NaCl}}\)
4. The reaction of Copper Sulphate with sodium hydroxide
   When sodium hydroxide solution is added dropwise to the solution of copper sulphate, it reacts with sodium hydroxide to form a pale-blue precipitate of copper hydroxide along with sodium sulphate. When an excess of sodium hydroxide is added to this precipitate, copper hydroxide is not soluble at all. The following chemical equation denotes the chemical reaction involved in this process:
   \({\rm{CuS}}{{\rm{O}}_4} + 2{\rm{NaOH}} \to {\rm{Cu}}{({\rm{OH}})_2} \downarrow + {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}\)
5. The reaction of Zinc Sulphate with sodium hydroxide
   When sodium hydroxide solution is added dropwise to the solution of zinc sulphate, it reacts with sodium hydroxide to form a white-gelatinous precipitate of zinc hydroxide along with sodium sulphate. When an excess of sodium hydroxide is added to this precipitate, zinc hydroxide is soluble by forming a complex compound. The following chemical equation denotes the chemical reaction involved in this process: \({\rm{ZnS}}{{\rm{O}}_4} + 2{\rm{NaOH}} \to {\rm{Zn}}{({\rm{OH}})_2} \downarrow + {\rm{N}}{{\rm{a}}_2}{\rm{S}}{{\rm{O}}_4}\) \({\rm{Zn}}{({\rm{OH}})_2}({\rm{s}}) + {\rm{excessNaOH}}({\rm{aq}}.) \to {\rm{N}}{{\rm{a}}_2}\left[ {{\rm{Zn}}{{({\rm{OH}})}_4}} \right]({\rm{aq}}.)\) or \({\rm{Zn}}{({\rm{OH}})_2}({\rm{s}}) + 2{\rm{NaOH}}({\rm{aq}}.) \to {\rm{N}}{{\rm{a}}_2}{\rm{Zn}}{{\rm{O}}_2}({\rm{aq}}.) + 2{{\rm{H}}_2}{\rm{O}}\)
6. The reaction of Lead Nitrate with sodium hydroxideWhen sodium hydroxide solution is added dropwise to the solution of lead nitrate, it reacts with sodium hydroxide to form a white precipitate of lead hydroxide along with sodium nitrate. When an excess of sodium hydroxide is added to this precipitate, lead hydroxide becomes soluble. The following chemical equation denotes the chemical reaction involved in this process: \({\rm{Pb}}{\left( {{\rm{N}}{{\rm{O}}_3}} \right)_2} + 2{\rm{NaOH}} \to {\rm{Pb}}{({\rm{OH}})_2} \downarrow + 2{\rm{NaN}}{{\rm{O}}_3}\)
7. The reaction of Barium Chloride with sodium hydroxide
   When sodium hydroxide solution is added dropwise to the solution of barium chloride, it reacts with sodium hydroxide to form a white precipitate of barium hydroxide along with sodium chloride. The following chemical equation denotes the chemical reaction involved in this process: \({\rm{BaC}}{{\rm{l}}_2} + 2{\rm{NaOH}} \to {\rm{Ba}}{({\rm{OH}})_2} \downarrow + 2{\rm{NaCl}}\)
8. The reaction of Magnesium Chloride with sodium hydroxide
   When sodium hydroxide solution is added dropwise to the aqueous solution of magnesium chloride, it reacts with sodium hydroxide to form a solid, insoluble white precipitate of magnesium hydroxide along with sodium chloride as sodium displaces magnesium from its chloride salt as it is more reactive. The following chemical equation denotes the chemical reaction involved in this process: \({\rm{MgC}}{{\rm{l}}_2} + 2{\rm{NaOH}} \to {\rm{Mg}}{({\rm{OH}})_2} \downarrow + 2{\rm{NaCl}}\)

All these reactions are examples of double displacement reactions as two reactants exchange their ions to form new compounds.

Summary

Qualitative analysis is done to identify unknown salts in the chemical laboratories. In order to identify metal ions present in salt, metallic salts are reacted with sodium hydroxide, and based on the colour and solubility of the precipitate in the presence of an excess of sodium hydroxide gives us a clear identification of the metal ion present in that metallic salt solution. These reactions are basically double displacement reactions as both the ions exchange their positions. These reactions may also be called precipitation reactions as a precipitate of different colours and textures are formed.

Some examples of the reaction of metallic salts with sodium hydroxide are the reaction of calcium nitrate with sodium hydroxide to form calcium hydroxide and sodium nitrate, ferrous sulphate with sodium hydroxide to form ferrous hydroxide and sodium sulphate, ferric chloride with sodium hydroxide to form ferric hydroxide and sodium chloride, lead nitrate with sodium hydroxide to form lead hydroxide and sodium nitrate. It also involves zinc chloride with sodium hydroxide to form zinc hydroxide and sodium chloride, magnesium chloride with sodium hydroxide to form magnesium hydroxide and sodium chloride, etc. All the precipitates have different colours, textures, and solubilities in excess of sodium hydroxide solutions.

FAQs

Q.1. What does sodium hydroxide do to metal salt?
Ans: When sodium hydroxide reacts with a metal salt, it forms metal hydroxide as a precipitate with a salt solution. The general chemical equation is represented as:
\({\rm{Metalic}}\,{\rm{Salt}} + {\rm{Sodium}}\,{\rm{Hydroxide}} \to {\rm{Metal}}\,{\rm{hydroxide}} + {\rm{Salt}}\,{\rm{formed}}\,{\rm{in}}\,{\rm{the}}\,{\rm{solution}}\)

Q.2. Does NaOH react with metal salt?
Ans: Yes, sodium hydroxide reacts with a metal salt, forming metal hydroxide and sodium salt solution. There are various examples for such reactions as the reaction of calcium nitrate with sodium hydroxide to form calcium hydroxide and sodium nitrate, ferrous sulphate with sodium hydroxide to form ferrous hydroxide and sodium sulphate. The examples also involve ferric chloride with sodium hydroxide to form ferric hydroxide and sodium chloride, lead nitrate with sodium hydroxide to form lead hydroxide and sodium nitrate. The examples also involve zinc chloride with sodium hydroxide to form zinc hydroxide and sodium chloride, magnesium chloride with sodium hydroxide to form magnesium hydroxide and sodium chloride, etc.

Q.3. What happens when ammonium hydroxide is treated with a metal salt?
Ans: When ammonium hydroxide is added to the metallic salts, ammonium hydroxide solutions form precipitates of metal hydroxides and are identified by their distinct colours. The precipitated metallic hydroxides are soluble when excess amounts of ammonium hydroxide are added.

Q.4. What happens when a base reacts with a metallic salt?
Ans: When a base reacts with a metal, a salt is formed along with the evolution of hydrogen gas. The salt has a positive ion composed of metal and oxygen. For example, zinc reacts with sodium hydroxide to form sodium zincate along with hydrogen gas. This hydrogen gas can be identified by bringing a burning matchstick near it; as soon as the burning matchstick comes near the gas, it explodes with a pop sound, and this clarifies the evolution of hydrogen gas during the reaction.

Q.5. Which metals can dissolve in excess sodium hydroxide solution?
Ans: Excess sodium hydroxide can be used to dissolve the solution of lead hydroxide and calcium hydroxide (sparingly soluble). Calcium nitrates react with sodium hydroxide to form a precipitate of calcium hydroxide and sodium nitrate, and when an excess of sodium hydroxide is added, it gets sparingly dissolved. Similarly, when sodium hydroxide solution is added dropwise to the solution of lead sulphate, it reacts with sodium hydroxide to form a white precipitate of lead hydroxide along with sodium sulphate. When an excess of sodium hydroxide is added to this precipitate, lead hydroxide becomes soluble.

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