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Methods of Purification of Metals

Contents

Surface chemistry focuses on the behavior of chemical species at the interfaces of solid, liquid, and gas phases, contributing to various Chemistry Topics.

Refining Process of Metals Against Impurities

The chemical effect of electric current is used in the purification of impure metals (which are extracted from their naturally occurring compounds called ores). In the purification of an impure metal by using the chemical effect of current (or electrolysis) :
(a) A thick rod of impure metal is made positive electrode (or anode) : It is connected to the positive terminal of the battery.
(b) A thin strip of pure metal is made negative electrode (or cathode): It is connected to the negative terminal of the battery.
(c) A water soluble salt of the metal to be purified is taken as electrolyte.

On passing electric current, the metal dissolves from the impure anode and goes into electrolyte solution. The metal present in dissolved form in electrolyte gets deposited on the cathode in the pure form. The impurities are left behind in the electrolyte solution. The metals like copper, zinc and aluminium, etc., are purified by the process of electrolysis by using the chemical effect of electric current.

Activity 5

To Purify Impure Copper Metal

Take 250 mL of distilled water in a clean beaker. Dissolve two teaspoonfuls of copper sulphate in it. Add a few drops of dilute sulphuric acid to copper sulphate solution. A thick rod of impure copper metal is made positive electrode (anode) by connecting it to the positive terminal of the battery (see Figure).

A thin plate of pure copper metal is made negative electrode (cathode) by connecting it to the negative terminal of the battery (see Figure).
Methods of Purification of Metals 1
Switch on the electric current by closing the switch. Allow the current to pass for about half an hour. It will be observed that the impure copper rod goes on becoming thinner and thinner whereas the pure copper plate goes on becoming thicker and thicker.

This is because the impure copper metal of ‘anode’ goes on dissolving in copper sulphate solution whereas the pure metal from copper sulphate solution goes on depositing on copper plate’cathode’. Impurities present in impure rod of copper fall to the bottom of the beaker.

Please note that the process of purification of impure copper is like electroplating ‘copper metal’ on ‘copper metal’. This is because the copper metal of impure copper rod (anode) gets deposited on the pure copper plate (cathode).

Production of Metals

The chemical effect of electric current is used in the production (or extraction) of certain metals from their naturally occurring compounds called ores. Actually, the chemically reactive metals are produced by this method. For example, the reactive metals such as sodium, aluminium and magnesium, etc., are produced by passing electric current (or electricity) through their compounds in molten state (melted state).

Sodium metal is produced by the electrolysis of molten sodium chloride whereas aluminium metal is produced by the electrolysis of molten aluminium oxide. In all the cases, metal is produced at the negatively charged electrode (or cathode). This is because the positively charged ions of metals present in their molten compounds are attracted by the negatively charged (or oppositely charged) electrode.

Production of Compounds

The chemical effect of electric current (or electrolysis) is used in the production of various chemical compounds. For example, sodium hydroxide (or caustic soda) is produced by the electrolysis of an aqueous solution (water solution) of sodium chloride.

Decomposition of Compounds

The chemical effect of electric current (or electrolysis) is used to decompose various chemical compounds into their elements. For example, water can be decomposed by passing electric current (or electrolysis) into two elements : hydrogen and oxygen.

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