A Thin Coat of Silver Can Be Laid Down Using Nothing but Electric Current
See how an object is coated with nickel or silver by electrolysis, why nickel sulphate and sodium argentocyanide are the chosen electrolytes, how impure copper is purified by electrorefining, and why industry depends on both processes for everything from taps to electrical wire.
How does an ordinary metal object end up with a shiny silver or nickel surface?
A bicycle bell gleams like polished silver, yet underneath it is plain steel. A set of silver-coloured spoons given as a gift is often brass or copper with a coat of silver finer than a sheet of paper. Nobody painted those coats on; they were built up atom by atom by an electric current.
The process is electroplating, and it uses exactly the chemistry of Part 2 — copper sulphate electrolysed with copper electrodes. There, the copper anode dissolved and the same amount of copper appeared on the cathode. Now replace the cathode with the object to be coated, and the anode with a block of the coating metal. The coating metal leaves the anode and settles evenly over the object.
Turn the idea around and it purifies metal instead. Make the anode a lump of impure copper and the cathode a thin sheet of pure copper. Copper moves from the impure block to the pure sheet, but most impurities cannot follow — some stay dissolved, others fall to the bottom. That is electrorefining.
So this part covers:
- Electroplating with nickel and with silver — the arrangement of electrodes and the equations
- Why the electrolytes are chosen — nickel sulphate for nickel, and a cyanide complex for silver
- Electrorefining of copper — electrodes, electrolyte, reactions and what happens to impurities
- Why industry depends on both
Both processes appear around you every day. Chrome-finished taps, nickel-plated handlebars, gold-plated connectors inside electronic devices and the copper in every electric wire at home all rely on electrolysis for their surface or their purity.
The single idea to hold on to. In both processes, the anode is made of the metal that is moving, and it dissolves; the metal ions in the electrolyte are replaced as fast as they are deposited. The concentration of the electrolyte therefore stays steady, which is what makes long, controlled operation possible.
This page covers the third part of the ICSE Class 10 Chemistry chapter on electrolysis: electroplating with nickel and silver, the choice of electrolytes, electrorefining of copper, and industrial importance.
The process is electroplating, and it uses exactly the chemistry of Part 2 — copper sulphate electrolysed with copper electrodes. There, the copper anode dissolved and the same amount of copper appeared on the cathode. Now replace the cathode with the object to be coated, and the anode with a block of the coating metal. The coating metal leaves the anode and settles evenly over the object.
Turn the idea around and it purifies metal instead. Make the anode a lump of impure copper and the cathode a thin sheet of pure copper. Copper moves from the impure block to the pure sheet, but most impurities cannot follow — some stay dissolved, others fall to the bottom. That is electrorefining.
So this part covers:
- Electroplating with nickel and with silver — the arrangement of electrodes and the equations
- Why the electrolytes are chosen — nickel sulphate for nickel, and a cyanide complex for silver
- Electrorefining of copper — electrodes, electrolyte, reactions and what happens to impurities
- Why industry depends on both
Both processes appear around you every day. Chrome-finished taps, nickel-plated handlebars, gold-plated connectors inside electronic devices and the copper in every electric wire at home all rely on electrolysis for their surface or their purity.
The single idea to hold on to. In both processes, the anode is made of the metal that is moving, and it dissolves; the metal ions in the electrolyte are replaced as fast as they are deposited. The concentration of the electrolyte therefore stays steady, which is what makes long, controlled operation possible.
This page covers the third part of the ICSE Class 10 Chemistry chapter on electrolysis: electroplating with nickel and silver, the choice of electrolytes, electrorefining of copper, and industrial importance.
How is an object electroplated with nickel and with silver?
The object is made the cathode, a block of the plating metal is the anode, and the electrolyte contains ions of the plating metal, so metal dissolves from the anode and deposits evenly on the object.
Electroplating is the process of depositing a thin, firm layer of a superior metal on the surface of an article by electrolysis.
The general arrangement:
- Cathode: the article to be plated, thoroughly cleaned, connected to the negative terminal
- Anode: a block of the pure plating metal, connected to the positive terminal
- Electrolyte: a solution containing ions of the plating metal
- Current: a low, steady direct current passed for a longer time, giving a smooth deposit
1. Nickel plating — for example, an iron key.
- Cathode: the iron article
- Anode: a plate of pure nickel
- Electrolyte: aqueous nickel sulphate solution
Ionisation:
At the cathode — nickel deposits on the article:
At the anode — the nickel plate dissolves:
2. Silver plating — for example, a brass spoon.
- Cathode: the brass spoon
- Anode: a plate of pure silver
- Electrolyte: sodium argentocyanide, , solution
Ionisation:
At the cathode — silver deposits on the spoon:
At the anode — the silver plate dissolves:
Conditions for a good deposit:
- The article must be clean and free of grease, or the coat will not stick
- A low current gives a slow, even, firmly adhering layer
- The article is often rotated, so that every side is coated equally
Worked example — comparing the two metals electron by electron. The same quantity of electricity passes through a nickel-plating cell and a silver-plating cell. **For moles of electrons:
- Nickel**: needs electrons per atom, so mole of nickel is deposited
- Silver: needs electron per atom, so moles of silver are deposited
The same current deposits twice as many silver atoms as nickel atoms, because each silver ion needs only one electron.
An everyday example. Many steel bicycle handlebars and bathroom fittings are nickel-plated and then given a thin chromium finish. The nickel layer underneath protects the steel from rust, while the outer layer adds shine and hardness.
The trap that reverses the whole process. If the article is connected as the anode instead of the cathode, it will not be coated — it will lose metal and be eaten away, because oxidation happens at the anode. The article always goes at the cathode.
Electroplating is the process of depositing a thin, firm layer of a superior metal on the surface of an article by electrolysis.
The general arrangement:
- Cathode: the article to be plated, thoroughly cleaned, connected to the negative terminal
- Anode: a block of the pure plating metal, connected to the positive terminal
- Electrolyte: a solution containing ions of the plating metal
- Current: a low, steady direct current passed for a longer time, giving a smooth deposit
1. Nickel plating — for example, an iron key.
- Cathode: the iron article
- Anode: a plate of pure nickel
- Electrolyte: aqueous nickel sulphate solution
Ionisation:
At the cathode — nickel deposits on the article:
At the anode — the nickel plate dissolves:
2. Silver plating — for example, a brass spoon.
- Cathode: the brass spoon
- Anode: a plate of pure silver
- Electrolyte: sodium argentocyanide, , solution
Ionisation:
At the cathode — silver deposits on the spoon:
At the anode — the silver plate dissolves:
Conditions for a good deposit:
- The article must be clean and free of grease, or the coat will not stick
- A low current gives a slow, even, firmly adhering layer
- The article is often rotated, so that every side is coated equally
Worked example — comparing the two metals electron by electron. The same quantity of electricity passes through a nickel-plating cell and a silver-plating cell. **For moles of electrons:
- Nickel**: needs electrons per atom, so mole of nickel is deposited
- Silver: needs electron per atom, so moles of silver are deposited
The same current deposits twice as many silver atoms as nickel atoms, because each silver ion needs only one electron.
An everyday example. Many steel bicycle handlebars and bathroom fittings are nickel-plated and then given a thin chromium finish. The nickel layer underneath protects the steel from rust, while the outer layer adds shine and hardness.
The trap that reverses the whole process. If the article is connected as the anode instead of the cathode, it will not be coated — it will lose metal and be eaten away, because oxidation happens at the anode. The article always goes at the cathode.
Why are nickel sulphate and sodium argentocyanide chosen as electrolytes?
Nickel sulphate supplies nickel ions in a stable solution whose anions are not discharged, while sodium argentocyanide releases silver ions slowly, keeping their concentration low so the silver deposits smoothly and firmly.
The basic requirement for any plating electrolyte. It must contain ions of the plating metal, because only those ions can be deposited as that metal at the cathode.
1. Why nickel sulphate for nickel plating.
- It is soluble and gives a good supply of ions
- Sulphate ions are not discharged at the anode, so no unwanted product forms there — the nickel anode dissolves instead
- The nickel anode replaces every nickel ion deposited, so the concentration of the solution stays constant
A boundary case with the activity series. Nickel lies above hydrogen, so from Part 2 you might expect hydrogen rather than nickel at the cathode. The high concentration of nickel ions in the plating bath makes nickel discharge in preference — the concentration factor overriding the series, exactly as chloride ions win in concentrated brine.
2. Why sodium argentocyanide, and not silver nitrate, for silver plating.
Silver nitrate would seem the obvious choice — it is soluble and full of silver ions. But it gives a poor coat, for two reasons.
- Too many silver ions at once. Silver nitrate dissociates completely, so the concentration of is high. Silver deposits too quickly, forming a rough, loose, uneven layer that does not stick well
- Direct displacement. A brass or copper article placed in silver nitrate solution reacts even without any current, because copper is more reactive than silver:
Silver displaced this way forms a loose, powdery layer instead of a controlled coating.
How sodium argentocyanide solves both problems. Silver in this compound is held in the complex ion , which breaks up only slightly:
- **The concentration of free stays low, so silver deposits slowly and evenly
- As silver ions are used up, more complex ions break up to replace them, so the supply remains steady
- The article is not attacked by displacement, because so few free silver ions are present
The result is a smooth, bright, firmly adhering silver coat.
Worked example — reasoning from the equilibrium. A student uses silver nitrate to plate a copper spoon and finds a dark, flaky deposit that rubs off. Explain, and suggest a correction.
The high concentration of silver ions gave rapid, uneven deposition, and some silver was displaced by the copper directly. Replacing the electrolyte with sodium argentocyanide lowers the free silver ion concentration and gives a slow, firm deposit.
The safety line. Cyanide compounds are extremely poisonous, so plating baths of this kind are handled only under strict industrial controls.
An everyday example. Cheap silver-coloured jewellery whose coat peels or rubs off within weeks has usually been given too thin or too hastily deposited a layer. The slow, controlled deposition that a complex electrolyte provides** is what separates a lasting finish from a peeling one.
The basic requirement for any plating electrolyte. It must contain ions of the plating metal, because only those ions can be deposited as that metal at the cathode.
1. Why nickel sulphate for nickel plating.
- It is soluble and gives a good supply of ions
- Sulphate ions are not discharged at the anode, so no unwanted product forms there — the nickel anode dissolves instead
- The nickel anode replaces every nickel ion deposited, so the concentration of the solution stays constant
A boundary case with the activity series. Nickel lies above hydrogen, so from Part 2 you might expect hydrogen rather than nickel at the cathode. The high concentration of nickel ions in the plating bath makes nickel discharge in preference — the concentration factor overriding the series, exactly as chloride ions win in concentrated brine.
2. Why sodium argentocyanide, and not silver nitrate, for silver plating.
Silver nitrate would seem the obvious choice — it is soluble and full of silver ions. But it gives a poor coat, for two reasons.
- Too many silver ions at once. Silver nitrate dissociates completely, so the concentration of is high. Silver deposits too quickly, forming a rough, loose, uneven layer that does not stick well
- Direct displacement. A brass or copper article placed in silver nitrate solution reacts even without any current, because copper is more reactive than silver:
Silver displaced this way forms a loose, powdery layer instead of a controlled coating.
How sodium argentocyanide solves both problems. Silver in this compound is held in the complex ion , which breaks up only slightly:
- **The concentration of free stays low, so silver deposits slowly and evenly
- As silver ions are used up, more complex ions break up to replace them, so the supply remains steady
- The article is not attacked by displacement, because so few free silver ions are present
The result is a smooth, bright, firmly adhering silver coat.
Worked example — reasoning from the equilibrium. A student uses silver nitrate to plate a copper spoon and finds a dark, flaky deposit that rubs off. Explain, and suggest a correction.
The high concentration of silver ions gave rapid, uneven deposition, and some silver was displaced by the copper directly. Replacing the electrolyte with sodium argentocyanide lowers the free silver ion concentration and gives a slow, firm deposit.
The safety line. Cyanide compounds are extremely poisonous, so plating baths of this kind are handled only under strict industrial controls.
An everyday example. Cheap silver-coloured jewellery whose coat peels or rubs off within weeks has usually been given too thin or too hastily deposited a layer. The slow, controlled deposition that a complex electrolyte provides** is what separates a lasting finish from a peeling one.
How is impure copper purified by electrorefining?
Impure copper is made the anode and a thin sheet of pure copper the cathode in acidified copper sulphate solution; copper dissolves from the anode and deposits as pure copper on the cathode, while impurities stay in solution or fall as anode mud.
The arrangement:
- Anode: a thick block of impure copper
- Cathode: a thin sheet of pure copper
- Electrolyte: aqueous copper sulphate acidified with a little dilute sulphuric acid, which improves conduction
Ionisation:
At the anode — copper from the impure block dissolves:
At the cathode — pure copper deposits:
What happens to the impurities — decided by the activity series from Part 2:
- More reactive metals, such as zinc and iron, dissolve from the anode as ions. Their ions are harder to discharge than copper ions, so they remain in solution and never reach the cathode
- Less reactive metals, such as silver and gold, do not dissolve at all. As the copper around them is removed, they fall to the bottom beneath the anode as anode mud
Observations over time:
- The anode becomes thinner
- The cathode grows thicker with pure copper
- Anode mud collects below the anode
- The blue colour of the solution stays almost unchanged
Worked example — tracing the mass. During refining, the cathode gains of pure copper, atomic mass .
**The anode loses at least of copper — and in practice loses more mass than the cathode gains, because impurities also leave it, either dissolving as ions or dropping as mud.
The boundary case — why reactive impurities never contaminate the cathode. Zinc ions and iron ions are present in the solution throughout, yet the cathode stays pure. Copper ions, lower in the series, are always discharged in preference, so the cathode receives copper and nothing else as long as copper ions are plentiful.
An everyday example. The copper in the wiring of every home is refined this way. Even small amounts of impurity raise the electrical resistance of copper noticeably, so wire made from unrefined copper would waste energy as heat.
The link back to Part 2. This is the copper-electrode electrolysis of copper sulphate, with one change: the anode is impure.** The chemistry at each electrode is unchanged, and the purification follows from nothing more than the order of the activity series.
The arrangement:
- Anode: a thick block of impure copper
- Cathode: a thin sheet of pure copper
- Electrolyte: aqueous copper sulphate acidified with a little dilute sulphuric acid, which improves conduction
Ionisation:
At the anode — copper from the impure block dissolves:
At the cathode — pure copper deposits:
What happens to the impurities — decided by the activity series from Part 2:
- More reactive metals, such as zinc and iron, dissolve from the anode as ions. Their ions are harder to discharge than copper ions, so they remain in solution and never reach the cathode
- Less reactive metals, such as silver and gold, do not dissolve at all. As the copper around them is removed, they fall to the bottom beneath the anode as anode mud
Observations over time:
- The anode becomes thinner
- The cathode grows thicker with pure copper
- Anode mud collects below the anode
- The blue colour of the solution stays almost unchanged
Worked example — tracing the mass. During refining, the cathode gains of pure copper, atomic mass .
**The anode loses at least of copper — and in practice loses more mass than the cathode gains, because impurities also leave it, either dissolving as ions or dropping as mud.
The boundary case — why reactive impurities never contaminate the cathode. Zinc ions and iron ions are present in the solution throughout, yet the cathode stays pure. Copper ions, lower in the series, are always discharged in preference, so the cathode receives copper and nothing else as long as copper ions are plentiful.
An everyday example. The copper in the wiring of every home is refined this way. Even small amounts of impurity raise the electrical resistance of copper noticeably, so wire made from unrefined copper would waste energy as heat.
The link back to Part 2. This is the copper-electrode electrolysis of copper sulphate, with one change: the anode is impure.** The chemistry at each electrode is unchanged, and the purification follows from nothing more than the order of the activity series.
Why are electroplating and electrorefining important in industry?
Electroplating protects metals from corrosion, improves their appearance and hardness and saves costly metals, while electrorefining supplies the very pure metals industry needs and recovers precious metals as a by-product.
Uses of electroplating:
- Protection from corrosion — a coat of nickel, chromium or tin keeps air and moisture away from iron and steel. Many food cans are made from steel electroplated with tin, which does not react with the food
- Decoration — silver and gold plating give an attractive, costly-looking finish to jewellery, utensils and ornaments
- Hardness and wear resistance — chromium plating on tools and machine parts resists scratching and wear
- Economy — a cheap, strong base metal carries only a thin layer of an expensive metal, instead of the whole article being made of it
- Electronics — electrical connectors are often gold-plated because gold does not tarnish, so the contact stays reliable
Uses of electrorefining:
- High-purity metal — especially copper for electrical wires and cables, where purity controls conductivity
- Recovery of valuable metals — silver, gold and other precious metals are recovered from the anode mud, which helps pay for the refining
- Refining of other metals — silver, gold, zinc and several others are purified by the same principle, with an electrolyte containing ions of the metal concerned
- Recycling — scrap copper can be refined back to pure copper
Worked comparison — the two processes side by side.
- Aim: electroplating coats an object; electrorefining purifies a metal
- Anode: electroplating uses pure plating metal; electrorefining uses impure metal
- Cathode: electroplating uses the article; electrorefining uses a thin sheet of pure metal
- Electrolyte: both use a solution of ions of the metal being moved
- Anode behaviour: dissolves in both
- Electrolyte concentration: stays nearly constant in both
- By-product: none in electroplating; anode mud in electrorefining
Worked example — identifying the process. A cell has a thick block of impure silver as the anode, a thin strip of pure silver as the cathode and silver nitrate solution with a little nitric acid as the electrolyte. Which process is it, and what forms below the anode?
Electrorefining of silver. Copper impurities dissolve and stay in solution because copper ions are harder to discharge than silver ions; gold impurities fall as anode mud.
An everyday example. A chrome-finished tap in a kitchen stays bright for years in daily contact with water. The plated layer is a barrier between the steel or brass underneath and the moisture that would otherwise corrode it.
The boundary case. A plated coat protects only while it is unbroken. If a tin-plated steel can is scratched deeply, the exposed iron can rust faster than bare iron would, because tin is less reactive than iron and the two metals together in moisture speed up the corrosion of the iron. Plating is protection by covering, not by chemical sacrifice.
Uses of electroplating:
- Protection from corrosion — a coat of nickel, chromium or tin keeps air and moisture away from iron and steel. Many food cans are made from steel electroplated with tin, which does not react with the food
- Decoration — silver and gold plating give an attractive, costly-looking finish to jewellery, utensils and ornaments
- Hardness and wear resistance — chromium plating on tools and machine parts resists scratching and wear
- Economy — a cheap, strong base metal carries only a thin layer of an expensive metal, instead of the whole article being made of it
- Electronics — electrical connectors are often gold-plated because gold does not tarnish, so the contact stays reliable
Uses of electrorefining:
- High-purity metal — especially copper for electrical wires and cables, where purity controls conductivity
- Recovery of valuable metals — silver, gold and other precious metals are recovered from the anode mud, which helps pay for the refining
- Refining of other metals — silver, gold, zinc and several others are purified by the same principle, with an electrolyte containing ions of the metal concerned
- Recycling — scrap copper can be refined back to pure copper
Worked comparison — the two processes side by side.
- Aim: electroplating coats an object; electrorefining purifies a metal
- Anode: electroplating uses pure plating metal; electrorefining uses impure metal
- Cathode: electroplating uses the article; electrorefining uses a thin sheet of pure metal
- Electrolyte: both use a solution of ions of the metal being moved
- Anode behaviour: dissolves in both
- Electrolyte concentration: stays nearly constant in both
- By-product: none in electroplating; anode mud in electrorefining
Worked example — identifying the process. A cell has a thick block of impure silver as the anode, a thin strip of pure silver as the cathode and silver nitrate solution with a little nitric acid as the electrolyte. Which process is it, and what forms below the anode?
Electrorefining of silver. Copper impurities dissolve and stay in solution because copper ions are harder to discharge than silver ions; gold impurities fall as anode mud.
An everyday example. A chrome-finished tap in a kitchen stays bright for years in daily contact with water. The plated layer is a barrier between the steel or brass underneath and the moisture that would otherwise corrode it.
The boundary case. A plated coat protects only while it is unbroken. If a tin-plated steel can is scratched deeply, the exposed iron can rust faster than bare iron would, because tin is less reactive than iron and the two metals together in moisture speed up the corrosion of the iron. Plating is protection by covering, not by chemical sacrifice.
Exam tip
What must an electroplating or electrorefining answer include?
Name the anode, cathode and electrolyte for the specific metal, write both electrode equations, and state what happens to the anode, the cathode and the solution.
- Put the article at the cathode in electroplating and the impure metal at the anode in electrorefining
- Name the electrolyte exactly: nickel sulphate solution; sodium argentocyanide solution; copper sulphate with dilute sulphuric acid
- Write the dissociation of the electrolyte, including the complex ion for silver
- Write both electrode reactions with electrons: and
- State that the anode dissolves and that the electrolyte concentration stays constant
- Mention a low current and a clean article as conditions for a good coat
- Give both reasons for rejecting silver nitrate: fast rough deposition and direct displacement
- Say where impurities go in electrorefining: reactive ones stay in solution, less reactive ones form anode mud
- Name the metals recovered from anode mud
- List at least three uses with the purpose of each
The misconception to name. The anode in electroplating is not an inert electrode. It must be made of the plating metal, because its dissolving is what keeps the electrolyte supplied. A platinum anode would let the silver or nickel ions run out and release gas instead.
A second trap. Writing that impurities in electrorefining are deposited at the cathode. Reactive impurities remain in solution and unreactive ones fall as mud — the cathode receives only pure copper, which is the whole point of the process.
- Put the article at the cathode in electroplating and the impure metal at the anode in electrorefining
- Name the electrolyte exactly: nickel sulphate solution; sodium argentocyanide solution; copper sulphate with dilute sulphuric acid
- Write the dissociation of the electrolyte, including the complex ion for silver
- Write both electrode reactions with electrons: and
- State that the anode dissolves and that the electrolyte concentration stays constant
- Mention a low current and a clean article as conditions for a good coat
- Give both reasons for rejecting silver nitrate: fast rough deposition and direct displacement
- Say where impurities go in electrorefining: reactive ones stay in solution, less reactive ones form anode mud
- Name the metals recovered from anode mud
- List at least three uses with the purpose of each
The misconception to name. The anode in electroplating is not an inert electrode. It must be made of the plating metal, because its dissolving is what keeps the electrolyte supplied. A platinum anode would let the silver or nickel ions run out and release gas instead.
A second trap. Writing that impurities in electrorefining are deposited at the cathode. Reactive impurities remain in solution and unreactive ones fall as mud — the cathode receives only pure copper, which is the whole point of the process.
Did you know
Why does a silver-plated spoon turn black, and how can it be made bright again?
Leave a silver-plated spoon in a drawer, or use it to eat a boiled egg, and its surface slowly darkens to a dull brown-black. The silver has not worn away; it has reacted with a gas present in tiny amounts in the air.
That gas is hydrogen sulphide, the same rotten-egg gas you identified with lead acetate paper in the acids chapter. It is released by eggs, some foods and decaying matter, and silver reacts with it and with oxygen to form a thin film of black silver sulphide:
Checking the balance: silver ; hydrogen ; sulphur ; oxygen . Balanced.
Eggs darken silver spoons especially quickly because cooked eggs release hydrogen sulphide from the sulphur in their proteins.
Polishing removes the black film — but it also removes a little silver each time, which matters for a plated article whose silver layer is very thin.
There is a gentler, chemical way that gives the silver back. Line a bowl with aluminium foil, add hot water with a spoonful of baking soda, and place the tarnished spoon so that it touches the foil. The black film fades within minutes. Aluminium, being far more reactive than silver, takes the sulphur and returns the silver to metal:
Checking the balance: silver ; sulphur ; aluminium . Balanced. The baking soda solution simply lets ions move between the spoon and the foil, turning the bowl into a small electrochemical cell — the same kind of electron transfer that runs through this whole chapter.
The same reactivity order explains the plating choice. Silver resists oxygen and water, which is why it is used as a decorative coat at all; it is only the trace of sulphur in the air that tarnishes it, and the activity series tells you both why silver survives and how aluminium can rescue it.
That gas is hydrogen sulphide, the same rotten-egg gas you identified with lead acetate paper in the acids chapter. It is released by eggs, some foods and decaying matter, and silver reacts with it and with oxygen to form a thin film of black silver sulphide:
Checking the balance: silver ; hydrogen ; sulphur ; oxygen . Balanced.
Eggs darken silver spoons especially quickly because cooked eggs release hydrogen sulphide from the sulphur in their proteins.
Polishing removes the black film — but it also removes a little silver each time, which matters for a plated article whose silver layer is very thin.
There is a gentler, chemical way that gives the silver back. Line a bowl with aluminium foil, add hot water with a spoonful of baking soda, and place the tarnished spoon so that it touches the foil. The black film fades within minutes. Aluminium, being far more reactive than silver, takes the sulphur and returns the silver to metal:
Checking the balance: silver ; sulphur ; aluminium . Balanced. The baking soda solution simply lets ions move between the spoon and the foil, turning the bowl into a small electrochemical cell — the same kind of electron transfer that runs through this whole chapter.
The same reactivity order explains the plating choice. Silver resists oxygen and water, which is why it is used as a decorative coat at all; it is only the trace of sulphur in the air that tarnishes it, and the activity series tells you both why silver survives and how aluminium can rescue it.
Exam relevance
How do electroplating and refining calculations appear in JEE and NEET?
This is foundation work for Class 12 Electrochemistry, examined in both JEE Main and NEET Chemistry, and for Class 12 Coordination Compounds.
Where electroplating leads. Class 12 introduces Faraday's first law — the mass of substance deposited is proportional to the charge passed — and Faraday's second law, which compares the masses of different substances deposited by the same charge. The worked example on this page, where the same charge deposits twice as many silver atoms as nickel atoms, is the second law in words. Numericals asking for the mass deposited, the time needed or the current required for a plating job appear in both exams.
Where the electrode equations lead. Every Faraday's-law calculation depends on the number of electrons per ion: one for silver, two for nickel and copper, three for aluminium. Writing the electrode equation first is how that number is found, and it is the step most often skipped under time pressure.
Where sodium argentocyanide leads. The ion is a complex ion of the kind studied in Class 12 Coordination Compounds. Its name, the oxidation state of silver in it and the number of ligands are typical questions, and the idea that a complex releases metal ions only slightly connects to the stability of complexes.
Where electrorefining leads. The fate of impurities — reactive ones staying in solution, less reactive ones falling as anode mud — is explained in Class 12 by standard electrode potentials. Predicting which metals end up in the anode mud is a reasoning question that uses exactly the activity series argument here.
Question types to expect. At this level: arrangement of electrodes, electrolyte choice with reasons, equations and uses. In competitive papers: Faraday's-law numericals on mass, time and current, comparisons of masses deposited in cells connected in series, complex-ion naming and electrode-potential reasoning.
The single trap that costs marks. Using the wrong number of electrons per ion in a Faraday's-law problem. Silver needs one electron and copper two, so for the same charge the moles deposited differ by a factor of two — and options built on the wrong factor are common.
A second trap. Treating the anode in electroplating as inert. The dissolving anode is what keeps the electrolyte concentration constant, and assertion-reason questions test exactly that.
Board versus competitive emphasis. The ICSE paper marks the arrangement, the chosen electrolyte with its justification and the equations; a competitive paper marks a calculated mass or time. The transferable habit is always writing the electrode equation before doing any arithmetic, because the electrons in it decide every number that follows.
Where electroplating leads. Class 12 introduces Faraday's first law — the mass of substance deposited is proportional to the charge passed — and Faraday's second law, which compares the masses of different substances deposited by the same charge. The worked example on this page, where the same charge deposits twice as many silver atoms as nickel atoms, is the second law in words. Numericals asking for the mass deposited, the time needed or the current required for a plating job appear in both exams.
Where the electrode equations lead. Every Faraday's-law calculation depends on the number of electrons per ion: one for silver, two for nickel and copper, three for aluminium. Writing the electrode equation first is how that number is found, and it is the step most often skipped under time pressure.
Where sodium argentocyanide leads. The ion is a complex ion of the kind studied in Class 12 Coordination Compounds. Its name, the oxidation state of silver in it and the number of ligands are typical questions, and the idea that a complex releases metal ions only slightly connects to the stability of complexes.
Where electrorefining leads. The fate of impurities — reactive ones staying in solution, less reactive ones falling as anode mud — is explained in Class 12 by standard electrode potentials. Predicting which metals end up in the anode mud is a reasoning question that uses exactly the activity series argument here.
Question types to expect. At this level: arrangement of electrodes, electrolyte choice with reasons, equations and uses. In competitive papers: Faraday's-law numericals on mass, time and current, comparisons of masses deposited in cells connected in series, complex-ion naming and electrode-potential reasoning.
The single trap that costs marks. Using the wrong number of electrons per ion in a Faraday's-law problem. Silver needs one electron and copper two, so for the same charge the moles deposited differ by a factor of two — and options built on the wrong factor are common.
A second trap. Treating the anode in electroplating as inert. The dissolving anode is what keeps the electrolyte concentration constant, and assertion-reason questions test exactly that.
Board versus competitive emphasis. The ICSE paper marks the arrangement, the chosen electrolyte with its justification and the equations; a competitive paper marks a calculated mass or time. The transferable habit is always writing the electrode equation before doing any arithmetic, because the electrons in it decide every number that follows.
Key takeaways
What must you be able to do from this part?
Two plating cells, one refining cell, two electrolyte choices and a set of uses.
- Electroplating deposits a thin layer of a superior metal on an article by electrolysis
- Article at the cathode, pure plating metal at the anode, electrolyte containing ions of the plating metal, low current, clean surface
- Nickel plating: nickel sulphate electrolyte; at the cathode, at the anode
- Silver plating: sodium argentocyanide electrolyte; at the cathode, at the anode
- The anode dissolves and keeps the electrolyte concentration constant
- Nickel sulphate supplies nickel ions, its sulphate ions are not discharged, and high nickel ion concentration lets nickel deposit
- Silver nitrate is unsuitable: too many silver ions give a rough loose deposit, and copper or brass displaces silver directly
- Sodium argentocyanide releases silver ions slowly from , giving a smooth, firm coat
- Same charge: twice as many silver atoms as nickel atoms are deposited
- Electrorefining of copper: impure copper anode, pure copper cathode, acidified copper sulphate electrolyte
- Anode: ; cathode:
- Reactive impurities such as zinc and iron stay in solution; less reactive ones such as silver and gold form anode mud
- ** of copper deposited** needs mol of electrons; the anode loses more mass than the cathode gains
- Electroplating uses: corrosion protection, decoration, hardness, economy, reliable electrical contacts
- Electrorefining uses: pure copper for wires, recovery of precious metals, refining other metals, recycling
The sharpest self-test is a single sketch done twice. Draw one cell set up to silver-plate a spoon and another to refine impure copper, label every electrode and electrolyte, write both electrode equations for each — then list every way the two cells differ.
- Electroplating deposits a thin layer of a superior metal on an article by electrolysis
- Article at the cathode, pure plating metal at the anode, electrolyte containing ions of the plating metal, low current, clean surface
- Nickel plating: nickel sulphate electrolyte; at the cathode, at the anode
- Silver plating: sodium argentocyanide electrolyte; at the cathode, at the anode
- The anode dissolves and keeps the electrolyte concentration constant
- Nickel sulphate supplies nickel ions, its sulphate ions are not discharged, and high nickel ion concentration lets nickel deposit
- Silver nitrate is unsuitable: too many silver ions give a rough loose deposit, and copper or brass displaces silver directly
- Sodium argentocyanide releases silver ions slowly from , giving a smooth, firm coat
- Same charge: twice as many silver atoms as nickel atoms are deposited
- Electrorefining of copper: impure copper anode, pure copper cathode, acidified copper sulphate electrolyte
- Anode: ; cathode:
- Reactive impurities such as zinc and iron stay in solution; less reactive ones such as silver and gold form anode mud
- ** of copper deposited** needs mol of electrons; the anode loses more mass than the cathode gains
- Electroplating uses: corrosion protection, decoration, hardness, economy, reliable electrical contacts
- Electrorefining uses: pure copper for wires, recovery of precious metals, refining other metals, recycling
The sharpest self-test is a single sketch done twice. Draw one cell set up to silver-plate a spoon and another to refine impure copper, label every electrode and electrolyte, write both electrode equations for each — then list every way the two cells differ.