When items of jewellery made of metals such as copper or nickel are placed in a solution having a salt of gold, a thin film of gold is deposited by

1. Chemical Effects of Electric Current (basic)

To understand the chemical effects of electric current, we must first look at how electricity interacts with different materials. We know that materials like silver, copper, and gold are excellent conductors, which is why copper is the standard choice for household wiring (Science-Class VII, Electricity: Circuits and their Components, p.36). However, electricity behaves uniquely when it passes through liquids. While a solid wire might simply get hot (the heating effect) or create a magnetic field (Science, Class VIII, Electricity: Magnetic and Heating Effects, p.58), passing a current through a conducting liquid—known as an electrolyte—actually triggers a chemical reaction. This process is driven by the movement of ions (charged particles) within the liquid. When the current flows, it can cause gas bubbles to form at the electrodes, change the color of the solution, or result in the deposition of metal on one of the electrodes. This last application is widely used in industry as electroplating. For example, by using the chemical effect of current, we can deposit a very thin, uniform layer of a precious metal like gold onto a less expensive metal like copper or nickel. Unlike a simple magnetic effect that disappears when the current stops (Science, Class VIII, Electricity: Magnetic and Heating Effects, p.48), the chemical effects often result in permanent changes to the materials involved. This is the same principle used in rechargeable batteries: we use an external current to drive a chemical reaction that 'stores' energy for later use (Science, Class VIII, Electricity: Magnetic and Heating Effects, p.58).

Effect of Current	What Happens?	Common Application
Heating Effect	Energy is lost as heat due to resistance.	Electric Irons, Geyers
Magnetic Effect	A magnetic field is created around the conductor.	Electric Bells, Cranes
Chemical Effect	Chemical bonds are broken or formed in a solution.	Electroplating, Metal extraction

Sources: Science-Class VII, Electricity: Circuits and their Components, p.36; Science, Class VIII, Electricity: Magnetic and Heating Effects, p.58; Science, Class VIII, Electricity: Magnetic and Heating Effects, p.48

2. Mechanism of an Electrolytic Cell (basic)

Hello! To understand how we can coat a piece of jewelry with gold or purify a block of copper, we must first understand the Electrolytic Cell. Think of this cell as a chemical factory driven by an external "pump"—the battery. Unlike a standard battery (Voltaic cell) that produces electricity from chemicals, an electrolytic cell uses electrical energy to force a chemical change that wouldn't happen on its own.

The mechanism relies on three core components: a liquid electrolyte, two electrodes (solid conductors), and a DC power source. The electrolyte is a solution or a melt containing ions—atoms that have gained or lost electrons and thus carry a charge. When the battery is turned on, it creates a potential difference that acts like a magnet for these ions. Science, Class VIII, Electricity: Magnetic and Heating Effects, p.55

The movement of these ions is the "secret sauce" of the mechanism:

• The Anode (Positive Electrode): This is connected to the positive terminal of the battery. It attracts negatively charged ions (anions). In refining processes, the impure metal is often placed here; as it loses electrons, it dissolves into the solution.
• The Cathode (Negative Electrode): This is connected to the negative terminal. It is rich in electrons and attracts positively charged metal ions (cations). Science, Class X, Metals and Non-metals, p.52

As the positive metal ions reach the cathode, they gain electrons (a process called reduction) and transform into solid metal atoms. These atoms then "stick" to the surface, forming a thin, even layer. This is why, in both electroplating and metal refining, the pure metal always deposits on the cathode. Science, Class X, Metals and Non-metals, p.53

Feature	Anode (Positive)	Cathode (Negative)
Ion Attraction	Attracts negative ions (Anions)	Attracts positive ions (Cations)
Chemical Action	Oxidation (Loss of electrons)	Reduction (Gain of electrons)
Result	Metal dissolves or impurities settle	Pure metal is deposited

Sources: Science, Class VIII (NCERT 2025 ed.), Electricity: Magnetic and Heating Effects, p.55; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.52; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.53

3. Fundamentals of Redox Reactions (intermediate)

At its core, every Redox reaction is a chemical dance involving the transfer of "stuff"—traditionally oxygen, but more fundamentally, electrons. The term itself is a portmanteau of Reduction and Oxidation. These two processes are inseparable; you cannot have one without the other because if one substance gives something up, another must be there to receive it.

Historically, we defined these through oxygen: Oxidation is the gain of oxygen, while Reduction is the loss of oxygen Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12. For example, when Copper(II) oxide reacts with Hydrogen, the copper oxide loses oxygen (it is reduced) to become metallic copper, while the hydrogen gains oxygen (it is oxidized) to become water (H₂O). This perspective is vital in metallurgy, where obtaining pure metals from their natural ores is essentially a massive reduction process Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.51.

To master the intermediate level, we must look at the electronic perspective. Since electric current is the motion of electrons Science, Class X (NCERT 2025 ed.), Electricity, p.177, redox reactions are the bridge between chemistry and electricity. In this view, oxidation is the loss of electrons and reduction is the gain of electrons. This explains why different metals act as positive or negative electrodes in a battery (Voltaic cell); their varying "hunger" for electrons determines which one gets reduced and which one gets oxidized Curiosity — Science for Grade 8 (NCERT), Electricity, p.56.

Feature	Oxidation	Reduction
Oxygen Transfer	Gain of Oxygen	Loss of Oxygen
Electron Transfer	Loss of Electrons	Gain of Electrons
Common Example	Iron rusting (gaining O₂)	Extracting Iron from ore

Sources: Science, Class X (NCERT 2025 ed.), Chemical Reactions and Equations, p.12; Science, Class X (NCERT 2025 ed.), Metals and Non-metals, p.51; Science, Class X (NCERT 2025 ed.), Electricity, p.177; Curiosity — Science for Grade 8 (NCERT), Electricity: Magnetic and Heating Effects, p.56

4. Cells and Batteries: Energy Conversion (intermediate)

At its heart, a chemical cell is a device that acts as a bridge between chemistry and electricity. It works on a simple but profound principle: converting stored chemical energy into electrical energy. A single unit is called a cell, while a combination of two or more cells connected together is termed a battery Science, class X (NCERT 2025 ed.), Electricity, p.173. The magic happens through a potential difference created by chemical reactions within the cell, which pushes charges through a circuit even when no current is being actively drawn.

The classic Voltaic (or Galvanic) cell consists of two different metal plates called electrodes submerged in a liquid called an electrolyte (usually a weak acid or salt solution) Science, Class VIII, Electricity: Magnetic and Heating Effects, p.55. The chemical reaction between the electrodes and the electrolyte causes electrons to accumulate at one terminal (the negative terminal) and be depleted at the other (the positive terminal). When you complete the circuit, these electrons flow from the negative to the positive terminal, creating the electric current we use to power our world.

In modern life, we categorize these power sources into two main types based on their chemical 'stamina':

Feature	Primary Cells (Dry Cells)	Secondary Cells (Rechargeable)
Reusability	Single-use; disposed of once chemicals are exhausted.	Can be recharged and reused hundreds of times.
Electrolyte	A thick moist paste (hence 'dry').	Varies (liquid or gel/solid-state).
Common Examples	Zinc-Carbon batteries for remotes and torches Science, Class VIII, Electricity: Magnetic and Heating Effects, p.57.	Lithium-ion (phones), Lead-acid (inverters), or NiCd batteries Science, Class VIII, Electricity: Magnetic and Heating Effects, p.57.

It is vital to remember that batteries are not just trash when they stop working. Even a 'dead' battery contains heavy metals like Lead (Pb), Cadmium (Cd), or Lithium (Li), which can be hazardous to the environment if they leak into soil or water. However, these materials are also incredibly valuable; through e-waste recycling, they can be recovered and reused to make new batteries, protecting both the planet and our resources Science, Class VIII, Electricity: Magnetic and Heating Effects, p.61.

Sources: Science, class X (NCERT 2025 ed.), Electricity, p.173; Science, Class VIII, Electricity: Magnetic and Heating Effects, p.55, 57, 61

5. Corrosion and Metal Preservation (intermediate)

At its heart, corrosion is a natural process where refined metals return to their more stable chemical states (like oxides or sulfides) when exposed to environmental triggers like moisture and oxygen. For iron, this specific process is called rusting. To protect our infrastructure and everyday items, we use a range of preservation techniques that either create a physical barrier or change the chemical behavior of the metal.

The simplest methods involve barrier protection—applying paint, oil, or grease to prevent air and moisture from touching the metal surface Science-Class VII . NCERT(Revised ed 2025), The World of Metals and Non-metals, p.50. However, for long-term durability, we turn to electrochemical methods like Galvanisation. This involves coating iron or steel with a thin layer of Zinc. What makes galvanisation remarkable is that even if the zinc layer is scratched, the underlying iron remains protected because zinc is more reactive and 'sacrifices' itself to corrode instead of the iron Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.54.

Another sophisticated method is Anodising, which is primarily used for aluminium. While aluminium naturally forms a thin, protective oxide layer, anodising uses electrolysis (with dilute sulphuric acid) to force the formation of a much thicker, tougher oxide coat that resists further corrosion and can even be dyed for aesthetics Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.42. For items like jewellery or car bumpers, we use Electroplating. By passing an electric current through an electrolyte solution, we can deposit a uniform, thin film of a decorative or corrosion-resistant metal (like gold or chromium) onto a base metal.

Finally, we can modify the metal's internal properties through Alloying. Pure iron, for instance, is too soft for many uses, but mixing it with small amounts of carbon, nickel, or chromium creates stainless steel, which is hard and does not rust Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.54.

Method	Mechanism	Common Example
Galvanisation	Sacrificial Zinc coating	Iron pipes, roofing sheets
Anodising	Thickening the Oxide layer	Aluminium cookware, window frames
Alloying	Mixing metals to change properties	Stainless steel utensils

Sources: Science-Class VII . NCERT(Revised ed 2025), The World of Metals and Non-metals, p.50; Science , class X (NCERT 2025 ed.), Metals and Non-metals, p.54; Science , class X (NCERT 2025 ed.), Metals and Non-metals, p.42

6. The Science of Electroplating (exam-level)

Electroplating is an ingenious application of electrolysis where a thin layer of a superior metal (like gold or silver) is deposited onto the surface of a base metal (like copper or nickel). At its core, this is not just a cosmetic change but a precise electrochemical reaction. Unlike simple chemical dipping—known as displacement or immersion plating, which is self-limiting and thin—electroplating uses an external electric current to drive the deposition, allowing us to control the thickness and durability of the coating.

To understand how this works, imagine a circuit with three main components: the anode, the cathode, and the electrolyte. In a typical gold-plating setup for jewellery:

• The Cathode (Negative Electrode): This is the object you want to plate (e.g., a copper ring). It is connected to the negative terminal of a power source.
• The Anode (Positive Electrode): This is usually a bar of the pure metal you want to deposit (e.g., gold).
• The Electrolyte: A solution containing ions of the plating metal, such as potassium gold cyanide.

When current flows, gold atoms at the anode lose electrons (oxidation) and enter the solution as ions. Simultaneously, gold ions in the solution are attracted to the negatively charged cathode. Once they touch the jewellery, they gain electrons (reduction) and transform back into solid gold atoms, bonding to the surface. This principle of electrolytic reduction is similar to how reactive metals like sodium or magnesium are extracted in their pure form Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.52.

Why do we do this? Metals like gold and silver do not react with oxygen even at high temperatures, meaning they won't tarnish or corrode easily Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.42. However, 24-carat gold is too soft for durable jewellery Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.54. By electroplating a thin layer of gold over a harder alloy or base metal, we achieve a piece of jewellery that has the aesthetic brilliance and corrosion resistance of gold with the structural strength of the underlying metal.

Component	Role in Electroplating	Example (Silver Plating)
Anode	Source of metal ions (Positive)	Pure Silver Rod
Cathode	Object to be coated (Negative)	Steel Spoon
Electrolyte	Medium for ion transport	Silver Nitrate solution

Sources: Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.42; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.52; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.54; Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Distribution of World Natural Resources, p.34

7. Solving the Original PYQ (exam-level)

You have just mastered the fundamentals of electrolysis and redox reactions, and this question is a classic application of those principles. The core concept here is electroplating, a process where electrical energy is converted into chemical energy to deposit a layer of one metal onto another. In this scenario, the copper or nickel jewellery acts as the cathode (negative electrode), while the gold salt solution serves as the electrolyte containing gold ions. As noted in ScienceDirect: Gold Plating, when you pass an electric current, these gold ions are attracted to the jewellery's surface and reduced to metallic gold, forming a uniform and durable coating.

To arrive at the correct answer, you must identify the driving force behind the deposition. While simple immersion (Option D) can trigger a displacement reaction, it is self-limiting and produces a very thin, poorly adherent layer. UPSC often includes such distractors to test your understanding of industrial standards versus incidental reactions. Options A and B focus on temperature fluctuations; however, heating or cooling may change the rate of a reaction but cannot provide the flow of electrons necessary for metallic reduction. Therefore, (C) passing an electric current is the only method that ensures the adhesion and thickness control required for jewellery, making it the definitive answer according to the TAU Chemistry Lab Manual.