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Q1. Why does distilled water not conduct electricity, but tap water does?
Distilled water is pure water with no dissolved salts, so it lacks free ions to carry electric current. Tap water contains dissolved minerals and salts that dissociate into ions, making it a conductor of electricity. The presence of ions is essential for electrical conductivity in liquids.
Q2. In an LED, what happens if the longer lead is connected to the negative terminal and the shorter lead to the positive terminal?
An LED is a polarised device and will only glow when the longer lead (anode) is connected to the positive terminal and the shorter lead (cathode) to the negative terminal. If reversed, the LED does not glow because current cannot flow in the reverse direction. It is not damaged by this reverse connection at low voltage.
Q3. During electroplating of copper on an iron key, the iron key is made the cathode. Why?
In electroplating, the object to be plated is made the cathode (negative electrode). Positive copper ions (Cu²⁺) in the electrolyte are attracted to the negatively charged cathode, where they gain electrons and deposit as copper metal on the object. This is the fundamental principle of electroplating.
Q4. Which of the following statements about electrodes is correct?
The cathode is the negative electrode and attracts positive ions (cations) from the electrolyte. The anode is the positive electrode and attracts negative ions (anions). So the correct statement is that the cathode attracts positive ions. The anode attracts negative ions, not positive ions.
Q5. Why is a small amount of acid or base added to water during electrolysis of water?
Pure water is a poor conductor. Adding a small amount of acid (like sulphuric acid) or base provides ions that increase the conductivity of water, allowing the electric current to pass through and enabling the electrolysis process. The acid or base itself is not consumed in the reaction.
Q6. In electroplating, the thickness of the plated layer depends on:
The thickness of the electroplated layer depends on the amount of charge passed through the circuit, which is the product of current and time. A higher current or longer duration will deposit more metal, resulting in a thicker layer. The colour of the electrolyte and size of the container do not affect thickness.
Q7. An object is electroplated with silver. What is the likely composition of the electrolyte?
For silver electroplating, the electrolyte must contain silver ions. Silver nitrate (AgNO₃) is a soluble silver salt that provides Ag⁺ ions. Copper sulphate would be used for copper plating, sodium chloride for chlorine production, and zinc sulphate for zinc plating.
Q8. Why does an LED glow even with a weak current, but a bulb does not?
A bulb glows by heating its filament to a high temperature, which requires a strong current. An LED glows by electroluminescence—the emission of light when electrons recombine in a semiconductor—and this occurs even with a very weak current. This makes LEDs more sensitive for conductivity testing.
Q9. Which of the following is NOT a chemical effect of electric current?
Heating of a wire is a heating effect of electric current, not a chemical effect. Chemical effects include formation of gas bubbles (electrolysis), deposition of metal (electroplating), and change in colour of the solution due to chemical reactions. Heating effects are separate from chemical effects.
Q10. In the electroplating of an iron spoon with copper, the anode is made of copper. What happens to the copper anode over time?
In electroplating, the copper anode dissolves into the electrolyte as Cu²⁺ ions to replenish the copper deposited on the cathode. Over time, the mass of the anode decreases as copper atoms lose electrons and go into solution. The cathode gains mass, and the anode loses mass.
Q11. Why is a layer of zinc electroplated on iron objects like bridges and railings?
Zinc is more reactive than iron and acts as a sacrificial anode. It corrodes preferentially, protecting the iron underneath from rusting. This is called galvanisation. Zinc is not harder than iron, does not make iron magnetic, and is not primarily used because it is cheaper.
Q12. What is the role of the electrolyte in electroplating?
The electrolyte contains ions of the metal to be deposited. These ions move to the cathode (object being plated) and deposit as metal. The electrolyte also completes the electrical circuit by allowing ions to carry current between the electrodes. It does not dissolve the object or act as an insulator.
Q13. If you use a copper anode and an iron cathode in copper sulphate solution, what colour change do you observe in the solution after some time?
In copper electroplating, copper ions are removed from the solution at the cathode and added back at the anode. If the anode dissolves at the same rate as copper is deposited, the concentration of Cu²⁺ ions in the solution remains constant, so the blue colour of copper sulphate remains unchanged.
Q14. Which of the following liquids is a poor conductor but can become a good conductor if a little salt is added?
Distilled water is a poor conductor because it has no ions. Adding salt (like sodium chloride) dissolves into Na⁺ and Cl⁻ ions, making it a good conductor. Vegetable oil, petrol, and kerosene are non-polar and do not dissolve salts, so they remain poor conductors.
Q15. Why is a resistance (resistor) often used in series with an LED in a circuit?
LEDs have a low internal resistance and can be damaged by excessive current. A series resistor limits the current to a safe level, protecting the LED from burning out. It does not increase brightness, change colour, or make the LED work on AC.
Q16. In the electrolysis of water with a few drops of sulphuric acid, which gas is collected at the cathode?
In the electrolysis of acidified water, hydrogen gas (H₂) is collected at the cathode (negative electrode) because H⁺ ions gain electrons and are reduced to hydrogen gas. Oxygen gas is collected at the anode. Chlorine and nitrogen are not produced in this process.
Q17. Which of the following is an example of an object that is electroplated not for decoration but for a functional purpose?
Tin-plated food cans are electroplated primarily to prevent rusting and contamination of food, which is a functional purpose. Gold-plated watches, silver-plated spoons, and chrome-plated car logos are mainly for decorative or aesthetic purposes, though they also offer some corrosion protection.
Q18. A student tests a liquid with an LED conductivity tester. The LED glows dimly. What can be concluded?
A dim glow means the liquid conducts electricity but weakly. It has some ions, but not many. This indicates it is a poor conductor but not a complete insulator. Distilled water would not glow at all, and a broken circuit would not allow any current.
Q19. Why can’t we electroplate a plastic toy directly?
Electroplating requires the object to be plated to be a conductor (usually metal) so that it can act as an electrode. Plastic is an insulator and cannot carry current, so it cannot be electroplated directly. Plastics must first be coated with a conductive layer (like graphite or copper) before electroplating.
Q20. In electroplating, if the object to be plated is made the anode instead of the cathode, what will happen?
If the object to be plated is made the anode (positive electrode), instead of getting plated, it will dissolve into the electrolyte as metal ions. Plating only occurs at the cathode where positive ions gain electrons. Making the object the anode would cause it to corrode, not be plated.
Q21. Which of the following is a good conductor of electricity in liquid state but a poor conductor in solid state?
Sodium chloride (NaCl) is an ionic compound. In solid state, ions are held in a rigid lattice and cannot move, so it does not conduct. When dissolved in water, the ions become free to move and conduct electricity. Copper, iron, and graphite conduct in both solid and liquid states.
Q22. What is the purpose of using a LED instead of a bulb in a tester for liquids?
LEDs are more sensitive than bulbs and can detect weak currents. A bulb requires a higher current to heat its filament and glow. LEDs are also cheaper and longer lasting, but the main advantage in a conductivity tester is their sensitivity to weak currents.
Q23. During electroplating with copper, if the copper sulphate solution is not stirred, what might happen?
Without stirring, the concentration of Cu²⁺ ions near the cathode decreases as they are deposited, while the anode region gets saturated. This concentration gradient leads to poor, non-uniform plating. Stirring ensures a uniform ion concentration throughout the solution.
Q24. Which of the following statements about electrodes is INCORRECT?
This statement is incorrect. Graphite is a non-metal that is a good conductor of electricity and is commonly used as an inert electrode in many electrolysis experiments. It does not react with many electrolytes and is cheap. Graphite electrodes are widely used in batteries and electrolysis.
Q25. An iron nail is electroplated with nickel. The electrolyte is nickel sulphate solution. The anode should be made of:
In electroplating, the anode must be made of the metal to be deposited (nickel). As current flows, nickel from the anode dissolves into the electrolyte as Ni²⁺ ions, replacing the ions deposited on the cathode (the iron nail). If the anode were iron or copper, the electrolyte would become contaminated.
Q26. Why do we add a few drops of dilute sulphuric acid to water before performing electrolysis of water?
Pure water is a poor conductor of electricity. Adding a few drops of dilute sulphuric acid provides ions (H⁺ and SO₄²⁻) that make the water conductive enough to allow electrolysis. The acid itself is not consumed in the reaction; it only helps the current flow.
Q27. What is the advantage of electroplating with chromium over zinc?
Chromium plating is used because it is hard, corrosion-resistant, and provides a shiny, attractive finish that is also scratch-resistant. Zinc is used for galvanisation to prevent rusting, but chromium gives a more decorative and durable surface. Chromium is not cheaper and is less reactive than zinc.
Q28. In a circuit testing conductivity, the LED glows brightly for liquid A, dimly for liquid B, and not at all for liquid C. Which statement is correct?
Salt water contains many ions and is a good conductor, making the LED glow brightly. Tap water has fewer ions, so it conducts weakly (dim glow). Distilled water has no ions and does not conduct, so the LED does not glow. This matches the observation.
Q29. Which of the following is NOT a reason for electroplating?
Electroplating is done to prevent corrosion (rusting), improve appearance, and make objects more durable. It does not make metals more reactive—in fact, it often makes them less reactive by covering them with a less reactive metal. Increasing reactivity is not a goal of electroplating.
Q30. During the electrolysis of copper sulphate solution using copper electrodes, the colour of the solution remains blue. Why?
In this electrolysis setup, copper from the anode dissolves into the solution as Cu²⁺ ions at the same rate that Cu²⁺ ions are removed from the solution and deposited on the cathode. This maintains a constant concentration of copper ions, so the blue colour of copper sulphate solution remains unchanged.
Q31. An LED has a forward voltage of 2V and requires 10 mA current. If connected to a 6V battery without a resistor, what will happen?
LEDs have low internal resistance and cannot limit current on their own. A 6V battery would push excessive current through the LED, exceeding its maximum rating, causing it to overheat and be damaged. A series resistor is always needed to limit the current to a safe value.
Q32. Which of the following is an example of electroplating for economic reasons?
Tin plating iron is economically beneficial because it allows the use of cheaper iron while providing the corrosion resistance of tin. This reduces the cost of food cans compared to using expensive stainless steel. Gold and silver plating are for aesthetics, and chromium is for durability.
Q33. In an electroplating experiment, the mass of the cathode increased by 0.5 g. The mass of the anode:
In electroplating, the mass gained by the cathode equals the mass lost by the anode. This is because metal ions from the anode dissolve into the electrolyte and are deposited on the cathode. So if the cathode gains 0.5 g, the anode loses 0.5 g.
Q34. Why is carbon (graphite) used as an electrode in some electrolysis experiments?
Graphite is used as an inert electrode because it does not react with most electrolytes and is a good conductor of electricity. It is useful when you want to avoid contamination of the electrolyte by metal ions from the electrode. It is not shiny, decorative, or easily melted.
Q35. Which liquid among the following will show the highest deflection in a galvanometer when used in a conductivity test?
Salt solution is a strong electrolyte that dissociates completely into Na⁺ and Cl⁻ ions, providing high conductivity. Sugar and alcohol do not dissociate into ions and are poor conductors. Vinegar is a weak electrolyte and conducts less than salt solution. Therefore, salt solution gives the highest deflection.
Q36. A student electroplates a copper key with silver. The electrolyte is silver nitrate. After some time, the solution becomes blue. What is the most likely reason?
Copper is more reactive than silver. In this setup, some copper from the key (anode/cathode depending on connection) may dissolve into the solution as Cu²⁺ ions, turning the solution blue. Silver nitrate is colourless, and a dirty beaker or reversed battery would not produce the blue colour typically associated with copper ions.
Q37. Why is it important that the object to be electroplated is thoroughly cleaned before the process?
Cleaning removes oil, grease, and oxide layers from the surface. If the surface is not clean, the electroplated layer will not adhere properly and may peel off. A clean surface ensures good adhesion and a uniform, durable coating. It does not affect current flow or dissolve the object.
Q38. In the electrolysis of acidified water, the volume of hydrogen collected at the cathode is double that of oxygen at the anode. Why?
Electrolysis of water produces two molecules of hydrogen gas (H₂) for every one molecule of oxygen gas (O₂) because each water molecule contains two hydrogen atoms and one oxygen atom. This ratio is 2:1 by volume, corresponding to the chemical formula H₂O.
Q39. Which of the following is NOT an electroplated object?
Aluminium foil is not electroplated; it is made by rolling aluminium metal. Iron nails with zinc coating are galvanised, steel rims with chromium are plated, and tin-coated food cans are electroplated. Aluminium foil is simply aluminium metal, not a coated object.
Q40. What happens to the pH of copper sulphate solution during electroplating with copper electrodes?
In copper electroplating with copper electrodes, the overall reaction is the transfer of copper from anode to cathode. The concentration of H⁺ ions (which affect pH) does not change significantly because no acid or base is produced or consumed. The pH remains nearly constant during the process.
Q41. Why is it dangerous to touch an electrical switch with wet hands?
Water containing dissolved salts (from sweat) is a good conductor of electricity. When you touch a switch with wet hands, the current can flow through your body to the ground, increasing the risk of electric shock. Dry skin has higher resistance, but wet skin reduces resistance, making it more dangerous.
Q42. In a conductivity tester using an LED, if the LED does not glow even when the liquid is a good conductor, what could be the possible fault?
LEDs have polarity: the longer lead must be connected to the positive terminal and the shorter lead to the negative terminal. If the leads are reversed, the LED will not glow even if the liquid is a good conductor. Temperature, beaker size, or coldness do not affect the LED in this way.
Q43. Which metal is commonly electroplated on iron objects used in marine environments (ships) to prevent rusting?
In marine environments, zinc is used as a sacrificial anode to protect iron ships from rusting. Zinc is more reactive than iron and corrodes preferentially, protecting the iron hull. This is a form of cathodic protection, not decorative electroplating. Zinc is also used in galvanisation.
Q44. During electroplating, if the current is too high, what defect may occur?
High current causes rapid deposition, which can lead to poor quality plating. The metal deposits unevenly, resulting in a rough, dark, or powdery surface. This is because the rate of ion deposition is too fast for a smooth, dense layer to form. Lower current gives smoother, brighter plating.
Q45. Which of the following statements is true about distilled water and rainwater?
Rainwater is not pure; it dissolves carbon dioxide and other gases from the atmosphere, forming weak acids that provide some ions, making it a weak conductor. Distilled water is pure and contains no ions, so it is a poor conductor. Rainwater has more conductivity than distilled water but less than tap water or saltwater.
Q46. In the electroplating of a silver spoon with gold, the electrolyte should be:
For gold electroplating, the electrolyte must contain gold ions, which are provided by gold chloride (AuCl₃) or gold cyanide complexes. Silver nitrate would plate silver, copper sulphate would plate copper, and sodium chloride would not plate any metal. Gold salts are used to deposit a gold layer.
Q47. Why are cyanide-based electrolytes used in gold and silver electroplating even though they are toxic?
Cyanide-based electrolytes produce high-quality, smooth, bright, and well-adherent deposits of gold and silver. They also allow plating at lower current densities and have good throwing power. Despite their toxicity, they are still used in many industrial plating processes because of their superior results.
Q48. A student sets up an electroplating experiment but forgets to add any electrolyte; only distilled water is used. What will happen?
Distilled water is a poor conductor because it lacks ions. Without an electrolyte, the circuit is incomplete, and no current flows. Therefore, no electroplating occurs. An electrolyte with ions of the metal to be plated is essential for the process to work.
Q49. In the chemical effect of electric current, the amount of metal deposited on the cathode depends on:
The amount of metal deposited in electroplating is directly proportional to the total charge passed through the circuit, which is current (I) multiplied by time (t). This is Faraday’s first law of electrolysis. Voltage alone does not determine the amount deposited; the charge passed and the equivalent weight of the metal are the factors.
Q50. Why is a layer of silver electroplated on copper wires used in some high-frequency electronic applications?
Silver has the highest electrical conductivity of all metals. Plating copper wire with silver reduces the overall resistance, especially at high frequencies where the skin effect causes current to flow near the surface. This improves signal transmission in high-frequency applications. It is not for aesthetics, magnetism, or weight.