Chemical Effects of Electric Current – Case-based Questions with Answers
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CBSE Class 8 Science – Chapter 14: Chemical Effects of Electric Current | Case-Based Questions
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CBSE Class 8 Science – Chapter Wise Study Materials Based on NCERT
Chapter 14: Chemical Effects of Electric Current – Case-Based Questions
Suitable for:
Annual Examination • Periodic Tests • Class Tests • Pre-Board / School Level Board Exam Pattern
These Case-Based Questions with Answers are designed strictly as per the NCERT syllabus, making them ideal for CBSE Class 8 board exams standard.
Below is a comprehensive, topic-wise set of 20 Case-Based Questions from
Chapter 14: Chemical Effects of Electric Current with clear and easy-to-understand answers
to help students practice application-based questions for exams.
Case-Based Questions – Chapter 14: Chemical Effects of Electric Current
Section A – Conductors, Insulators & Testing of Liquids
Case Study 1 – Testing Kitchen Liquids
Riya decided to test the electrical conductivity of some common kitchen liquids. She arranged a simple circuit with a battery,
an LED, two iron nails as electrodes and a switch. She tested four liquids: lemon juice, sugar solution, tap water and cooking oil.
The LED glowed brightly in lemon juice, glowed faintly in tap water, did not glow in sugar solution and did not glow in cooking oil.
- Which of the tested liquids are good conductors of electricity?
- Why did the LED glow only faintly in tap water?
- Why did sugar solution and cooking oil not allow the LED to glow?
- Name any other household liquid that is likely to conduct electricity.
Ans 1. Lemon juice is a good conductor, and tap water is a weak conductor.
Ans 2. Tap water contains dissolved salts and minerals but in limited amount, so it allows only a weak current to pass, causing the LED to glow faintly.
Ans 3. Sugar solution and cooking oil do not produce many ions in solution, so they cannot provide enough charge carriers; therefore, the LED does not glow.
Ans 4. Vinegar, baking soda solution or lime water are other household liquids that can conduct electricity.
Case Study 2 – Distilled Water vs Tap Water
In a school lab, a teacher gave two beakers to a group of students. Beaker A contained distilled water and Beaker B contained tap water.
The students used the same circuit (battery, LED and electrodes) to test both beakers. The LED did not glow in Beaker A but glowed faintly in Beaker B.
When a pinch of salt was added to Beaker A and the test was repeated, the LED started glowing.
- What can you conclude about the conduction of distilled water?
- Why did the LED glow faintly in tap water even before adding any salt?
- What effect did adding salt to distilled water have on its conductivity?
- Explain why adding salt helps the solution conduct electricity.
Ans 1. Distilled water is a very poor conductor of electricity.
Ans 2. Tap water contains dissolved salts and minerals that produce ions, so it allows a small current to flow and the LED glows faintly.
Ans 3. Adding salt to distilled water produced ions in the solution, making it a conductor so the LED glowed.
Ans 4. Salt (such as NaCl) dissolves in water and splits into positive and negative ions, which move in the solution and carry electric charge, allowing current to pass.
Case Study 3 – Use of LED and Bulb
Two friends, Arun and Meera, set up circuits to test the conductivity of a weak salt solution.
Arun used a small filament bulb in his circuit, while Meera used an LED. When they dipped their electrodes into the same weak salt solution,
Arun’s bulb did not glow at all, but Meera’s LED glowed faintly. Both circuits and batteries were otherwise similar.
- Why did Arun conclude that the solution is a poor conductor?
- Why was Meera’s conclusion different from Arun’s?
- Which device is more suitable for testing weak currents? Why?
- What precaution should be taken when connecting an LED in a circuit?
Ans 1. Arun’s bulb did not glow, so he thought that no current was passing and concluded that the solution is a poor conductor.
Ans 2. Meera’s LED glowed faintly, showing that a small current was flowing. She concluded that the solution is a weak conductor, not a complete insulator.
Ans 3. An LED is more suitable because it can glow even with a very small current, making it ideal for detecting weak conduction in liquids.
Ans 4. The LED should be connected in the correct direction (polarity) and not directly to a very high voltage, otherwise it may get damaged.
Case Study 4 – Magnetic Effect to Detect Current
A science teacher used a coil of wire and a compass needle to show the conduction in various liquids.
The coil was connected in series with a battery, switch and two electrodes dipped in a liquid.
When the switch was turned on with tap water as the liquid, the compass needle deflected.
When the same setup was used with kerosene, there was no deflection.
- Why did the compass needle deflect when tap water was used?
- What does the deflection of the compass needle indicate?
- Why was there no deflection when kerosene was used?
- What is the advantage of using a compass needle in such experiments?
Ans 1. Tap water conducts electricity, so current passed through the coil, creating a magnetic field which caused the compass needle to deflect.
Ans 2. The deflection indicates that current is flowing in the circuit and hence through the liquid.
Ans 3. Kerosene does not conduct electricity, so no current flowed and the coil did not produce a magnetic field, resulting in no deflection.
Ans 4. A compass needle is very sensitive to magnetic fields and can detect even small currents that might not light a bulb.
Section B – Chemical Effects & Changes in Solutions
Case Study 5 – Gas Bubbles at Electrodes
During an experiment, students passed electric current through a solution of an acid in water.
Two graphite rods were used as electrodes. After a few minutes, they observed gas bubbles forming at both electrodes.
The teacher explained that this was due to the chemical effect of electric current.
- What term is used for the chemical changes that occur when current is passed through a solution?
- Why did gas bubbles form at both electrodes?
- What does the formation of gas bubbles indicate about the solution?
- Mention any one more observable effect of electric current in a solution, other than gas formation.
Ans 1. These changes are called the chemical effects of electric current.
Ans 2. The current caused the solution to decompose into its components; gases were produced at both electrodes due to reactions of ions there.
Ans 3. It indicates that the solution conducts electricity and undergoes chemical decomposition (electrolysis).
Ans 4. Change in colour of the solution or deposition of a metal on one of the electrodes are other observable effects.
Case Study 6 – Colour Change in Solution
A blue coloured solution of a salt was taken in a beaker and two metal electrodes were dipped in it.
When electric current was passed for some time, one electrode became coated with a metal and the intensity of blue colour of the solution decreased slowly.
- What type of change is taking place in the solution: physical or chemical? Why?
- What is happening to the ions of the salt in the solution?
- Why does the intensity of the blue colour decrease?
- What effect of electric current is demonstrated in this activity?
Ans 1. A chemical change is taking place, because a new substance (metal deposit) is formed and the composition of the solution is changing.
Ans 2. The metal ions in the solution move towards one of the electrodes and get deposited there by gaining electrons.
Ans 3. As metal ions are removed from the solution and deposited on the electrode, fewer ions remain in the solution, so the colour becomes lighter.
Ans 4. This activity demonstrates the chemical effect of electric current.
Case Study 7 – Mass Change at Electrodes
In an experiment, a copper plate and a carbon rod were used as electrodes in a salt solution.
After passing current for about 30 minutes, it was found that the copper plate had lost some mass while the carbon rod had gained mass.
- Which electrode is acting as the anode in this experiment?
- What happens to the copper atoms at the electrode that loses mass?
- Why does the other electrode gain mass?
- What conclusion about electric current can you draw from this activity?
Ans 1. The electrode that loses mass (copper plate) is acting as the anode.
Ans 2. Copper atoms at the anode lose electrons, become Cu2+ ions and go into the solution.
Ans 3. The Cu2+ ions from the solution move towards the other electrode (cathode), gain electrons and deposit as copper, increasing its mass.
Ans 4. Electric current can cause chemical changes and transfer material from one electrode to another.
Case Study 8 – Identifying Electrolyte
Three transparent liquids were given to a student: A, B and C (unlabelled).
The student was allowed to use only an electric circuit with electrodes and an LED to identify which liquid is an electrolyte.
The LED glowed in liquids A and B but not in liquid C.
- What is meant by an electrolyte?
- Which of the three liquids (A, B, C) are electrolytes?
- What can you conclude about liquid C?
- Give one possible example of a liquid that behaves like liquid C.
Ans 1. An electrolyte is a conducting liquid or solution that allows electric current to pass due to the presence of ions.
Ans 2. Liquids A and B are electrolytes because the LED glowed in them.
Ans 3. Liquid C does not conduct electricity; it is a non-electrolyte or a very poor conductor.
Ans 4. Kerosene, pure alcohol or cooking oil could behave like liquid C.
Section C – Copper Sulphate Activity & Related Concepts
Case Study 9 – Copper Sulphate with Copper Electrodes
In a classic experiment, two copper plates are dipped in copper sulphate (CuSO4) solution.
One plate is connected to the positive terminal of the battery and the other to the negative terminal.
After passing current for 20 minutes, the negative electrode appears thicker and the positive electrode appears thinner, while the blue colour of the solution remains almost the same.
- Which plate is the cathode in this experiment?
- What happens to copper at the cathode?
- Why does the anode become thinner?
- Why does the blue colour of the copper sulphate solution remain almost unchanged?
Ans 1. The plate connected to the negative terminal is the cathode.
Ans 2. Cu2+ ions from the solution move towards the cathode, gain electrons and deposit as copper metal on its surface, making it thicker.
Ans 3. Copper atoms at the anode lose electrons, become Cu2+ ions and dissolve into the solution, making the anode thinner.
Ans 4. The number of Cu2+ ions leaving the anode is nearly equal to the number of Cu2+ ions deposited at the cathode, so the concentration of copper ions in the solution remains almost constant and the colour does not change much.
Case Study 10 – Copper Sulphate with Iron Electrode
In another experiment, an iron nail and a copper plate are dipped in copper sulphate solution.
The iron nail is connected to the negative terminal and the copper plate to the positive terminal of a battery.
After some time, the iron nail gets coated with a reddish-brown layer, and the blue colour of the solution becomes lighter.
- What is the reddish-brown layer deposited on the iron nail?
- Is the iron nail acting as an anode or cathode?
- Why does the blue colour of the solution become lighter?
- Which effect of electric current is being shown here?
Ans 1. The reddish-brown layer is a deposit of copper metal.
Ans 2. The iron nail is connected to the negative terminal, so it is acting as the cathode.
Ans 3. Cu2+ ions from the solution are deposited on the iron nail as copper metal, so the concentration of copper ions decreases and the blue colour fades.
Ans 4. This shows the chemical effect of electric current, specifically deposition of a metal on another object.
Case Study 11 – Change of Electrodes and Solution
A student first used copper electrodes in copper sulphate solution and observed little change in colour of the solution.
Then the student replaced one copper electrode with a carbon rod and repeated the experiment.
This time the blue solution slowly became lighter in colour and a layer of copper was seen on the copper electrode.
- Why did the colour of the solution not change much when both electrodes were copper?
- Why did the colour become lighter when one electrode was carbon?
- Which electrode receives copper deposition in the second experiment?
- What does this show about the role of electrodes?
Ans 1. When both electrodes are copper, copper lost by the anode is almost equal to the copper gained by the cathode, so the solution’s copper ion concentration stays nearly the same.
Ans 2. With a carbon rod, copper ions from the solution deposit on the copper electrode but there is no copper anode to replace them, so the concentration of Cu2+ ions decreases and the colour becomes lighter.
Ans 3. The copper electrode receives copper deposition and becomes thicker.
Ans 4. It shows that the material of the electrodes affects the way ions move and where deposition occurs in an electrolytic cell.
Section D – Electroplating & Its Applications
Case Study 12 – Electroplating a Key
In a school activity, students electroplated an iron key with copper. The key was cleaned thoroughly and connected to the negative terminal of a battery.
A copper strip was connected to the positive terminal. Both were dipped in copper sulphate solution. After 20 minutes of passing current,
the key appeared to be coated with a shining reddish-brown metal.
- Which electrode acts as the cathode in this setup?
- Why must the key be cleaned before electroplating?
- From where does the copper deposited on the key come?
- State one use of electroplating in everyday life other than plating a key.
Ans 1. The iron key connected to the negative terminal acts as the cathode.
Ans 2. Cleaning removes dust, grease and rust so that the deposited metal layer sticks properly and uniformly to the surface.
Ans 3. Copper atoms from the copper strip (anode) go into the solution as Cu2+ ions and then deposit on the key (cathode) as copper metal.
Ans 4. Electroplating is used for gold-plating artificial jewellery or chrome-plating bicycle and car parts.
Case Study 13 – Chrome Plating of Bike Parts
Many parts of bicycles and motorcycles are shiny and do not rust easily.
These parts are usually made of iron or steel but are coated with a thin layer of chromium metal by electroplating.
Chromium is expensive but has a bright appearance and resists corrosion.
- Why are the bike parts not made entirely of chromium metal?
- How does electroplating help to save expensive metals like chromium?
- What advantages do bike parts gain by being coated with chromium?
- In chromium electroplating, what will you choose as anode, cathode and electrolyte?
Ans 1. Chromium is very expensive and dense, so making the whole part from chromium would be costly and heavy.
Ans 2. Electroplating uses only a very thin layer of chromium on the outside while the inner part is made of cheaper iron or steel, thus saving a lot of chromium.
Ans 3. The parts look shiny and attractive, resist rust and corrosion, and have a hard surface that wears slowly.
Ans 4. The bike part (iron or steel) is the cathode, a chromium rod is the anode, and a solution of a chromium salt is used as the electrolyte.
Case Study 14 – Tin Plating of Food Cans
Iron cans used for storing food are often coated with a thin layer of tin metal.
Tin is less reactive than iron and does not rust easily. This coating is done by electroplating in factories
so that food does not come into direct contact with iron.
- Why is it necessary to coat iron cans with tin for food storage?
- What might happen if the tin coating on a can is broken or scratched?
- Which metal acts as the anode in tin plating and which as the cathode?
- How does this process show the importance of chemical effects of electric current in daily life?
Ans 1. Iron can rust and react with food, which may spoil the food and make it unsafe. Tin does not corrode easily, so it protects the food and the can.
Ans 2. If the tin coating breaks, iron below may rust and react with food; brown flakes of rust may appear and food quality may decrease.
Ans 3. The tin metal acts as the anode, and the iron can acts as the cathode in electroplating.
Ans 4. It shows that by using the chemical effect of electric current (electroplating), we can protect metals and keep food safe, which is very useful in daily life.
Case Study 15 – Gold Plating of Jewellery
Artificial jewellery made of cheap metals is often coated with a thin layer of gold to make it look like real gold jewellery.
This is done by electroplating in jewellery workshops. A solution containing gold salt is used as electrolyte,
and the article to be plated is connected to the negative terminal of the power supply.
- Why is gold plating preferred for artificial jewellery?
- Explain how electroplating makes artificial jewellery economical.
- What will happen to the appearance of such jewellery after long use if the gold layer wears off?
- State the role of chemical effects of electric current in this process.
Ans 1. Gold plating gives a bright, shiny and attractive appearance similar to real gold, increasing the jewellery’s appeal.
Ans 2. Only a very thin layer of gold is deposited on a cheap base metal, so little gold is used, reducing cost compared to solid gold items.
Ans 3. If the gold layer wears off, the base metal becomes visible, and the jewellery may look dull or different in colour.
Ans 4. The chemical effect of electric current allows gold ions from the solution to deposit on the object’s surface as a solid gold layer.
Section E – Safety, Higher Order & Mixed Applications
Case Study 16 – Wet Hands and Electric Shock
Aman came home from playing in the rain and tried to switch on the television with wet hands.
His mother quickly stopped him and explained that it can be very dangerous to touch electrical switches with wet hands or bare feet on a wet floor.
- Why is it dangerous to touch switches with wet hands?
- What role does water play in this situation?
- How is this related to the idea that the human body is a conductor?
- State one safety rule that should always be followed at home regarding electricity.
Ans 1. Wet hands reduce the resistance of the body and make it easier for electric current to pass, which can cause an electric shock.
Ans 2. Water (especially with dissolved salts) conducts electricity and forms a path for current from the switch through the body to the ground.
Ans 3. The human body contains salty water and behaves as a conductor. With wet skin, current can pass more easily through the body and cause harm.
Ans 4. Never touch switches, plugs or electrical appliances with wet hands or while standing on a wet floor.
Case Study 17 – School Lab Precautions
In the science lab, students were performing experiments on chemical effects of electric current.
The teacher strictly instructed them to use only small cells or batteries and not to connect any apparatus directly to the mains supply.
She also asked them to avoid spilling solutions near the battery and switches.
- Why should only small cells or batteries be used in these experiments?
- What is the danger of using mains electricity for such experiments?
- Why should students avoid spilling solutions near electrical connections?
- How does following these precautions help in safe learning?
Ans 1. Small cells and batteries provide low voltage and limited current, which are safer for students.
Ans 2. Mains electricity has high voltage and can cause serious shocks, burns or fires if there is any mistake or short circuit.
Ans 3. Spilled solutions can conduct electricity, cause short circuits and increase the risk of electric shock.
Ans 4. These precautions reduce the risk of accidents and allow students to learn about chemical effects of current safely.
Case Study 18 – Mistaken Conclusion About a Liquid
A student tested a very dilute salt solution using a simple circuit with a small bulb. The bulb did not glow.
The student immediately recorded that the solution is a non-conductor. Later, the teacher repeated the test using an LED and noticed a faint glow.
- Was the student’s conclusion correct? Why or why not?
- What did the teacher’s observation with the LED indicate?
- Why did the bulb not glow in the student’s experiment?
- What lesson can we learn about testing conductivity of weak solutions?
Ans 1. The student’s conclusion was not fully correct. The solution was a weak conductor, not a complete non-conductor.
Ans 2. The faint glow of the LED indicated that a small current was passing, so the solution conducts weakly.
Ans 3. The bulb needed a larger current to glow, but the weak solution allowed only a small current, so it did not glow.
Ans 4. For weak solutions, we should use sensitive devices like LEDs or a compass needle instead of a bulb to test conductivity.
Case Study 19 – Electroplating and Resource Saving
A factory manufactures steel spoons and then electroplates them with silver.
This makes the spoons look like pure silver spoons, but they are cheaper.
The manager explains that electroplating helps in saving expensive metals and still keeps customers happy.
- Why are the spoons made of steel instead of pure silver?
- How does electroplating help in saving silver?
- What effect of electric current is used in this process?
- Besides saving resources, mention one more advantage of electroplating in such products.
Ans 1. Steel is strong and much cheaper than silver, so spoons made of steel are affordable.
Ans 2. Only a thin layer of silver is deposited on the surface of the steel spoon, so very little silver is used, saving a lot of metal.
Ans 3. The chemical effect of electric current (electroplating) is used.
Ans 4. Electroplating makes the spoons look attractive and protects the base metal from corrosion and blackening.
Case Study 20 – Simple Exhibition Project on Chemical Effects
For a science exhibition, a group of Class 8 students prepared a working model showing electroplating.
They used a small battery, copper sulphate solution, a copper strip, and a small iron object.
They also displayed charts explaining how ions move in the solution and how copper is transferred from the copper strip to the iron object.
- What scientific concept from Chapter 14 are the students demonstrating?
- Which object in their setup is acting as the cathode?
- How does this model help visitors understand chemical effects of electric current?
- Suggest one improvement that could make their project more informative.
Ans 1. They are demonstrating the chemical effect of electric current, especially electroplating.
Ans 2. The small iron object connected to the negative terminal of the battery acts as the cathode.
Ans 3. The model shows that when current passes, copper from the copper strip goes into solution and then deposits on the iron object, clearly illustrating how electric current causes chemical changes.
Ans 4. They could add labelled diagrams of an electrolytic cell, show “before and after” weights of the electrodes, or include a note on real-life electroplating uses like jewellery and car parts.
These 20 case-based questions with detailed answers cover key applications and concepts of
Chapter 14: Chemical Effects of Electric Current and are ideal for practice of
competency-based and reasoning-type questions for CBSE Class 8 examinations.