By Subhash Kumar | Science Educator | UREducator
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Chemical Effects of Electric Current
Complete Teaching Resource for CBSE Class 8 Science Chapter 11
Class: VIII
Subject: Science
Chapter: 11
Duration: 4 Hours
What Students Will Achieve
Knowledge
- Understand the concept of electrical conductivity in liquids
- Identify good and poor conductors among common liquids
- Explain the chemical effects of electric current
- Describe the process and applications of electroplating
Understanding
- Analyze why some liquids conduct electricity while others don't
- Explain the working principle of LED testers
- Understand the chemical changes during electrolysis
- Evaluate the importance of electroplating in daily life
Skills
- Construct and use simple conductivity testers
- Perform electroplating experiments safely
- Test various household liquids for conductivity
- Design simple electroplating setups
Prior Knowledge Assessment
Starter Questions:
- Why is it dangerous to touch electrical appliances with wet hands?
- Name three materials that conduct electricity well
- What happens to a compass needle when placed near a current-carrying wire?
Testing Electrical Conductivity in Liquids
Building a Simple Tester
Bulb Tester
- Materials: Bulb, battery, wires
- Works for strong conductors
- Bulb glows when circuit is complete
- Limitation: Doesn't detect weak currents
Magnetic Compass Tester
- More sensitive than bulb tester
- Detects weak currents through needle deflection
- Can be made with matchbox tray and compass
- Shows direction of current flow
LED Tester
- LEDs glow with very small current
- More efficient than traditional bulbs
- Polarity sensitive (longer lead to positive)
- Available in multiple colors
Conductivity of Common Liquids
| Liquid | Conductivity | Reason |
|---|---|---|
| Distilled Water | Poor conductor | No free ions present |
| Tap Water | Good conductor | Contains dissolved salts |
| Lemon Juice | Good conductor | Contains citric acid (H+ ions) |
| Vegetable Oil | Poor conductor | No free ions, non-polar molecules |
Experiment: Testing Household Liquids
Objective: Classify liquids as good or poor conductors
- Collect small samples of various household liquids
- Test each with your conductivity tester
- Record observations (bulb glow/LED light/needle deflection)
- Classify as good or poor conductors
- Analyze patterns (acids/bases/salts conduct better)
Chemical Changes Due to Electric Current
Observable Chemical Effects
Gas Bubbles
Formation at electrodes during electrolysis
- Example: H₂ and O₂ from water
- More bubbles = stronger current
- Different gases at anode and cathode
Color Changes
Indication of chemical reactions
- Example: Blue copper sulfate fades
- New compounds may form colored solutions
- Can indicate pH changes
Metal Deposits
Visible plating on electrodes
- Example: Copper on negative electrode
- Thickness depends on current duration
- Used in electroplating industries
Electrolysis of Water Demonstration
- Set up two carbon electrodes in water with a little acid
- Connect to 6V battery and observe bubbles
- Test gases: Hydrogen pops with flame, oxygen relights glowing splint
- Note volume ratio (2:1 for H₂:O₂)
- Equation: 2H₂O → 2H₂ + O₂
Activity: Potato Electrochemical Cell
Objective: Observe chemical effects in organic material
- Insert copper and zinc electrodes into a potato
- Connect to a voltmeter to show voltage generation
- Leave for 30 minutes and observe color changes
- Explain the electrochemical reactions occurring
Principles and Applications of Electroplating
Electroplating Process
| Component | Role | Example |
|---|---|---|
| Anode | Source of metal ions (positive electrode) | Copper plate for copper plating |
| Cathode | Object to be plated (negative electrode) | Iron key for copper plating |
| Electrolyte | Solution containing metal ions | Copper sulfate solution |
| Power Source | Provides direct current | Battery or rectifier |
Common Electroplating Applications
Chrome Plating
- Used on automobile parts
- Provides shiny, corrosion-resistant surface
- Example: Bumper guards, wheel rims
Tin Plating
- Used for food cans and containers
- Prevents iron corrosion
- Non-toxic and food-safe
Gold/Silver Plating
- Used in jewelry and decorative items
- Provides expensive look at lower cost
- Example: Costume jewelry, trophies
Experiment: Copper Electroplating
Objective: Demonstrate basic electroplating process
- Prepare copper sulfate solution (2 tsp in 250ml water)
- Clean copper anode and iron cathode (key/nail)
- Connect to 6V battery (copper to positive, iron to negative)
- Immerse in solution without touching (15-20 minutes)
- Observe copper deposition on iron object
- Compare with unplated object
Evaluation Strategies
Formative Assessments
- Conductivity prediction and testing
- Electroplating process diagrams
- Chemical reaction equations
- Lab notebook checks
Summative Assessments
- Chapter test (MCQs + short answer)
- Electroplating experiment report
- Case study analysis of industrial applications
- Presentation on electroplating safety
Extended Learning: Visit an Electroplating Unit
Objective: Connect classroom learning to real-world applications
- Arrange visit to local electroplating workshop
- Observe industrial-scale electroplating process
- Interview technicians about safety measures
- Research waste disposal methods
- Prepare report comparing with classroom experiment
Frequently Asked Questions
Why does distilled water not conduct electricity while tap water does?
Key differences:
- Distilled Water: Pure H₂O with no dissolved salts or ions - no charge carriers
- Tap Water: Contains dissolved minerals (Na⁺, K⁺, Ca²⁺, Cl⁻ etc.) that act as charge carriers
- Adding even a pinch of salt makes distilled water conductive
Why is chromium used for electroplating on automobile parts?
Chromium plating is preferred because:
- Highly corrosion resistant
- Extremely hard surface resists scratches
- Attractive shiny appearance
- More economical than making whole parts from chromium
