Chemical Effects of Electric Current

Chemical Effects of Electric Current - Class 8 CBSE Science Notes

This chapter teaches how electric current causes chemical changes in solutions. Students learn about conductors, insulators, electrolysis, and its applications in electroplating. Access NCERT Solutions for Class 8 Science to understand step-by-step solutions for numerical and conceptual questions. Review Class 8 Notes for easy learning and summaries. Through Home Tuition for Class 8, students receive expert help to perform experiments safely and understand chemical reactions involving electricity. Tutors simplify tough terms and processes, making the subject enjoyable. Understanding this topic helps in higher-level studies related to electricity and chemistry.

Introduction to Electricity and Electric Current

Electricity is a fundamental form of energy that powers our modern world. At its core, electricity involves the flow of electric charges through conductors. Understanding how electric current interacts with different materials especially conducting liquids opens up fascinating applications from electroplating jewelry to purifying metals.

What is Electric Current?

Electric current is the flow of electric charges (electrons) through a conductor. It represents the amount of charge passing through a given point in a conductor per unit time.

Mathematical Definition: If a charge Q (in coulombs) flows through a conductor in time t (seconds), then:

I = Q/t

Where:

  • I = Electric current (Ampere)
  • Q = Electric charge (Coulomb)
  • t = Time (seconds)

SI Unit: Ampere (A)

  • 1 Ampere = 1 Coulomb/1 second

An ammeter is used to measure electric current and must be connected in series in a circuit.

Understanding Electric Charge

Electric charge is a fundamental property of matter. There are two types:

  1. Positive Charge - Associated with protons
  2. Negative Charge - Associated with electrons

Properties of Electric Charge:

  • Like charges repel; unlike charges attract
  • Charge is quantized: Q = ±ne (where n is an integer, e = 1.6 × 10⁻¹⁹ C)
  • Charge is a scalar quantity
  • Charge is always conserved in a system

Important Fact: 1 coulomb = charge on 6.25 × 10¹⁸ electrons

Conductors, Insulators, and Semiconductors

Materials are classified based on their ability to conduct electricity:

Conductors

Substances that allow electricity to pass through them easily. Most metals are good conductors because they have loosely bound electrons.

Examples: Silver, copper, gold, aluminum

Why copper is used in electric wires: Copper has very low resistance, minimizing energy loss during transmission.

Insulators

Substances that do not allow electricity to pass through them easily.

Examples: Rubber, plastic, wood, glass, vacuum (the best insulator)

Safety Application: Electricians wear rubber gloves because rubber is an excellent insulator and protects against electric shock.

Semiconductors

Substances whose conductivity lies between conductors and insulators. Their conductivity can be increased by raising temperature.

Examples: Silicon, germanium

Why Do Solutions Conduct Electricity?

Conductivity of Pure Water vs. Impure Water

Pure water (distilled water) is a poor conductor of electricity because it contains very few ions.

Impure water (tap water, pond water, seawater) contains dissolved salts and minerals, making it a good conductor.

Why only solutions conduct electricity, not pure liquids:

For a liquid to conduct electricity, it must contain free-moving ions (charged particles). When chemical compounds like acids, bases, or salts dissolve in water, they dissociate into positive ions (cations) and negative ions (anions). These mobile ions carry electric current through the solution.

Pure liquids like distilled water, vegetable oil, or alcohol lack sufficient ions, making them poor conductors.

Example:

  • Pure water: Poor conductor
  • Water + Salt (NaCl): Good conductor (because Na⁺ and Cl⁻ ions are formed)

Solutions That Conduct Electricity

Most electrolytes conduct electricity:

  1. Aqueous solutions of acids (HCl, HNO₃, H₂SO₄)
  2. Aqueous solutions of bases (NaOH, KOH)
  3. Aqueous solutions of salts (NaCl, CuSO₄, ZnCl₂)

Non-Electrolytes

Solutions that do NOT conduct electricity:

  • Petrol, kerosene, diesel
  • Vegetable oil
  • Alcohol, ether
  • Distilled water
  • Sugar solution

Chemical Effects of Electric Current

When electric current passes through a conducting solution, it produces chemical reactions. These effects include:

  1. Formation of gas bubbles on electrodes
  2. Deposition of metals on electrodes
  3. Change in color of the solution

The specific chemical change depends on the nature of the conducting solution (electrolyte) and the electrodes used.

Understanding Electrolysis

What is Electrolysis?

Electrolysis is the process by which a chemical compound conducts electric current and simultaneously undergoes chemical decomposition.

Key Terms in Electrolysis

Electrolyte: A solution that conducts electricity and undergoes chemical change during the passage of current.

Electrodes: Metal wires, plates, or rods through which current enters or leaves the electrolyte.

Anode: The electrode connected to the positive terminal of the battery. Anions (negative ions) move toward it.

Cathode: The electrode connected to the negative terminal of the battery. Cations (positive ions) move toward it.

Ions: Electrically charged atoms or groups of atoms formed when compounds dissolve in water.

Cations: Positively charged ions (e.g., Cu²⁺, Na⁺, H⁺)

Anions: Negatively charged ions (e.g., Cl⁻, SO₄²⁻, OH⁻)

Voltameter: The complete apparatus containing electrodes, electrolyte, and vessel used for electrolysis.

Electrolysis with Examples and Equations

Example 1: Electrolysis of Acidified Water

When electric current is passed through water containing a small amount of sulfuric acid (H₂SO₄):

Setup:

  • Electrolyte: Water + H₂SO₄
  • Electrodes: Carbon rods or platinum electrodes

Chemical Reactions:

At Cathode (negative electrode): Hydrogen ions gain electrons and form hydrogen gas:

2H⁺ + 2e⁻ → H₂ (gas) ↑

At Anode (positive electrode): Hydroxyl ions lose electrons and form oxygen gas:

4OH⁻ → 2H₂O + O₂ (gas) ↑ + 4e⁻

Overall Reaction:

2H₂O → 2H₂ + O₂

Observation:

  • Hydrogen gas collected at cathode (twice the volume of oxygen)
  • Oxygen gas collected at anode
  • This demonstrates the chemical decomposition of water

Example 2: Electrolysis of Copper Sulphate Solution

Setup:

  • Electrolyte: Copper sulfate (CuSO₄) solution
  • Anode: Copper plate
  • Cathode: Any metal object (e.g., iron spoon)

Ions present in solution:

  • Cations: Cu²⁺ (from CuSO₄), H⁺ (from water)
  • Anions: SO₄²⁻ (from CuSO₄), OH⁻ (from water)

Chemical Reactions:

At Cathode: Copper ions gain electrons and deposit as copper metal:

Cu²⁺ + 2e⁻ → Cu (deposited on cathode)

At Anode: Copper atoms lose electrons and form copper ions:

Cu → Cu²⁺ + 2e⁻

Observation:

  • Copper deposits on the cathode (object being plated)
  • Copper dissolves from the anode
  • Blue color of solution remains constant

Example 3: Electrolysis of Sodium Chloride Solution (Brine)

Setup:

  • Electrolyte: NaCl solution
  • Electrodes: Carbon rods

Ions present:

  • Cations: Na⁺, H⁺
  • Anions: Cl⁻, OH⁻

Chemical Reactions:

At Cathode:

2H⁺ + 2e⁻ → H₂ (gas) ↑

At Anode:

2Cl⁻ → Cl₂ (gas) ↑ + 2e⁻

Observation:

  • Hydrogen gas at cathode
  • Chlorine gas at anode
  • Solution becomes alkaline (NaOH formed)

Experiments to Demonstrate Gas Formation at Electrodes

Experiment 1: Testing Hydrogen and Oxygen from Water Electrolysis

Materials Required:

  • Distilled water with dilute H₂SO₄
  • Two test tubes (inverted over electrodes)
  • Carbon electrodes
  • Battery
  • Connecting wires

Procedure:

  1. Fill a vessel with acidified water
  2. Invert two test tubes filled with water over the electrodes
  3. Pass electric current through the solution
  4. Collect gases in test tubes

Observations:

  • At cathode: Gas collected (hydrogen) - double the volume
    • Test: Brings burning splinter near gas - burns with "pop" sound
  • At anode: Gas collected (oxygen) - half the volume
    • Test: Brings glowing splinter near gas - splinter rekindles

Conclusion: Water decomposes into hydrogen (at cathode) and oxygen (at anode) in the ratio 2:1 by volume.

Chemical Equation:

2H₂O → 2H₂ + O₂
Electrolysis

Experiment 2: Testing for Chlorine Gas

Setup:

  • NaCl solution as electrolyte
  • Carbon electrodes
  • Invert test tube over anode

Test for Chlorine:

  1. Bring wet starch-iodide paper near the anode
  2. Observation: Paper turns blue-black (confirms chlorine gas)

Or:

  1. Smell carefully (in a well-ventilated area)
  2. Observation: Pungent, greenish-yellow gas (chlorine)

How Are Metals Deposited on Cathode During Electrolysis?

The Mechanism of Metal Deposition

When current passes through an electrolyte containing metal ions:

Step 1: Ionization The metal salt dissolves in water and forms positive metal ions (cations):

CuSO₄ → Cu²⁺ + SO₄²⁻

Step 2: Migration When current flows:

  • Positive ions (Cu²⁺) move toward the cathode (negative electrode)
  • Negative ions (SO₄²⁻) move toward the anode (positive electrode)

Step 3: Discharge at Cathode At the cathode, metal ions gain electrons and convert to neutral metal atoms:

Cu²⁺ + 2e⁻ → Cu (solid metal deposited)

Step 4: Deposition The neutral copper atoms deposit on the surface of the cathode, forming a thin, uniform layer.

Factors Affecting Metal Deposition

  1. Amount of current: More current = faster deposition
  2. Time duration: Longer time = thicker deposit
  3. Concentration of electrolyte: Proper concentration ensures uniform coating
  4. Nature of electrodes: Clean surface ensures good adhesion

Faraday's Laws of Electrolysis govern the quantitative relationship between current, time, and amount of metal deposited.

What is Electroplating and How is it Done?

Definition of Electroplating

Electroplating is the process of depositing a thin layer of one metal over another metal using electric current. This is done to:

  1. Improve appearance (shiny, attractive finish)
  2. Prevent corrosion (protect base metal from rusting)
  3. Resist scratches (harder coating)
  4. Provide special properties (conductivity, reflectivity)

How is Electroplating Done? - Complete Process

Components Required:

  1. Electrolyte: A solution containing a salt of the metal to be deposited
  2. Anode: A plate or bar of the metal to be deposited
  3. Cathode: The object to be electroplated
  4. Battery/DC power source
  5. Connecting wires

Step-by-Step Electroplating Process

Example: Copper Plating on an Iron Spoon

Step 1: Preparation

  • Clean the iron spoon thoroughly (cathode)
  • Remove grease, oil, or oxide layers using dilute acid

Step 2: Setup the Circuit

  • Prepare copper sulfate (CuSO₄) solution as electrolyte
  • Connect copper plate to positive terminal (anode)
  • Connect iron spoon to negative terminal (cathode)
  • Immerse both electrodes in the electrolyte

Step 3: Pass Electric Current

  • Switch on the DC power supply
  • Maintain steady current for required time

Step 4: Electrochemical Reactions

At Cathode (Iron Spoon):

Cu²⁺ + 2e⁻ → Cu (deposited on spoon)

Copper ions from solution gain electrons and deposit as copper metal on the spoon.

At Anode (Copper Plate):

Cu → Cu²⁺ + 2e⁻

Copper atoms from the plate lose electrons and dissolve into the solution as ions.

Step 5: Result

  • A uniform, thin layer of copper deposits on the iron spoon
  • The concentration of CuSO₄ solution remains constant (copper dissolves from anode as it deposits on cathode)

Step 6: Final Treatment

  • Remove the plated object
  • Wash and dry
  • Polish if needed

Important Points for Good Electroplating

  1. Object to be plated = Cathode (negative electrode)
  2. Metal to be deposited = Anode (positive electrode)
  3. Electrolyte = Water-soluble salt of the metal to be deposited
  4. Clean surface of the object ensures uniform coating
  5. Controlled current prevents rough or uneven deposits
  6. Adequate time ensures sufficient thickness

Common Examples of Electroplating

Application Base Metal Plated Metal Purpose
Jewelry Brass/Copper Gold/Silver Attractive appearance, high value
Car parts (bumpers, handles) Iron/Steel Chromium Shiny appearance, corrosion resistance
Bicycle handlebars Iron Chromium Scratch resistance, shine
Tin cans for food Iron Tin Prevent corrosion, non-toxic
Bridge parts Iron Zinc (Galvanization) Rust protection
Electronic components Copper Gold Better conductivity
Utensils Copper/Brass Silver/Nickel Aesthetic appeal

Why Specific Metals are Used for Plating

Chromium Plating:

  • Shiny, attractive appearance
  • Does not corrode
  • Resists scratches
  • Hard and durable

Tin Plating (Tin Cans):

  • Less reactive than iron
  • Non-toxic (safe for food storage)
  • Prevents food from contacting iron
  • Protects from spoilage

Zinc Coating (Galvanization):

  • Protects iron from corrosion
  • Forms protective layer
  • Used in bridges, railings, pipes

Gold/Silver Plating:

  • Beautiful appearance
  • Expensive metals used economically
  • Makes costume jewelry affordable

Formulas and Equations

Concept Formula Explanation
Electric Current I = Q/t Current (A) = Charge (C) / Time (s)
Charge on Electron e = 1.6 × 10⁻¹⁹ C Fundamental unit of charge
Coulomb Definition 1 C = Charge on 6.25 × 10¹⁸ electrons Standard unit of charge
Ampere Definition 1 A = 1 C/1 s Current when 1 coulomb flows in 1 second
Quantization of Charge Q = ±ne Total charge is integral multiple of e
Electrolysis of Water 2H₂O → 2H₂ + O₂ Water decomposes into hydrogen and oxygen
Copper Deposition (Cathode) Cu²⁺ + 2e⁻ → Cu Copper ions reduced to copper metal
Copper Dissolution (Anode) Cu → Cu²⁺ + 2e⁻ Copper metal oxidized to copper ions

Practical Applications in Daily Life

1. Electroplating Industry

  • Jewelry making (gold/silver plating)
  • Automobile industry (chromium plating)
  • Cutlery and utensils (silver/nickel plating)

2. Metal Purification

Impure metals can be purified using electrolysis:

  • Impure metal = Anode
  • Pure metal = Cathode
  • Metal salt solution = Electrolyte

Pure metal deposits at cathode; impurities remain in solution.

3. Extraction of Metals

Aluminum is extracted from its ore (bauxite) using electrolysis.

4. Water Treatment

Electrolysis helps in water purification and splitting for hydrogen production.

5. Safety Measures

Understanding conductivity of water explains why:

  • We shouldn't touch electrical appliances with wet hands
  • Firemen shut off electricity before using water hoses
  • Electrical appliances need proper earthing

Safety Precautions

  1. Never touch electrical appliances with wet hands - Water (especially impure) conducts electricity
  2. Earthing is essential - Protects from electric shock by providing alternative path for leakage current
  3. Rubber gloves for electricians - Rubber is an excellent insulator
  4. Turn off power during fires - Water used in firefighting conducts electricity
  5. Handle electrolysis carefully - Gases produced (H₂, Cl₂) can be dangerous

Summary

  1. Electric current is the flow of electric charge; measured in Amperes
  2. Conductors allow easy flow of electricity; insulators resist it
  3. Pure liquids generally don't conduct; solutions with ions do conduct
  4. Electrolysis is chemical decomposition caused by electric current
  5. Cations move to cathode; anions move to anode during electrolysis
  6. Metals deposit at cathode by gaining electrons
  7. Electroplating deposits one metal over another for protection and beauty
  8. Chemical effects include gas formation, metal deposition, color changes
  9. Always follow safety precautions when working with electricity
  10. Practical applications include purification, extraction, and coating of metals

Conclusion

The chemical effects of electric current demonstrate the powerful connection between electricity and chemistry. From the simple decomposition of water into hydrogen and oxygen to the intricate process of electroplating jewelry, these phenomena have revolutionized industry and technology. Understanding these concepts not only helps in academic success but also explains countless applications we encounter daily—from the chromium shine on your bicycle to the zinc protection on bridges.

Related Topics for Further Study:

For CBSE Class 8 Students: Practice drawing circuit diagrams, writing balanced chemical equations for electrolysis reactions, and solving numerical problems on current and charge. Conduct experiments safely with teacher supervision to observe these fascinating chemical effects firsthand.