Class 8 Notes

Class 8 Science Notes Chapter 14 (Chapter 14) – Examplar Problems Book

Philoid

Philoid

— 5 min read

Examplar Problems
Alright class, let's focus on Chapter 14: Chemical Effects of Electric Current from your Science Exemplar book. This is an important chapter, especially for competitive exams, as it deals with the interaction between electricity and chemical substances. Pay close attention to the concepts and their applications.

Chapter 14: Chemical Effects of Electric Current - Detailed Notes

1. Electrical Conductivity in Liquids

  • Concept: Not all materials allow electric current to pass through them. Solids like metals are good conductors, while rubber and plastic are insulators. Similarly, liquids also vary in their ability to conduct electricity.
  • Conductors (Liquids): Liquids that allow electric current to pass through them are called conducting liquids or electrolytes.
    • Examples: Tap water, saltwater, solutions of acids (like lemon juice, vinegar), solutions of bases (like soap solution), copper sulphate solution.
  • Insulators (Liquids): Liquids that do not allow electric current to pass through them are called poor conductors or insulators.
    • Examples: Distilled water, sugar solution, alcohol, kerosene, vegetable oil.
  • Why do some liquids conduct? Conduction in liquids is due to the movement of charged particles called ions (positive ions - cations, negative ions - anions).
    • Acids, bases, and salts dissociate into ions when dissolved in water, making the solution conductive.
    • Distilled water lacks these dissolved salts/minerals (ions) and hence is a poor conductor. Tap water contains dissolved salts, making it conductive.

2. Testing Conductivity of Liquids

  • Tester: A simple circuit can be used as a tester. It typically consists of a battery (cell), connecting wires, and an indicator (like a bulb or LED).
  • Procedure: The free ends of the tester are dipped into the liquid (without touching each other). If the indicator glows, the liquid is conducting.
  • Using LED: An LED (Light Emitting Diode) is often preferred over a bulb because it glows even when a weak current flows. LEDs have two terminals (leads); one slightly longer (positive terminal) than the other (negative terminal). The longer lead is always connected to the positive terminal of the battery, and the shorter lead to the negative terminal.
  • Using Magnetic Compass: Even a very weak current produces a magnetic effect. If the current is too weak to make a bulb or LED glow, the deflection of a magnetic compass needle placed near the wire can indicate the flow of current.

3. Chemical Effects of Electric Current

  • Definition: The passage of an electric current through a conducting liquid (electrolyte) causes chemical reactions. This phenomenon is known as the chemical effect of electric current.
  • Electrolysis: The process of decomposition of an electrolyte solution into its constituent ions when an electric current is passed through it is called electrolysis.
    • Electrodes: The metal rods or plates through which current enters and leaves the electrolyte are called electrodes.
      • Anode: The electrode connected to the positive terminal of the battery.
      • Cathode: The electrode connected to the negative terminal of the battery.
  • Examples of Chemical Effects:
    • Formation of gas bubbles: When current passes through water (acidulated water, to make it conductive), bubbles of oxygen gas are formed at the anode (+ve electrode) and hydrogen gas at the cathode (-ve electrode). (Electrolysis of Water: 2H₂O → 2H₂ (at cathode) + O₂ (at anode))
    • Deposition of metal: When current passes through a copper sulphate solution using copper electrodes, copper metal gets deposited on the cathode (-ve electrode).
    • Change in colour of the solution: The chemical reactions can sometimes lead to a change in the colour of the electrolyte solution.

4. Electroplating

  • Definition: Electroplating is a process of depositing a thin layer of any desired metal onto another material (usually metallic) by means of electrolysis.
  • Purpose:
    • Protection: To protect metals from corrosion (e.g., zinc coating on iron - galvanization).
    • Decoration: To give objects a shiny, attractive appearance (e.g., chromium plating on taps, silver/gold plating on jewellery).
    • Repair: To restore worn-out parts.
    • Improve Properties: To enhance hardness or conductivity.
  • Process:
    • Object to be plated: Made the cathode (-ve electrode).
    • Metal to be deposited: Made the anode (+ve electrode).
    • Electrolyte: A water-soluble salt solution of the metal that is to be deposited (e.g., copper sulphate solution for copper plating, silver nitrate solution for silver plating).
  • Mechanism: When current passes:
    1. The metal from the anode dissolves into the electrolyte in the form of positive ions.
    2. These positive metal ions from the electrolyte are attracted towards the cathode (the object).
    3. The ions gain electrons at the cathode and get deposited as a thin layer of pure metal onto the object.
    4. The loss of metal from the electrolyte is replenished by the dissolution of the anode, keeping the concentration constant. (This happens if the anode is made of the same metal being plated).
  • Common Examples:
    • Chromium plating: Done on car parts, bath taps, bicycle handlebars for a shiny appearance and resistance to scratches and corrosion. Chromium is expensive, so only a thin layer is deposited on cheaper metals.
    • Zinc plating (Galvanization): Iron objects (like bridges, car bodies, buckets) are coated with zinc to protect them from rusting.
    • Gold/Silver plating: Used on artificial jewellery to give it the appearance of real gold or silver.
    • Tin plating: Tin cans used for storing food are made by electroplating tin onto iron. Tin is less reactive than iron, preventing food spoilage.

Key Terms to Remember:

  • Conductor: Material allowing electric current to pass.
  • Insulator: Material resisting the flow of electric current.
  • Electrolyte: A conducting liquid/solution containing ions.
  • Electrolysis: Decomposition of an electrolyte by electric current.
  • Electrodes: Conductors through which current enters/leaves the electrolyte (Anode +, Cathode -).
  • Ions: Charged particles (Cations +, Anions -) responsible for conduction in liquids.
  • Electroplating: Depositing a layer of one metal onto another using electrolysis.
  • LED: Light Emitting Diode, sensitive indicator for electric current.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on the concepts discussed. Try to answer them yourself first.

  1. Which of the following liquids is expected to be a poor conductor of electricity?
    (a) Tap water
    (b) Salt solution
    (c) Distilled water
    (d) Lemon juice

  2. The process of depositing a layer of zinc on iron is known as:
    (a) Electroplating
    (b) Electrolysis
    (c) Galvanization
    (d) Refining

  3. During the electrolysis of water, hydrogen gas is produced at the:
    (a) Anode (positive electrode)
    (b) Cathode (negative electrode)
    (c) Both electrodes
    (d) In the solution itself

  4. An LED is often used in testers instead of a bulb because:
    (a) It consumes more electricity
    (b) It glows even with a weak current
    (c) It is much larger than a bulb
    (d) It only works with AC current

  5. In the process of electroplating an iron spoon with copper, which arrangement is correct?
    (a) Iron spoon as anode, Copper plate as cathode, Copper sulphate solution as electrolyte
    (b) Copper plate as anode, Iron spoon as cathode, Iron sulphate solution as electrolyte
    (c) Iron spoon as anode, Copper plate as cathode, Iron sulphate solution as electrolyte
    (d) Copper plate as anode, Iron spoon as cathode, Copper sulphate solution as electrolyte

  6. The passage of electric current through a solution causes _________ effects.
    (a) Magnetic
    (b) Heating
    (c) Chemical
    (d) All of the above

  7. Which of the following is NOT a reason for electroplating?
    (a) To make objects look attractive
    (b) To protect metals from corrosion
    (c) To increase the weight of the object significantly
    (d) To coat with a less reactive metal

  8. When electric current is passed through copper sulphate solution, copper gets deposited on the plate connected to the:
    (a) Positive terminal of the battery
    (b) Negative terminal of the battery
    (c) Either positive or negative terminal
    (d) No deposition occurs

  9. Why is chromium used for electroplating bath taps?
    (a) It is very cheap.
    (b) It has a dull appearance.
    (c) It resists corrosion and scratches.
    (d) It dissolves easily in water.

  10. Which of the following statements about conduction in liquids is TRUE?
    (a) All liquids conduct electricity equally well.
    (b) Conduction is due to the movement of free electrons.
    (c) Pure water is a very good conductor.
    (d) Dissolving salts or acids in water increases its conductivity.

Answers to MCQs:

  1. (c) Distilled water
  2. (c) Galvanization (Note: Galvanization is a specific type of electroplating where zinc is deposited, usually on iron/steel).
  3. (b) Cathode (negative electrode)
  4. (b) It glows even with a weak current
  5. (d) Copper plate as anode, Iron spoon as cathode, Copper sulphate solution as electrolyte
  6. (d) All of the above (While the chapter focuses on chemical effects, current also produces heating and magnetic effects).
  7. (c) To increase the weight of the object significantly (The deposited layer is very thin).
  8. (b) Negative terminal of the battery (Cathode)
  9. (c) It resists corrosion and scratches.
  10. (d) Dissolving salts or acids in water increases its conductivity.

Study these notes thoroughly. Remember the definitions, the setup for electrolysis and electroplating, and the common applications. Good luck with your preparation!

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