Class 8 Science Notes Chapter 14 (Chemical Effects of Electric Current) – Science Book
We brought to you a detailed Notes blog post of Chapter 14 'Chemical Effects of Electric Current' from the NCERT Class 8 Science textbook, tailored for government exam preparation, followed by 10 MCQs.
Chapter 14: Chemical Effects of Electric Current - Detailed Notes
1. Introduction: Conductors and Insulators
- Electric Current: The flow of electric charge.
- Conductors: Materials that allow electric current to pass through them easily. Examples: Metals (copper, aluminum, iron, silver), graphite.
- Insulators (Poor Conductors): Materials that do not allow electric current to pass through them easily. Examples: Rubber, plastic, wood, glass, pure water.
2. Testing Conductivity
- A simple tester can be made using a battery (cell), connecting wires, and a small bulb or an LED (Light Emitting Diode).
- Why LED? LEDs glow even when a weak current flows, making them suitable for testing materials/liquids with low conductivity. Remember the longer lead of an LED is connected to the positive terminal and the shorter lead to the negative terminal of the battery.
- To test a material/liquid, it is placed in the gap of the tester circuit. If the bulb/LED glows, the material/liquid is a conductor. If it doesn't, it's an insulator.
3. Do Liquids Conduct Electricity?
- Pure (Distilled) Water: Is a poor conductor of electricity because it lacks free ions.
- Tap Water, Pond Water, Well Water: Conduct electricity because they contain dissolved salts and minerals, which provide ions.
- Solutions of Acids, Bases, and Salts: These substances dissociate into ions when dissolved in water, making the solution conductive.
- Examples: Lemon juice (citric acid), vinegar (acetic acid), common salt (NaCl) solution, copper sulphate (CuSO₄) solution, sodium hydroxide (NaOH) solution.
- Electrolytes: Liquids/solutions that conduct electricity and undergo decomposition (chemical change) in the process.
- Non-electrolytes: Liquids/solutions that do not conduct electricity (e.g., distilled water, sugar solution, alcohol).
4. 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.
- Observations: These effects may include:
- Formation of gas bubbles on the electrodes.
- Deposition of metal on the electrodes.
- Changes in the colour of the solution.
- Electrolysis: The process of decomposition of an electrolyte when electricity is passed through it.
5. Electrolysis Explained
- Electrodes: The conductors (usually metal rods or plates) through which current enters or leaves the electrolyte.
- Anode: The electrode connected to the positive (+) terminal of the battery.
- Cathode: The electrode connected to the negative (-) terminal of the battery.
- Mechanism: When current passes, the electrolyte dissociates into positive ions (cations) and negative ions (anions).
- Cations (Positive ions): Move towards the cathode (negative electrode) and gain electrons (reduction).
- Anions (Negative ions): Move towards the anode (positive electrode) and lose electrons (oxidation).
- Example: Electrolysis of Water (Acidulated)
- Pure water is a poor conductor, so a few drops of acid (like sulphuric acid) are added to make it conductive.
- When current passes:
- Oxygen gas bubbles are formed at the anode (+).
- Hydrogen gas bubbles are formed at the cathode (-).
- Example: Electrolysis of Copper Sulphate (CuSO₄) Solution using Copper Electrodes
- Copper sulphate dissociates into Cu²⁺ (copper ions) and SO₄²⁻ (sulphate ions).
- At the cathode (-): Copper ions (Cu²⁺) gain electrons and deposit as pure copper metal (Cu).
Cu²⁺ + 2e⁻ → Cu - At the anode (+): Copper atoms from the copper anode lose electrons and dissolve into the solution as copper ions (Cu²⁺).
Cu → Cu²⁺ + 2e⁻ - Net Result: Copper transfers from the anode to the cathode. The concentration of CuSO₄ in the solution remains relatively constant. This principle is used in the purification of copper.
6. Electroplating
- Definition: The process of depositing a thin layer of any desired metal onto another material (usually metallic) by means of electricity (electrolysis).
- Purpose:
- Protection: To protect metals from corrosion (e.g., zinc coating on iron - galvanization, chromium plating on bath taps).
- Appearance: To give objects a shiny, attractive appearance (e.g., silver or gold plating on jewellery, chromium plating on car parts).
- Process:
- The object to be plated is made the cathode (connected to the negative terminal).
- The metal to be deposited (e.g., copper, chromium, gold) is made the anode (connected to the positive terminal). Often, an inert anode is used, and the metal ions come solely from the electrolyte. However, for plating with metals like copper, a copper anode is typically used.
- The electrolyte is a salt solution of the metal to be deposited (e.g., copper sulphate solution for copper plating).
- When current passes, metal ions from the electrolyte move to the cathode, gain electrons, and deposit onto the object.
- If the anode is made of the same metal, it dissolves to replenish the metal ions in the electrolyte.
- Common Examples:
- Chromium Plating: Chromium has a shiny appearance, resists scratches, and doesn't corrode. Used on bath taps, car parts, bicycle handlebars. Chromium is expensive, so only a thin layer is deposited on cheaper metals.
- Zinc Plating (Galvanization): Zinc is deposited on iron objects to protect them from rusting. Used in bridges, automobiles, and making containers (like tin cans, which are actually iron electroplated with tin).
- Gold/Silver Plating: Used on less expensive metals to make jewellery look attractive.
7. Applications of Chemical Effects of Electric Current
- Electroplating: Widely used in industry for protective and decorative coatings.
- Purification of Metals (Electrorefining): Example: Refining of copper. Impure copper is the anode, pure copper is the cathode, and copper sulphate solution is the electrolyte. Pure copper deposits on the cathode.
- Extraction of Metals: Highly reactive metals like sodium, potassium, aluminum are extracted from their molten ores/salts by electrolysis.
- Production of Chemicals: Electrolysis is used to produce chemicals like chlorine gas, hydrogen gas, and sodium hydroxide (from brine - NaCl solution).
Multiple Choice Questions (MCQs)
-
Which of the following liquids is likely to be the poorest conductor of electricity?
(A) Tap Water
(B) Lemon Juice
(C) Distilled Water
(D) Salt Solution -
The process of depositing a layer of zinc on iron to protect it from corrosion is called:
(A) Electrorefining
(B) Electrolysis
(C) Galvanization (Electroplating with Zinc)
(D) Anodizing -
During the electrolysis of copper sulphate solution using copper electrodes, what happens at the cathode?
(A) Copper dissolves into the solution.
(B) Oxygen gas is released.
(C) Copper metal gets deposited.
(D) Hydrogen gas is released. -
An LED is often preferred over a bulb in a simple tester because:
(A) It consumes more electricity.
(B) It can glow even with a weak current.
(C) It is much larger in size.
(D) It only works with AC current. -
Which of the following is NOT a chemical effect of electric current?
(A) Formation of gas bubbles at electrodes.
(B) Heating of the wire carrying the current.
(C) Deposition of metal on an electrode.
(D) Change in colour of the electrolyte solution. -
In the process of electroplating an iron spoon with copper, the iron spoon should be connected to:
(A) The positive terminal of the battery (Anode)
(B) The negative terminal of the battery (Cathode)
(C) Either terminal of the battery
(D) A separate voltmeter -
The electrode connected to the positive terminal of the battery is called the:
(A) Cathode
(B) Anode
(C) Electrolyte
(D) Ion -
Which of the following materials is commonly used for electroplating bath taps to give them a shiny, corrosion-resistant finish?
(A) Zinc
(B) Copper
(C) Chromium
(D) Tin -
When electric current is passed through acidulated water, hydrogen gas is produced at:
(A) The anode (positive electrode)
(B) The cathode (negative electrode)
(C) Both electrodes
(D) Within the solution -
A liquid that conducts electricity and undergoes decomposition is called:
(A) An insulator
(B) A semiconductor
(C) An electrolyte
(D) A non-electrolyte
Answers to MCQs:
- (C) Distilled Water
- (C) Galvanization (Electroplating with Zinc)
- (C) Copper metal gets deposited.
- (B) It can glow even with a weak current.
- (B) Heating of the wire carrying the current. (This is the heating effect, not chemical)
- (B) The negative terminal of the battery (Cathode)
- (B) Anode
- (C) Chromium
- (B) The cathode (negative electrode)
- (C) An electrolyte