Chemical Effects of Electric Current

Overview & Fundamental Concept

Definitive AISSEE Class 9 General Science guide for Chemical Effects of Electric Current (Topic 10). Contains pointwise electrochemistry rules, electrolysis comparison tables, electroplating setup schematics, solved models, and 10 practice drills.

• Conduction in Solids vs Liquids:
  • In solids (metals like copper, aluminium), electric current is carried by the flow of free mobile electrons.
  • In liquids, electric current is carried by the migration of freely moving charged ions (positively charged cations and negatively charged anions).
• Classification of Liquids by Electrical Conductivity:
  • Good Conducting Liquids (Electrolytes): Liquids that conduct electricity due to the presence of dissolved ions. Examples: Tap water, vinegar (dilute acetic acid), lemon juice (citric acid), dilute hydrochloric acid, sodium hydroxide solution, copper sulphate solution, salt water (NaCl).
  • Poor Conducting Liquids (Insulators / Non-Electrolytes): Liquids that do not conduct electricity because they do not dissociate into free ions. Examples: Pure distilled water, sugar solution, kerosene oil, mustard oil, alcohol, liquid paraffin.
• How to Make Distilled Water Conducting: Pure distilled water lacks dissolved mineral salts and is a poor conductor. Adding a pinch of common salt (NaCl), a few drops of an acid ($H_2SO_4$, $HCl$), or a base ($NaOH$) provides free ions, transforming it into a good conductor.
• Testing Weak Currents in Liquids:
  • Standard Bulb Tester: A standard incandescent torch bulb may not glow if the liquid conducts electricity weakly, because the feeble electric current does not produce enough heat to make the tungsten filament incandescent.
  • 1. LED Tester (Light Emitting Diode): An LED glows brightly even when an extremely weak electric current flows through it. An LED has two wires (leads): the longer lead is connected to the positive (+) terminal of the battery (Anode); the shorter lead is connected to the negative (−) terminal (Cathode).
  • 2. Magnetic Compass Tester: Wrapping a copper wire around a discarded matchbox containing a magnetic compass needle detects minute currents. Even a faint current produces a magnetic field that deflects the compass needle.

• Chemical Effect of Electric Current: When an electric current is passed through a conducting ionic solution, it causes chemical changes and reactions. This phenomenon is known as the chemical effect of current.
• Manifestations of Chemical Effects:
  • 1. Evolution of gas bubbles on the submerged electrodes.
  • 2. Deposition of solid metal on the electrodes.
  • 3. Change in color of the electrolyte solution (e.g. greening around positive electrode in a potato).
• Electrolysis of Water (William Nicholson's Discovery, 1800):
  • In 1800, British chemist William Nicholson immersed two carbon electrodes in acidified water and passed an electric current from a battery.
  • Anode (+ve Electrode): Bubbles of Oxygen gas ($O_2$) were evolved.
  • Cathode (−ve Electrode): Bubbles of Hydrogen gas ($H_2$) were evolved.
  • Volume Ratio: The volume of hydrogen gas collected is twice ($2\times$) the volume of oxygen gas:
  $$\text{Volume of } H_2 : \text{Volume of } O_2 = 2 : 1$$
  (Because each water molecule $H_2O$ contains 2 hydrogen atoms for every 1 oxygen atom).
• The Potato Conduction Experiment: When copper wires from a battery are inserted into a cut potato, a greenish-blue spot develops around the wire connected to the positive (+) terminal. This chemical reaction occurs due to the formation of copper salts and serves to identify the positive terminal of a concealed battery.

• Electroplating Definition: The industrial process of depositing a thin, uniform protective layer of a desired metal on another material by passing an electric current through an electrolyte solution.
• Three Non-Negotiable Rules of Electroplating Setup:
  • 1. Anode (+ve terminal): Must be made of a clean plate of the pure metal that is to be deposited (e.g. a pure Copper plate if copper plating; pure Chromium if chromium plating).
  • 2. Cathode (−ve terminal): Must be the article to be electroplated (e.g. an iron spoon, brass key, or steel bumper).
  • 3. Electrolyte: Must be an aqueous solution of a water-soluble salt of the anode metal (e.g. Copper Sulphate solution $CuSO_4$ when plating copper).
• Mechanism of Copper Electroplating:
  • When current passes through the copper sulphate ($CuSO_4$) solution, it dissociates into copper ions ($Cu^{2+}$) and sulphate ions ($SO_4^{2-}$).
  • Positively charged copper ions ($Cu^{2+}$) are electrostatically attracted to the negative cathode (iron spoon), where they gain electrons and deposit as a smooth reddish layer of metallic copper.
  • Simultaneously, an equal amount of copper dissolves from the positive copper anode into the solution as copper ions ($Cu \rightarrow Cu^{2+} + 2e^-$).
  • Crucial Consequence: The concentration of copper sulphate in the electrolyte solution remains completely constant throughout the process! The copper lost from solution is continuously replenished by the sacrificial anode.

• Electroplating is widely used in modern industry for two primary purposes: (1) Protection against corrosion/rust, and (2) Enhancing decorative appearance.
• 1. Chromium Plating:
  • Chromium has a brilliant, shining lustrous finish, does not corrode, and is extremely scratch-resistant.
  • Because solid chromium metal is expensive, the entire article is made of cheap iron or steel, and only a thin coating of chromium is electroplated on top.
  • Applications: Car bumpers, motorcycle rims, bicycle handlebars, bathroom water taps, and gas burner knobs.
• 2. Tin Plating on Iron (Tin Cans):
  • Tin is less reactive than iron.
  • Iron cans used for storing edible food are electroplated with a thin layer of tin.
  • Prevents food from coming into direct contact with iron, preventing chemical spoilage and toxic contamination.
• 3. Gold and Silver Plating (Artificial Jewellery):
  • Cheap base metals (copper, brass) are electroplated with an ultra-thin surface layer of gold or silver.
  • Gives the jewelry the opulent appearance of solid gold/silver at a fraction of the cost.
• 4. Zinc Coating (Galvanisation):
  • Iron sheets, bridges, and automobile bodies are coated with zinc to protect them from rusting.
  • Zinc acts as a sacrificial anode; even if scratched, zinc oxidizes before iron.
• Disposal of Electroplating Waste: Electroplating factories produce large quantities of toxic, acidic, heavy-metal chemical effluents. These must be treated chemically according to environmental protection guidelines before discharge into water bodies.