Chemical Effects of Electric Current Class 8 Notes

Conduction of Electricity in Liquids

While we know that metals like copper and aluminum are good conductors of electricity, many liquids also conduct electricity. Liquids that conduct electricity are usually solutions of acids, bases, or salts dissolved in water. When electric current passes through these conducting liquids, it causes chemical changes. This phenomenon is called the chemical effect of electric current. Standard bulb testers may not glow if the current passing through the liquid is too weak. To detect weak currents, we use highly sensitive devices like an LED (Light Emitting Diode) or a magnetic compass needle tester which utilizes the magnetic effect of electric current to deflect and indicate flow.

Key Definitions & Terminology

Electrolyte

A liquid or solution that conducts electricity and undergoes chemical decomposition when an electric current passes through it.

Electrode

A conductor (like a carbon or metal rod) through which electric current enters or leaves an electrolyte.

Electrolysis

The chemical decomposition of an electrolyte caused by the passage of electric current.

Anode

The electrode connected to the positive terminal of the battery.

Cathode

The electrode connected to the negative terminal of the battery.

Electroplating

The process of depositing a thin layer of any desired metal on another material by means of electricity.

LED (Light Emitting Diode)

A semiconductor light source that glows even when a very weak electric current flows through it.

Comparison of Liquid Conductivity

The Step-by-Step Process of Electroplating

1. Preparation of Materials — Clean copper plates and the object to be plated (e.g., an iron nail) with sandpaper to remove dirt and oxide layers.
2. Electrolyte Solution Formulation — Prepare a solution of Copper Sulphate ($CuSO_4$) in a clean beaker using distilled water. Add a few drops of dilute sulphuric acid to increase conductivity.
3. Connecting to Power Supply — Connect the copper plate to the positive terminal (Anode) of the battery and the iron nail to the negative terminal (Cathode).
4. Passing Electric Current — Immerse the electrodes in the copper sulphate solution and pass current for about 15-20 minutes.
5. Deposition of Metal — Copper ions ($Cu^{2+}$) from the solution get drawn to the negative electrode (Cathode) and deposit on the iron nail as a shiny reddish-brown layer.
6. Solution replenishment — Simultaneously, an equal amount of copper dissolves from the positive copper plate (Anode) back into the solution, maintaining the concentration.

Must Remember: Key Points

• {"title":"Charge Carriers in Liquids","bodyMarkdown":"Unlike solids (where free electrons conduct electricity), liquids conduct electricity using positively and negatively charged ions."}
• {"title":"Chemical Changes Observed","bodyMarkdown":"When current passes through an electrolyte, it can cause gas bubbles at electrodes, deposits of metal on electrodes, or a change in color of the solution."}
• {"title":"LED Polarity","bodyMarkdown":"An LED has two leads: the longer lead is always connected to the positive terminal of the battery, and the shorter lead to the negative terminal."}
• {"title":"Nicholson's Experiment (1800)","bodyMarkdown":"British chemist William Nicholson showed that passing current through water yields oxygen bubbles at the positive terminal and hydrogen bubbles at the negative terminal."}
• {"title":"Common Applications of Electroplating","bodyMarkdown":"Used for coating chromium on car parts/taps (prevents scratches), tin on iron cans for storing food (prevents food-metal reaction), and gold/silver on cheap metal jewelry."}
• {"title":"Safety Warning","bodyMarkdown":"Never perform conductivity tests using the mains electricity supply. Always use a battery/dry cells to prevent electric shocks."}

Exam Cues and High-Yield Tips

• The Polarity Trap: In electroplating questions, always remember that the metal to be deposited must be the Anode (Positive Terminal) and the object to be coated must be the Cathode (Negative Terminal).
• Why Chromium? Exams often ask why chromium is used for electroplating despite being expensive. Your answer must include three points: shiny appearance, corrosion resistance, and resistance to scratches.
• Explain Distilled Water Conductivity: If asked how to make distilled water conductive, write: 'Add a small amount of salt, acid, or base to provide the necessary free ions for conduction.'

Practice Questions with Solutions

• Q: Why does a compass needle show deflection when a tester is dipped in a solution of vinegar? A: Vinegar contains acetic acid, which dissociates into free ions when dissolved in water. These free ions allow electric current to pass through. The current generates a magnetic field that deflects the compass needle.
• Q: Why are iron bridges and pipes coated with zinc? A: Iron is highly prone to rusting and corrosion. Coating it with a layer of zinc (through galvanization/electroplating) protects the underlying iron from reacting with moisture and oxygen, increasing its lifetime.
• Q: In the electrolysis of water, why is the volume of gas collected at one electrode double that of the other? A: Water ($H_2O$) is made of hydrogen and oxygen in a 2:1 ratio. During electrolysis, hydrogen gas collected at the cathode (negative electrode) is twice the volume of oxygen gas collected at the anode (positive electrode).
• Q: What is the purpose of adding dilute sulphuric acid to copper sulphate solution during electroplating? A: Adding dilute sulphuric acid makes the copper sulphate solution more acidic, which increases its conductivity by facilitating a higher flow of ions, making the electroplating process faster and smoother.

Frequently Asked Questions