Chemical Effects of Electric Current Class 8 NCERT Study Guide

Conductivity of Liquids: Why and How Liquids Conduct

Unlike solids like copper and aluminum which conduct electricity via free electrons, liquids conduct electricity through free-floating charged particles called ions. When substances like acids, bases, or salts dissolve in water, they split into positively and negatively charged ions. When we dip electrodes into these ionic solutions and connect them to a battery, these ions move towards the oppositely charged electrodes, completing the electrical circuit. Pure water (distilled water) lacks these dissolved salts, making it a poor conductor. However, tap water, well water, and pond water contain dissolved minerals and salts, turning them into good conductors of electricity. This is why we must always avoid handling electrical equipment with wet hands.

How Electric Current Causes Chemical Changes (Electrolysis)

1. Preparation of Electrolyte — Prepare a conducting liquid solution, such as water mixed with a few drops of dilute sulphuric acid or common salt, to increase its ionic concentration and conductivity.
2. Insertion of Electrodes — Immerse two clean carbon rods or metal plates (electrodes) into the conducting solution, keeping them separated so they do not touch each other directly.
3. Circuit Connection — Connect the electrodes to the terminals of a battery. The electrode linked to the positive terminal becomes the anode, and the one connected to the negative terminal is the cathode.
4. Chemical Reaction — As current flows, chemical reactions occur. You will observe bubbles of gas (oxygen at the anode, hydrogen at the cathode) forming on the electrodes, deposits of metal on electrodes, or changes in the color of the solution.

Essential Terms to Know

Electrode

A conductor through which electricity enters or leaves an object, substance, or liquid solution.

Electrolyte

A liquid solution that contains free ions and decomposes when an electric current passes through it.

Electroplating

The chemical process of depositing a thin layer of a desired metal onto another metal object using electricity.

Pro-Tips and Common Mistakes to Avoid in Exams

Many students mistakenly assume that distilled water is a good conductor because 'water conducts electricity'. Remember: distilled water is a poor conductor of electricity because it lacks dissolved ions! To make it conduct, you must add salt, acid, or base. Another common mistake is misidentifying the terminal deposits: during electroplating, the metal that gets deposited (like copper) always deposits on the cathode (negative terminal) because the positive metal ions in the solution are attracted to the negative electrode.

Practice Questions with Solutions

• Q: Distilled water does not conduct electricity. What can we add to make it conducting?
  A: Step 1: Understand why distilled water doesn't conduct. It lacks free ions to carry charge.
  Step 2: Identify substances that release ions when dissolved. These include acids, bases, and common salts.
  Step 3: State the solution. We can add a pinch of common salt (NaCl), a few drops of lemon juice (citric acid), or dilute sulphuric acid to the distilled water.
  Final answer: To make distilled water conducting, we can dissolve soluble substances like common salt, lemon juice, or dilute sulphuric acid, which release free ions that transport electric current.

• Q: Explain the process of electroplating copper on an iron key.
  A: Step 1: Set up the apparatus. Use copper sulphate solution as the electrolyte. Take a copper plate as the anode (positive electrode) and the iron key as the cathode (negative electrode).
  Step 2: Turn on the electricity. When current passes through the copper sulphate solution, copper sulphate dissociates into copper ions and sulphate ions.
  Step 3: Observe ion movement. The positively charged copper ions are drawn to the negative electrode (the iron key), where they gain electrons and deposit as a thin layer of copper.
  Step 4: Balance the solution. Simultaneously, the copper plate (anode) dissolves to replenish the copper ions in the solution, keeping the concentration constant.
  Final answer: By connecting the iron key to the negative terminal and a copper plate to the positive terminal in a copper sulphate bath, copper is transferred from the plate to coat the iron key.

• Q: A student tests three liquids A, B, and C with an LED tester. The LED glows brightly for A, glows dimly for B, and does not glow for C. Classify the liquids.
  A: Step 1: Analyze liquid A. Since the LED glows brightly, liquid A allows current to flow easily, indicating it is an excellent conductor.
  Step 2: Analyze liquid B. The LED glows dimly, which means current is flowing but it is weak. Thus, liquid B is a poor or weak conductor of electricity.
  Step 3: Analyze liquid C. Since the LED does not glow at all, liquid C does not conduct electricity, making it an insulator (or very poor conductor under normal voltage).
  Final answer: Liquid A is a good conductor, Liquid B is a weak/poor conductor, and Liquid C is an insulator.

• Q: Why does a tester with a magnetic compass needle deflect when dipped in lemon juice?
  A: Step 1: Identify the properties of lemon juice. Lemon juice contains citric acid, which dissociates into ions in water.
  Step 2: Connect ion dissociation to current. These free ions allow electric current to pass through the lemon juice, completing the tester circuit.
  Step 3: Relate electricity to magnetism. Electric current flowing through the circuit wires creates a magnetic field around them (magnetic effect of electric current).
  Step 4: Explain the indicator. This magnetic field acts on the compass needle, causing it to deflect.
  Final answer: Lemon juice conducts electricity because of its acidic ions. The resulting current creates a magnetic field that deflects the compass needle.

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