Revision Notes – Chapter 14: Chemical Effects of Electric Current

1. Good Conductors and Poor Conductors of Electricity

From earlier classes, you know that materials are divided into conductors and insulators based on their ability to allow electric current to pass through them.

1.1 Conductors (Good Conductors of Electricity)

Conductors are substances that allow electric current to pass through them easily. This happens because they have many free electrons or ions which can move when a potential difference is applied.

Examples: Copper, aluminium, silver, iron, human body, earth, etc.

Metals are generally good conductors. That is why most electrical wires are made of copper or aluminium and covered with plastic insulation.

1.2 Insulators (Poor Conductors of Electricity)

Insulators (or poor conductors) are substances that do not allow electric current to pass through them easily because they do not have free electrons or ions.

Examples: Plastic, rubber, glass, dry wood, pure distilled water, etc.

1.3 Conductors and Poor Conductors Among Liquids

In this chapter, the focus is on liquids. Some liquids conduct electricity while others do not.

• Solutions of acids, bases and salts in water usually conduct electricity. (Example: lemon juice, vinegar, hydrochloric acid, sodium hydroxide solution, saltwater).
• Some liquids like distilled water, kerosene, alcohol, and cooking oil do not conduct electricity or conduct very poorly.

2. Testing Conductivity of Liquids – Use of LED and Magnetic Effect

2.1 Why Use an LED Instead of a Bulb?

Sometimes the electric current passing through a liquid is very weak. An ordinary filament bulb may not glow with such a small current. A Light Emitting Diode (LED) is more sensitive and can glow even with a small current.

Therefore, we often use an LED in the circuit to test whether a liquid can conduct electricity or not.

2.2 Using Magnetic Effect to Detect Current

Another method is to use the magnetic effect of electric current. When current flows through a coil of wire, it behaves like a magnet.

In the activity, a small compass needle is placed near the coil. If the needle deflects when the circuit is completed and the liquid is connected, it means that current is flowing through the liquid, and hence the liquid is a conductor.

3. Chemical Effects of Electric Current

When electric current is passed through certain liquids (conducting solutions), they may undergo chemical changes. This is called the chemical effect of electric current.

3.1 What Changes Can Occur?

Depending on the nature of the solution and electrodes, the following changes may be observed:

• Formation of gas bubbles at the electrodes (e.g., bubbles of hydrogen or oxygen).
• Change in colour of the solution (due to formation of new substances).
• Deposition of a metal on one of the electrodes.
• Change in mass of electrodes (gain or loss of material).

3.2 Electrolyte, Electrodes, Anode and Cathode

When we pass current through a conducting solution, the arrangement generally includes:

• Electrolyte: The conducting solution (e.g., copper sulphate solution, salt solution).
• Electrodes: Conducting rods dipped in the solution (metal or graphite).
• Anode: The electrode connected to the positive terminal of the battery.
• Cathode: The electrode connected to the negative terminal of the battery.

4. Electrolysis – Simple Understanding

Electrolysis is the process in which electric current is passed through a liquid (or solution) to bring about a chemical decomposition of that substance.

At NCERT Class 8 level, it is enough to understand that:

• When current passes through an electrolyte, it breaks down into ions (charged particles).
• Positive ions move towards the cathode and negative ions move towards the anode.
• These ions may gain or lose electrons at the electrodes, leading to chemical changes like gas formation or metal deposition.

5. Important Activity – Copper Sulphate and Copper Electrodes

NCERT discusses an important experiment using copper sulphate solution and copper electrodes.

5.1 Setup

• Take a beaker with copper sulphate solution (CuSO₄ in water).
• Dip two clean copper plates (electrodes) into the solution.
• Connect one plate to the positive terminal (anode) and the other to the negative terminal (cathode) of a battery.
• Let the current pass for some time (e.g., 15–20 minutes).

5.2 Observations

After some time:

• The cathode (negative electrode) may appear to have a layer of reddish-brown copper deposited on it.
• The anode (positive electrode) may slowly get thinner, losing some of its copper.
• The colour of the solution may not change much if the process is balanced, because copper taken from the anode goes into solution and is again deposited on the cathode.

5.3 Explanation

When current passes through CuSO₄ solution:

• CuSO₄ → Cu²⁺ + SO₄²⁻ (in solution)
• Cu²⁺ ions move towards the cathode and gain electrons to become copper metal that deposits on the cathode.
• Copper atoms at the anode lose electrons and go into solution as Cu²⁺ ions, making the anode thinner.

Thus, there is a transfer of copper from the anode to the cathode, showing the chemical effect of electric current.

6. Electroplating – A Useful Application

Electroplating is an important application of the chemical effects of electric current. It is the process of depositing a thin layer of one metal over another metal object using electricity.

6.1 Why Electroplating Is Done

• To protect a metal from corrosion and rusting (e.g., plating iron objects with zinc or chromium).
• To make objects look more attractive and shiny (e.g., gold-plated jewellery, silver-plated spoons).
• To use a cheap metal base and cover it with a thin layer of an expensive metal, reducing cost.

6.2 Basic Principle of Electroplating

Electroplating uses the same idea as the copper sulphate activity. The key points are:

• The object to be electroplated is always made the cathode (negative electrode).
• The metal which is to be deposited is made the anode (positive electrode).
• The electrolyte is a solution of a salt of the metal to be coated (e.g., copper sulphate, silver nitrate, nickel chloride solution).

6.3 Example – Electroplating a Key with Copper

In short: Object to be plated → Cathode, Metal source → Anode, Salt solution of metal → Electrolyte.

7. Electroplating in Everyday Life

Electroplating is widely used in industry and in our daily life. Some common examples:

• Chrome plating: Bumpers and parts of cars, motorcycles and cycles are plated with chromium to make them shiny and resistant to corrosion.
• Silver plating: Cheap metal spoons and utensils are silver-plated to look like real silver articles.
• Gold plating: Artificial jewellery is often coated with a thin layer of gold to make it attractive.
• Tin plating: Iron cans used for food storage are plated with tin to prevent iron from reacting with food.

Thus, electroplating helps in saving costly metals and protecting objects from damage.

8. Good and Poor Conductors Among Solutions – NCERT Perspective

From the activities in the NCERT textbook, we learn that:

• Solutions of acids, bases and salts in water are generally good conductors (electrolytes).
• Pure, distilled water is a poor conductor, but adding a small amount of salt or acid makes it a better conductor.
• Many non-metallic liquids like kerosene, oil and alcohol do not conduct electricity.

9. Safety and Precautions

While doing experiments related to electric current and solutions, some precautions are necessary:

• Never touch an open electric circuit with wet hands.
• Always use small cells or batteries, not mains electricity, in school lab activities.
• Do not taste or directly touch chemicals like acids, bases and salts with bare hands.
• Work under the supervision of your teacher and wear proper safety equipment when necessary.

10. Key Points to Revise for Exams

• Understand the difference between conductors and insulators, especially among liquids.
• Know how to use LED and compass needle to test whether a liquid conducts electricity.
• Be able to explain chemical effects of electric current with examples (gas formation, colour change, metal deposition).
• Remember the definitions of electrolyte, electrode, anode, cathode.
• Be clear about the basic idea of electroplating and its applications.
• Learn at least two examples where electroplating is used in daily life (e.g., gold-plated jewellery, chrome plating of car parts).
• Revise any diagrams from the NCERT textbook showing the electroplating setup and LED test circuit.