NCERT Science Notes - Class 8
Chapter 11 - Chemical Effects of Electric Current

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 8 | Science | Chapter 11 - Chemical Effects of Electric Current. The chapter explores how electric current can cause chemical changes in substances. This chapter covers key concepts such as electrolysis, electroplating, and the conductivity of liquids. Students will learn about the practical applications of these processes, including how metals are purified and how objects are coated with a layer of metal. Engaging experiments and activities, such as testing the conductivity of various liquids and observing the effects of electric current on different solutions, make this chapter both informative and interactive. This chapter aims to deepen students’ understanding of the chemical effects of electricity in everyday life.

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NCERT Science Notes - Class 8
Chapter 11 - Chemical Effects of Electric Current

11.1 - Do Liquids Conduct Electricity?

To find out if liquids conduct electricity, we can use a tester similar to the one used for solids. Here’s how:

1. Using a Tester:
   • Replace the cell in the tester with a battery for stronger power.
   • Before using the tester on any liquid, always check if the tester is working properly.
2. Testing Conductivity:
   • Once the tester is ready, you can place its ends in a liquid to observe whether it allows the electric current to pass through.

Activity 11.1 : Testing the Tester

Objective: To check if the tester is working properly before using it to test liquids for electrical conductivity.

Steps:

1. Complete the Circuit:
   • Join the free ends of the tester for a moment.
   • If the bulb glows, the circuit is complete, and the tester is working.
2. Troubleshooting:
   • If the bulb does not glow, the tester is not working. Possible reasons could be:
     • Loose connections: Check if the wires or other connections are loose.
     • Fused bulb: The bulb might be fused. Replace it with a new one.
     • Used-up cells: The battery cells might be weak or dead. Replace them with fresh ones.
3. Final Test:
   • After tightening connections, replacing the bulb, or changing the cells, test again to ensure the tester works.

Notes:

• Circuit Completion: The bulb will glow only when the circuit is complete, confirming the flow of current.
• Common Issues: Loose connections, a fused bulb, or drained cells can cause the tester to malfunction.
• Testing Liquids: Once the tester is functioning properly, it can be used to test whether different liquids conduct electricity.

Activity 11.2 : Testing the Conductivity of Lemon Juice or Vinegar

Objective: To test if liquids such as lemon juice or vinegar conduct electricity.

Materials Needed:

• Small plastic or rubber caps (from discarded bottles)
• Lemon juice or vinegar (one teaspoon)
• A working tester (as verified in Activity 11.1)

1. Preparation:
   • Clean the small caps.
   • Pour one teaspoon of lemon juice or vinegar into one of the caps.
2. Testing Conductivity:
   • Dip the ends of the tester into the liquid (lemon juice or vinegar) without letting the ends touch each other. Ensure the ends are about 1 cm apart (see Fig. 11.2).
3. Observation:
   • Check the bulb: Does it glow when the ends of the tester are in the liquid?
     • If the bulb glows, the liquid conducts electricity, meaning it is a good conductor.
     • If the bulb does not glow, the liquid may either be a poor conductor or the current might be too weak to make the bulb glow.
4. Reason for Non-glowing Bulb:
   • Even if the liquid conducts electricity, the bulb might not glow if the current is too weak.
   • This happens because the material (lemon juice or vinegar) does not conduct electricity as easily as metals. Hence, the filament inside the bulb may not heat up enough to glow.

• Good Conductor vs Poor Conductor:
  • Lemon juice and vinegar contain ions, which allow them to conduct electricity. However, their conductivity is weaker compared to metals.
  • Even if they conduct electricity, the current may be too weak to heat the filament sufficiently, and the bulb might not glow.
• Heating Effect of Electric Current:
  • A bulb glows due to the heating effect of the current. If the current is too weak, the filament does not heat up enough to glow.

• Current Flow: If the liquid allows electric current to pass, the circuit will complete, allowing the bulb to glow (if the current is strong enough).
• Weak Current: A liquid may still conduct electricity but may not produce a strong enough current to light up a bulb, indicating a weak conductor.

Objective: To modify the tester by using an LED instead of a bulb to detect weak electric currents more effectively.

Procedure:

1. Materials Needed:
   • LED (Light Emitting Diode)
   • Tester (as verified in Activity 11.1)
   • Lemon juice or vinegar (for testing)
2. Replacing the Bulb with an LED:
   • Remove the bulb from the tester and replace it with an LED.
   • Note the LED leads:
     • The longer lead of the LED must be connected to the positive terminal of the battery.
     • The shorter lead should be connected to the negative terminal of the battery.
3. Testing Conductivity:
   • Dip the ends of the tester (now equipped with an LED) into the liquid (lemon juice or vinegar) without letting the ends touch each other (see Fig. 11.2).
   • Observe the LED:
     • If the LED glows, it indicates that the liquid conducts electricity, even if the current is weak.
     • If the LED does not glow, the liquid might not be a good conductor or there is no current flowing through the circuit.

• LED Sensitivity:
  • Unlike a regular bulb, an LED can glow with a weaker current, making it more effective in detecting weakly conducting liquids like lemon juice or vinegar.
  • The LED will provide better results in cases where the current is too weak to light up a normal bulb.
• Connecting the LED:
  • Always ensure the longer lead is connected to the positive terminal, and the shorter lead to the negative terminal, or else the LED will not function.

• LED in a Circuit: An LED requires very little current to glow, making it ideal for testing liquids that are poor or moderate conductors of electricity.
• This setup enhances the sensitivity of the tester, allowing you to detect the presence of electric current more easily in weakly conducting liquids.

Activity 11.3: Investigating the Conductivity of Liquids Using a Magnetic Tester

Objective: To test the conductivity of various liquids by observing the deflection of a compass needle when electric current flows through the liquid.

Materials Needed:

• Tray from a discarded matchbox
• Compass needle
• Electric wire
• Battery
• Various liquids (lemon juice, vinegar, tap water, vegetable oil, milk, honey, etc.)

1. Prepare the Tester:
   • Take the tray from a matchbox and wrap an electric wire around it a few times.
   • Place a compass needle inside the tray.
   • Connect one free end of the wire to one terminal of a battery.
   • Take another wire and connect it to the other terminal of the battery.
2. Test the Compass Deflection:
   • Join the free ends of the two wires momentarily.
   • Observe if the compass needle deflects (indicating that electric current is flowing through the wire).
3. Test Different Liquids:
   • Using the tester, dip the free ends of the wire into each liquid (lemon juice, vinegar, tap water, vegetable oil, milk, honey, etc.).
   • Observe the deflection of the compass needle.
   • After each test, remove the ends from the liquid, wipe them dry, and repeat with the next liquid.
4. Record Observations:
   • For each liquid, note if the compass needle shows deflection and classify the liquid as either a good conductor or a poor conductor.

• Good Conductors: Lemon juice, vinegar, tap water, milk, and salt water allow electric current to pass through, causing deflection in the compass needle.
• Poor Conductors: Vegetable oil, honey, distilled water, and sugar solution do not cause deflection, indicating that they are poor conductors of electricity.

Conductivity of Air and Distilled Water

Boojho and Paheli are curious about how materials typically classified as poor conductors, like air and distilled water, may still allow electricity to pass under certain conditions.

• Air is usually a poor conductor of electricity, but during lightning, electric currents can pass through air. This indicates that while air is generally a poor conductor, it can conduct electricity under high-voltage conditions, such as in storms.
• Distilled water is another example. It is often classified as a poor conductor of electricity because it lacks dissolved ions, which are necessary for conduction. However, tap water conducts electricity due to the presence of dissolved salts and minerals.

1. Set up your tester with a battery and wires, similar to what was done in Activity 11.3.
2. Dip the free ends of the tester into distilled water.
3. Observe the deflection of the compass needle (if any) or the glow of an LED or bulb.

• Distilled water does not allow electricity to pass through because it lacks ions. Hence, the bulb or LED does not glow, and the compass needle does not deflect.

Conclusion: Most materials can conduct electricity under certain conditions. Therefore, classifying materials as good conductors or poor conductors is more appropriate than labeling them simply as conductors or insulators. Even materials like air and distilled water, usually poor conductors, can allow the passage of electric current in specific situations.

Activity 11.4 : Testing the Conductivity of Distilled Water and Salt Solution

Steps:

1. Test Distilled Water:
   • Take about two teaspoonfuls of distilled water in a clean plastic or rubber cap.
   • Use a tester (as described in previous activities) to check whether distilled water conducts electricity.
   • Observation: The bulb or LED does not glow and the compass needle does not show deflection, indicating that distilled water does not conduct electricity.
2. Dissolve Salt in Distilled Water:
   • Add a pinch of common salt to the distilled water and stir to create a salt solution.
   • Use the same tester to check the conductivity of the salt solution.
   • Observation: The bulb or LED glows or the compass needle deflects, indicating that the salt solution conducts electricity.

• Distilled water alone is a poor conductor of electricity because it lacks dissolved ions.
• Salt solution conducts electricity because the dissolved salt provides ions (charged particles) which help to carry the electric current.
• This explains why water from natural sources like taps, wells, and ponds (which contain dissolved salts) is a good conductor of electricity.

• Natural mineral salts in water are important for human health but also make water a good conductor of electricity. Therefore, it is dangerous to handle electrical appliances with wet hands or while standing on a wet surface, as the water can conduct electricity and cause electric shocks.

Activity 11.5 : Testing the Conductivity of Different Solutions

Materials Required:

• Three clean plastic or rubber caps
• Distilled water
• Lemon juice or dilute hydrochloric acid (acid)
• Caustic soda or potassium iodide (base)
• Sugar
• Tester (with LED or compass needle)

1. Prepare Three Solutions:
   • Pour two teaspoonfuls of distilled water into each of the three caps.
   • In the first cap, add a few drops of lemon juice or dilute hydrochloric acid (an acid).
   • In the second cap, add a few drops of a base like caustic soda or potassium iodide.
   • In the third cap, dissolve a little sugar in distilled water.
2. Test Conductivity:
   • Use the tester to check each of the solutions:
     • Dip the free ends of the tester into each solution one by one.
     • Observe whether the bulb glows or the compass needle shows deflection.
3. Observations:
   • Lemon juice or dilute hydrochloric acid (acid) solution:
     • The bulb glows or compass needle deflects, indicating it conducts electricity.
   • Caustic soda or potassium iodide (base) solution:
     • The bulb glows or compass needle deflects, indicating it conducts electricity.
   • Sugar solution:
     • The bulb does not glow and the compass needle does not deflect, indicating that the sugar solution does not conduct electricity.

• Solutions of acids and bases conduct electricity because they produce ions in water, which can carry electric current.
• Sugar solution does not conduct electricity because sugar molecules do not break into ions when dissolved in water.

• Most liquids that conduct electricity are solutions of acids, bases, or salts because they dissociate into ions in water, allowing current to pass through the solution.

11.2 - Chemical Effects of Electric Current

Electric current can produce various effects, including heating, magnetic, and chemical effects. In this section, we will focus on the chemical effects of electric current when it flows through a conducting solution.

Activity 11.6: Observing Chemical Changes

Materials Required:

• Carbon rods from discarded cells (or two iron nails)
• Sandpaper
• Copper wires
• Battery
• Glass or plastic bowl
• Cupful of water
• Salt or lemon juice (to enhance conductivity)

1. Prepare the Electrodes:
   • Carefully remove the carbon rods from two discarded cells.
   • Clean the metal caps of the carbon rods with sandpaper to ensure good electrical contact.
   • Wrap copper wires around the metal caps of the carbon rods.
2. Set Up the Circuit:
   • Connect the other ends of the copper wires to a battery.
   • These rods will act as electrodes.
3. Prepare the Conducting Solution:
   • Pour a cupful of water into the glass or plastic bowl.
   • Add a teaspoonful of salt or a few drops of lemon juice to the water to make it more conductive.
4. Immerse the Electrodes:
   • Carefully immerse the electrodes in the prepared solution, ensuring that the metal caps remain outside the water.
5. Observe:
   • Wait for 3-4 minutes.
   • Carefully observe the electrodes for any visible changes.

• Gas Bubbles:
  • You should notice gas bubbles forming near one or both of the electrodes. This indicates that a chemical reaction is occurring due to the passage of electric current.
• Chemical Changes:
  • The formation of gas bubbles and any change in the color of the solution or the deposition of metals on the electrodes indicates that a chemical change is taking place.

• The passage of electric current through the conducting solution causes chemical reactions. The type of gas produced and the nature of the chemical change will depend on the solution and the electrodes used.
• For example:
  • In a saltwater solution, hydrogen gas may be released at the cathode (negative electrode), while chlorine gas may be released at the anode (positive electrode).
• Changes in color or the deposition of metals on the electrodes can also occur, indicating a chemical reaction.

• The chemical effects of electric current demonstrate that electric current can drive chemical reactions in solutions, leading to the formation of gases, deposition of materials, or changes in the solution’s color.
• These observations provide clear evidence of the chemical changes that occur as a result of the electric current passing through a conducting solution.

Historical Context: Electrolysis and Water

In 1800, the British chemist William Nicholson made a significant discovery in the field of electrochemistry. He demonstrated that when electrodes are immersed in water and an electric current is passed through the setup, bubbles of gases are produced. This process is known as electrolysis.

Key Points of Nicholson's Experiment:

1. Electrodes and Electric Current:
   • Two electrodes are placed in water.
   • When an electric current is applied, it travels through the water.
2. Gas Production:
   • Oxygen Bubbles:
     • Form on the electrode connected to the positive terminal of the battery (anode).
   • Hydrogen Bubbles:
     • Form on the electrode connected to the negative terminal of the battery (cathode).
3. Chemical Reactions:
   • The electrolysis of water breaks down the water molecules (H₂O) into hydrogen and oxygen gases: $$2H_{2}O\to 2H_2(g)+O_2(g)$$
   • This reaction shows that water can be decomposed into its elemental gases when subjected to an electric current.

• Foundational Work in Electrochemistry: Nicholson's experiment laid the groundwork for future research in electrolysis, leading to a better understanding of the chemical effects of electric current.
• Application in Industry: This process is essential in various industrial applications, including the production of hydrogen gas and the purification of metals.

Discovery of Chemical Effects in Fruits and Vegetables

Boojho’s experiment with the potato serves as a fascinating example of how unexpected discoveries can lead to new insights in science. Here's a summary of his findings:

Experiment Overview:

1. Objective:
   • Boojho aimed to test whether fruits and vegetables, specifically a potato, conduct electricity.
2. Method:
   • He cut a potato in half and inserted the copper wires of a tester into it.
   • One wire was connected to the positive terminal of the battery, and the other wire was connected to the negative terminal.
3. Observation:
   • After returning to the experiment half an hour later, Boojho noticed a greenish-blue spot around the wire connected to the positive terminal.
   • No such spot was observed around the wire connected to the negative terminal.
4. Conclusion:
   • The greenish-blue spot indicated a chemical reaction occurring in the potato due to the passage of electric current, specifically around the positive terminal.
   • This effect could be attributed to the copper ions reacting with the substances in the potato, leading to a visible change.

• Chemical Effects of Electric Current:
  • The experiment highlighted the chemical effects of electric current on organic materials, showing that fruits and vegetables can conduct electricity and undergo chemical changes when an electric current passes through them.
• Identification of Battery Terminals:
  • Boojho and Paheli realized that this method could be used to identify the positive terminal of a cell or battery concealed in a box, offering a practical application for their discovery.
• Unexpected Findings in Science:
  • Boojho’s experience illustrates a fundamental aspect of scientific inquiry: while seeking specific knowledge, one may stumble upon unexpected and valuable information. This is a common theme in the history of science, leading to significant breakthroughs.

• Inspiration for Further Experiments:
  • Boojho and Paheli’s curiosity might inspire others to explore the conductivity of different fruits and vegetables, leading to a deeper understanding of organic chemistry and electrical conductivity in biological materials.
• Educational Value:
  • This experiment serves as an engaging educational activity, demonstrating the principles of electricity, chemical reactions, and the scientific method in a hands-on way.

11.3 - Electroplating

• Definition of Electroplating:
  • Process of depositing a layer of one metal onto another.
• Common Examples:
  • Shiny bicycle handlebars and wheel rims can get scratched, exposing a dull surface underneath.
  • Gold-coated ornaments may wear off with use, revealing silver or another metal beneath.
• Purpose:
  • To enhance appearance and protect underlying metal from corrosion or wear.
• Curiosity:
  • The process of how one metal can be deposited on another metal prompts experimentation.

Activity 11.7 : Electroplating

• Materials Needed:
  • Copper sulfate
  • Two copper plates (10 cm × 4 cm)
  • 250 mL distilled water
  • Dilute sulfuric acid
  • Battery
• Procedure:
  1. Dissolve 2 teaspoonfuls of copper sulfate in 250 mL of distilled water.
  2. Add a few drops of dilute sulfuric acid to enhance conductivity.
  3. Clean copper plates with sandpaper, rinse, and dry them.
  4. Connect the copper plates to the battery terminals.
  5. Immerse the plates in the copper sulfate solution.
  6. Allow the current to pass for 15 minutes.
  7. Remove electrodes and observe any differences. Look for coatings and note the battery terminal connection.
• Observations:
  • One electrode will have a coating, usually with a distinctive color.
  • Interchanging electrodes may result in different coatings on each plate.
• Chemical Reaction:
  • Copper sulfate dissociates into copper and sulfate ions.
  • Free copper is deposited on the electrode connected to the negative terminal.
  • Equal amounts of copper dissolve from the electrode connected to the positive terminal, maintaining the solution's copper content.

Boojho's Variation:

• Used a carbon rod in place of one copper plate connected to the negative terminal, successfully obtaining a copper coating on the carbon rod.

NCERT Science Notes - Class 8 | Science | Chapter 11 - Chemical Effects of Electric Current

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