Chapter 11: Chemical Effects of Electric Current

11.0 Introduction to Electric Conduction

Safety Warning: Do not touch electrical appliances with wet hands as water can conduct electricity.
Good Conductors: Materials that allow electric current to pass through them (e.g., metals like copper, aluminium).
Poor Conductors (Insulators): Materials that do not allow electric current to pass through them easily (e.g., rubber, plastic, wood).
Previous Learning: In Class VI, a tester was used to check conduction in solid materials.
Current Focus: Investigating whether liquids also conduct electricity.
Important Caution: Always use only electric cells for activities; do not experiment with mains supply, generator, or inverter.

11.1 Do Liquids Conduct Electricity?

11.1.1 Testing Setup and Checking the Tester

Tester Modification: For liquids, replace the single cell in a standard tester with a battery.
Tester Check (Activity 11.1): Join the free ends of the tester for a moment; the bulb should glow, completing the circuit.
Troubleshooting Non-glowing Bulb: Check for loose connections, a fused bulb, or used-up cells; ensure all connections are tight.
Bulb Replacement: If connections are tight, replace the bulb.
Cell Replacement: If the bulb still doesn't work, replace the cells with fresh ones.
Caution for Tester Check: Do not join free ends for more than a few seconds to avoid quickly draining the battery cells.

11.1.2 Testing Liquids with a Bulb-based Tester (Activity 11.2)

Procedure: Collect plastic/rubber caps, pour a teaspoon of liquid (e.g., lemon juice or vinegar).
Testing Method: Dip the free ends of the tester into the liquid, ensuring they are not more than 1 cm apart but do not touch each other.
Observation: Note if the tester bulb glows, indicating if the liquid conducts electricity.
Conduction Mechanism: If the liquid conducts, the circuit completes, current flows, and the bulb glows.
Non-conduction Mechanism: If the liquid does not conduct, the circuit remains incomplete, and the bulb does not glow.
Weak Current Issue: Sometimes, even if a liquid conducts, the current might be too weak to heat the bulb filament sufficiently to glow.
Solution for Weak Current Detection: Use an LED (Light Emitting Diode) instead of a bulb, as LEDs glow even with a weak current.
LED Connection: The longer lead of an LED connects to the positive terminal, and the shorter lead connects to the negative terminal of the battery.

11.1.3 Testing Liquids with a Magnetic Effect Tester (Activity 11.3)

Alternative Tester Principle: Electric current produces a magnetic effect; even a small current can cause a compass needle to deflect.
Magnetic Tester Construction: Wrap an electric wire around a matchbox tray, place a small compass needle inside, and connect one end to a battery terminal; connect another wire from the other battery terminal, leaving both free ends.
Tester Check: Momentarily join the free ends; the compass needle should show deflection.
Procedure: Repeat Activity 11.2 using the magnetic tester. Dip free ends into lemon juice and observe compass deflection.
Cleaning Protocol: After testing each liquid (e.g., tap water, vegetable oil, milk, honey), wash and wipe dry the tester ends.
Conclusion: Some liquids are good conductors (cause deflection), while others are poor conductors (no deflection).
Conductors vs. Insulators: Most materials can conduct electricity under certain conditions, making it more accurate to classify them as good or poor conductors rather than strict conductors and insulators.

11.1.4 Conduction through Distilled Water vs. Tap Water (Activity 11.4)

Distilled Water Test: Distilled water, being free of salts, is a poor conductor of electricity and will not cause a tester bulb to glow or a compass to deflect.
Salt Solution Test: Dissolving a pinch of common salt in distilled water makes it a good conductor of electricity.
Tap Water Conduction: Water from taps, hand pumps, wells, and ponds contains dissolved mineral salts, making it a good conductor of electricity.
Health Aspect: Small amounts of naturally present mineral salts in water are beneficial for human health.
Safety Precaution: Due to mineral salts, water is a good conductor; therefore, never handle electrical appliances with wet hands or while standing on a wet floor.

11.1.5 Conduction by Acids, Bases, and Salts (Activity 11.5)

Acidic Solutions: Distilled water with a few drops of lemon juice or dilute hydrochloric acid conducts electricity.
Basic Solutions: Distilled water with a few drops of a base (e.g., caustic soda or potassium iodide) conducts electricity.
Sugar Solution: Distilled water with dissolved sugar does not conduct electricity.
General Conclusion: Most liquids that conduct electricity are solutions of acids, bases, and salts.
Supervision: Activities involving acids require supervision from a teacher, parent, or elderly person.

11.2 Chemical Effects of Electric Current

11.2.1 Introduction to Chemical Effects

Review: Electric current produces various effects, including heating and magnetic effects (recalled from Class VII).
New Inquiry: Investigate the effects produced when an electric current flows through a conducting solution.

11.2.2 Electrolysis of Water (Activity 11.6)

Electrode Preparation: Carefully remove carbon rods from discarded cells, clean metal caps with sandpaper, and wrap copper wires around them. These rods are called electrodes.
Setup: Connect electrodes to a battery and immerse them in a glass/plastic bowl containing water with a teaspoon of salt or a few drops of lemon juice (to enhance conductivity). Ensure metal caps are above water.
Observation: After 3-4 minutes, gas bubbles are observed near the electrodes.
Chemical Change: The formation of gas bubbles indicates a chemical change taking place in the solution.
Historical Context: In 1800, William Nicholson showed that passing current through water produced oxygen bubbles on the positive electrode and hydrogen bubbles on the negative electrode.
Definition of Chemical Effects: The passage of electric current through a conducting solution causes chemical reactions, resulting in effects like gas bubble formation, metal deposits on electrodes, or color changes in the solution. These are known as chemical effects of electric current.

11.2.3 Potato Tester for Identifying Positive Terminal

Boojho's Discovery: Inserting a tester's copper wires into a potato revealed a greenish-blue spot around the wire connected to the positive terminal after some time.
Application: This observation can be used as a simple method to identify the positive terminal of a cell or battery, even if concealed.
Scientific Serendipity: This illustrates how scientific discoveries sometimes occur unexpectedly while looking for something else.

11.3 Electroplating

11.3.1 Introduction and Everyday Observations

Coated Objects: New bicycles have shiny handlebars/rims; ornaments might appear gold but wear off to reveal other metals.
Underlying Principle: These objects are coated with a layer of a different metal.
Inquiry: How can a layer of one metal be deposited on another?

11.3.2 Electroplating Copper (Activity 11.7)

Materials: Copper sulphate solution, two copper plates (electrodes), distilled water, dilute sulphuric acid, beaker.
Solution Preparation: Dissolve two teaspoonfuls of copper sulphate in 250 mL distilled water, add a few drops of dilute sulphuric acid to increase conductivity.
Electrode Preparation: Clean copper plates with sandpaper, rinse, and dry.
Setup: Connect copper plates to a battery's terminals and immerse them in the copper sulphate solution.
Procedure: Pass electric current for about 15 minutes.
Observation: Remove electrodes and observe; one electrode will have a coating, typically copper-colored, deposited on it. This deposition occurs on the electrode connected to the negative terminal.
Mechanism of Electroplating Copper: When current passes, copper sulphate (CuSO₄) dissociates into copper (Cu²⁺) and sulphate (SO₄^2-) ions.
Negative Electrode: Free copper ions (Cu²⁺) are attracted to the negatively charged electrode and get deposited as copper metal.
Positive Electrode: An equal amount of copper from the positive copper plate dissolves into the solution (as Cu²⁺ ions), replacing the copper lost from the solution.
Overall Process: This continuous process effectively transfers copper from the positive electrode to the negative electrode, resulting in a coating.
Variation: Copper can also be electroplated onto a carbon rod by connecting it to the negative terminal.

11.3.3 Definition and Applications of Electroplating

Definition: Electroplating is the process of depositing a thin layer of any desired metal onto another material using electricity.
Key Application: It is one of the most common applications of the chemical effects of electric current in industry.
Purpose of Coating: The deposited metal layer provides desired properties (e.g., shiny appearance, corrosion resistance, scratch resistance) that the base metal lacks.
Chromium Plating: Used on car parts, bath taps, kitchen gas burners, bicycle handlebars, and wheel rims due to its shiny appearance, corrosion resistance, and scratch resistance. Chromium is expensive, so it's only used as a coating over cheaper metals.
Jewellery: Less expensive metals are electroplated with silver or gold to give them the appearance of precious metals at a lower cost.
Tin Cans: Iron cans are electroplated with tin. Tin is less reactive than iron, preventing food from reacting with iron and getting spoilt.
Protection Against Corrosion: Iron objects (e.g., in bridges, automobiles) are electroplated with zinc to protect them from rusting and corrosion (this process is called galvanization).
Environmental Concern: Disposal of used conducting solutions from electroplating factories is a major environmental issue due to polluting waste; specific guidelines are followed for disposal.

What You Have Learnt

Liquid Conductivity: Some liquids are good conductors of electricity, while others are poor conductors.
Conducting Liquids Composition: Most liquids that conduct electricity are solutions of acids, bases, and salts.
Chemical Effects of Current: The passage of an electric current through a conducting liquid causes chemical reactions, and the resulting changes are called chemical effects of currents.
Electroplating Definition: Electroplating is the process of depositing a layer of any desired metal on another material by means of electricity.

Keywords

Electrode: A conductor through which electricity enters or leaves a nonmetallic medium.
Electroplating: The process of depositing a layer of any desired metal on another material by means of electricity.
Good Conductor: A material that allows electric current to pass through it easily.
LED (Light Emitting Diode): A semiconductor light source that emits light when current flows through it, glowing even with weak currents.
Poor Conductor: A material that does not allow electric current to pass through it easily; an insulator.