Chemical Effects Of Electric Current Class 8 Science NCERT

Understanding Electric Current and its Effects

Electric current is the flow of electric charges. When electricity flows through a circuit, it can produce various effects. You're likely familiar with the heating effect (think of a light bulb glowing or an electric heater warming up) and the magnetic effect (like how a compass needle deflects near a current-carrying wire). However, electricity can also bring about chemical changes, especially when it passes through certain liquids. This is known as the chemical effect of electric current.

To observe these chemical effects, we first need a medium that allows electricity to flow. These materials are called conductors. Metals are excellent conductors. But what about liquids? Many liquids contain dissolved salts or acids, which provide charged particles (ions) that can move and carry current. Liquids that conduct electricity are called electrolytes. Liquids that do not conduct electricity are called non-electrolytes or insulators. Understanding this distinction is crucial to observing and utilizing the chemical effects of electric current. For instance, distilled water is a poor conductor, but adding a pinch of salt makes it a good conductor.

Good and Poor Conductors of Electricity (Focus on Liquids)

Good Conductors

Materials that allow electric current to pass through them easily. In the context of liquids, these are often solutions containing dissolved acids, bases, or salts. Examples include tap water (due to dissolved salts), lemon juice, vinegar, and solutions of copper sulphate or common salt.

Poor Conductors (Insulators)

Materials that do not allow electric current to pass through them easily. Distilled water is a poor conductor because it lacks dissolved salts. Other examples include vegetable oil, sugar solution, and pure alcohol. Even though a liquid might be a poor conductor, a sensitive tester (like one using an LED) might detect a very weak current.

The Chemical Effect: Electrolysis Explained

1. Setting up the Circuit — Imagine taking a simple electrical circuit with a battery and two carbon rods (or metal strips) connected to its terminals. These rods are called electrodes. The carbon rod connected to the positive terminal is the anode, and the one connected to the negative terminal is the cathode. Dip these electrodes into a liquid, like a solution of common salt or copper sulphate, ensuring they don't touch each other. This liquid is called the electrolyte.
2. Passing Electric Current — When you switch on the current, the electric charges start flowing through the electrolyte. Unlike in solid conductors where electrons move, in liquid conductors (electrolytes), it's the movement of positively and negatively charged particles called ions that carries the current.
3. Observing Chemical Changes — As the current passes, you'll observe chemical changes taking place at the electrodes. For example, if you use a salt solution, tiny bubbles of gas might form at both electrodes. If you use a copper sulphate solution, you might see copper metal depositing on the electrode connected to the negative terminal (cathode). These changes indicate that new substances are being formed. This process of chemical decomposition by passing electric current is called electrolysis.
4. Understanding the Reactions — The current provides the energy needed to break down the compounds in the electrolyte. Positive ions are attracted to the negative electrode (cathode), and negative ions are attracted to the positive electrode (anode). At these electrodes, the ions gain or lose electrons, undergoing chemical reactions to form neutral atoms or molecules, which then appear as gases or deposits. For instance, in copper sulphate electrolysis, copper ions (Cu²⁺) gain electrons at the cathode to form solid copper (Cu), and sulphate ions (SO₄²⁻) react at the anode.

Real-World Applications: Electroplating and Purification

The chemical effects of electric current have many practical applications that you see and use every day. The most common and important application is electroplating. Electroplating is the process of coating one metal with a thin layer of another metal using electric current.

Why do we electroplate?

1. Corrosion Protection: Metals like iron rust easily. Coating them with a non-corroding metal like chromium or nickel protects them from rust.
2. Enhance Appearance: Many objects are electroplated with shiny metals like chromium or silver to make them more attractive. Think of bicycle handlebars, car parts, bath taps, or even jewellery.
3. Improve Hardness/Durability: Chromium plating provides a hard, scratch-resistant surface.
4. Cost-Effectiveness: It's often cheaper to make an object from an inexpensive metal and then electroplate it with a more expensive, desirable metal (e.g., silver-plated cutlery).

In electroplating, the object to be plated (e.g., an iron spoon) is connected to the negative terminal of the battery (cathode), and a block of the desired coating metal (e.g., copper or silver) is connected to the positive terminal (anode). The electrolyte is a solution of the salt of the coating metal. When current passes, the coating metal ions from the solution deposit onto the object at the cathode, and the anode replenishes these ions into the solution.

Another significant application is the purification of metals. Impure metals can be refined to a high degree of purity using electrolysis, which is crucial in industries like copper and aluminium production.

Key Takeaways and Common Misconceptions

• Not all liquids conduct electricity: Distilled water is a poor conductor. Tap water conducts due to dissolved impurities.
• LED testers are more sensitive: For detecting very weak currents in poor conductors, an LED (Light Emitting Diode) based tester is better than a bulb-based tester because LEDs glow even with a very small current.
• Chemical effects involve new substances: The key characteristic of the chemical effect is the formation of new products (gases, metal deposits) at the electrodes.
• Electroplating is for protection and aesthetics: Remember the two main reasons for electroplating: to protect an object from corrosion and to give it a shiny, attractive finish.
• Polarity matters in electrolysis: The type of chemical change occurring at each electrode (anode and cathode) depends on its connection to the positive or negative terminal of the battery.

Practice Questions with Solutions

• Q: What is electroplating? Give two applications of electroplating in our daily life. A: Step 1: Define electroplating. Electroplating is the process of depositing a thin layer of one metal onto another metal object using an electric current. Step 2: Provide two applications. Two applications are: 1. Protecting metals from corrosion, e.g., chrome plating on bicycle parts to prevent rust. 2. Enhancing the appearance of objects, e.g., gold plating on jewellery to make it look attractive. Final answer: Electroplating is coating one metal with another using electricity. It is used to protect metals from corrosion and to improve their appearance.
• Q: Why is it advised to use an LED tester instead of a bulb tester to check the conductivity of some liquids? A: Step 1: Understand the difference in current requirement. A bulb tester requires a significant amount of current to glow brightly because it relies on the heating effect of current to emit light. Step 2: Consider weak conductors. Many liquids are poor conductors and allow only a very weak current to flow through them. This weak current might not be sufficient to heat the filament of a bulb enough to make it glow. Step 3: Explain LED advantage. An LED (Light Emitting Diode) is designed to glow even when a very weak electric current passes through it. Therefore, an LED tester is more sensitive and can detect the conductivity of liquids that are poor conductors, which a bulb tester might fail to detect. Final answer: An LED tester is preferred over a bulb tester because LEDs glow even with a very weak current, making them more sensitive for testing the conductivity of poor liquid conductors where a bulb might not glow.
• Q: During the electrolysis of water, what kind of gases are formed at the positive and negative terminals? Name the gases. A: Step 1: Recall the components of water. Water (H₂O) is composed of hydrogen and oxygen. Step 2: Understand attraction to terminals. During electrolysis, water molecules break down into hydrogen ions (H⁺) and hydroxide ions (OH⁻). Hydrogen ions, being positive, are attracted to the negative terminal (cathode). Hydroxide ions, being negative, are attracted to the positive terminal (anode). Step 3: Identify gases formed. At the negative terminal (cathode), hydrogen ions gain electrons to form hydrogen gas (H₂). At the positive terminal (anode), hydroxide ions lose electrons to form oxygen gas (O₂). Final answer: During the electrolysis of water, hydrogen gas (H₂) is formed at the negative terminal (cathode), and oxygen gas (O₂) is formed at the positive terminal (anode).
• Q: A student wants to electroplate an iron key with copper. Describe the setup and materials required for this activity. A: Step 1: Identify the object to be plated and the coating material. The iron key is the object to be plated, so it will act as the cathode. Copper is the coating material, so a copper plate will act as the anode. Step 2: Choose the electrolyte. The electrolyte must be a solution of the salt of the coating metal. Therefore, a copper sulphate solution should be used. Step 3: Describe the electrical connections. Connect the iron key to the negative terminal of a battery and the copper plate to the positive terminal of the battery. Place both the key and the copper plate in the copper sulphate solution, ensuring they do not touch each other. Final answer: To electroplate an iron key with copper, you would need an iron key (connected to the negative terminal), a copper plate (connected to the positive terminal), a battery, and a beaker containing copper sulphate solution. Both the key and the copper plate should be immersed in the solution without touching each other.

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