All questions of Chemical Effects of Electric Current for Class 8 Exam

Distilled water is a poor conductor of electricity because it lacks the dissolved salts and minerals that are present in natural water sources, which contribute to its conductivity.

Plastic wire is an insulator because it doesn't conduct heat.

Understanding Conductivity in Solutions
When discussing the conductivity of a solution, it’s essential to understand that conductivity depends on the presence of ions in the solution. More ions typically mean better conductivity.
Why Add Sulphuric Acid?
Sulphuric acid (H2SO4) is a strong acid that dissociates completely in water, providing a significant number of hydrogen ions (H+) and sulfate ions (SO4^2-). This increase in ion concentration enhances the solution's ability to conduct electricity.
Comparison with Other Acids
- Hydrochloric Acid (HCl): While HCl is also a strong acid and would increase conductivity, it does not provide additional sulfate ions, which can further enhance the solution's properties.
- Nitric Acid (HNO3): Similar to HCl, nitric acid is a strong acid, but it introduces nitrate ions (NO3-) instead of sulfate ions, which may not be as effective in this specific context.
- Carbonic Acid (H2CO3): This is a weak acid that does not dissociate completely in solution, leading to fewer ions and, consequently, lower conductivity compared to strong acids.
Conclusion
Thus, the correct answer is option 'B' (Sulphuric acid), as it significantly increases the number of ions in the copper sulphate solution, enhancing its conductivity. This makes sulphuric acid the best choice among the options provided.

Understanding the Electrolysis Process
In the described activity, you are performing an electrolysis experiment using copper sulphate solution. When current is passed through the solution with copper plates, specific chemical reactions occur.
Role of Electrodes
- The two electrodes (copper plates) are connected to a battery.
- The plate connected to the negative terminal acts as the cathode, while the one connected to the positive terminal is the anode.
What Happens at the Cathode?
- When current flows, copper ions (Cu²⁺) from the copper sulphate solution migrate towards the cathode (negative plate).
- At the cathode, a reduction reaction occurs where Cu²⁺ ions gain electrons and are converted into solid copper.
Result of the Reaction
- This process leads to the deposition of copper metal on the negative plate.
- Therefore, as time passes, you will observe that the cathode plate becomes thicker due to the accumulation of copper.
Conclusion
- The correct answer is option 'C': Copper gets deposited on it.
- This phenomenon illustrates the principles of electrolysis and the behavior of ions in an electrolyte solution when exposed to an electric current.

Seawater is salty or saline so it is a good conductor of electricity

Understanding LED Leads
LEDs (Light Emitting Diodes) are semiconductor devices that emit light when an electric current passes through them. They have two leads, which connect to different terminals in a circuit.
Lead Length Significance
- The longer lead of an LED is known as the anode.
- The anode is always connected to the positive terminal of the power supply.
Why the Longer Lead is Positive
- Polarity in LEDs: LEDs are polarized components, meaning they must be connected to a power source in a specific way for them to function correctly. Connecting them incorrectly can damage the LED or prevent it from lighting up.
- Current Flow Direction: Current flows from the positive terminal (anode) to the negative terminal (cathode). The longer lead being the anode ensures that the current can flow into the LED correctly, allowing it to emit light.
Identifying LED Leads
- Long Lead: Connects to the positive terminal.
- Short Lead: This is the cathode, which connects to the negative terminal.
Conclusion
Understanding the polarity of LEDs is crucial for their proper operation. By ensuring the longer lead is connected to the positive terminal, users can enjoy the full functionality of their LED devices, whether in simple circuits or more complex applications.

The activity demonstrates that pure distilled water is a poor conductor, but after dissolving common salt, the resulting salt solution conducts electricity, making it a good conductor due to the presence of ions.

There are several metals that can be electroplated onto iron to improve its properties or appearance. Some common metals used for electroplating on iron include:

1. Nickel: Electroplating iron with nickel provides a protective layer that enhances corrosion resistance, hardness, and durability. It also offers a bright and attractive finish.

2. Chromium: Chromium electroplating on iron forms a thin layer of chrome, which not only enhances corrosion resistance but also provides a shiny, reflective surface. It is often used in decorative applications.

3. Zinc: Electroplating iron with zinc, known as galvanizing, is commonly used to prevent rust and corrosion. It provides a sacrificial layer that corrodes before the iron, protecting it from environmental exposure.

4. Copper: Copper electroplating on iron can improve electrical conductivity and thermal conductivity. It is also used for decorative purposes due to its reddish-brown color.

5. Silver: Silver plating on iron is occasionally used for its antimicrobial properties, as silver possesses natural germicidal effects. It is commonly used in the food and medical industries.

These are just a few examples, and other metals like gold, tin, brass, and bronze can also be electroplated onto iron for specific purposes.

An electrolyte is:

An electrolyte is a substance that conducts electricity when dissolved in water or melted. It is essential for various biological processes and is also used in many industrial applications.

Explanation:

1. Definition:
An electrolyte is a substance that contains free ions and can conduct electric current. It can be in the form of a solid, liquid, or solution. When an electrolyte dissolves in water or melts, its ions become mobile and are able to carry electric charge.

2. Conductivity of Electrolytes:
Electrolytes conduct electric current because they contain ions that are free to move. When a potential difference is applied across an electrolyte, the positive ions move towards the negative electrode (cathode) and the negative ions move towards the positive electrode (anode). This movement of ions constitutes the flow of electric current.

3. Types of Electrolytes:
There are two types of electrolytes:

- Strong Electrolytes: These electrolytes completely dissociate into ions when dissolved in water or melted. Examples include sodium chloride (NaCl), potassium hydroxide (KOH), and sulfuric acid (H2SO4).

- Weak Electrolytes: These electrolytes only partially dissociate into ions when dissolved in water or melted. Examples include acetic acid (CH3COOH) and ammonia (NH3).

4. Applications of Electrolytes:
Electrolytes have numerous applications in various fields, some of which include:

- Batteries: Electrolytes are used in batteries to facilitate the flow of ions between the electrodes and generate electrical energy.

- Electroplating: Electrolytes are used in electroplating processes to deposit a layer of metal onto a surface.

- Electrolysis: Electrolytes are used in electrolysis processes to separate compounds into their constituent elements.

- Medical Applications: Electrolytes are essential for the proper functioning of the human body. They help maintain fluid balance, nerve function, and muscle contraction.

Conclusion:
An electrolyte is a substance that conducts electric current when dissolved in water or melted. It is essential for various biological processes and has numerous industrial applications. Understanding the properties and behavior of electrolytes is crucial in various scientific and industrial fields.

When testing various liquids with the tester, ensure that the ends of the tester do not touch each other to avoid short-circuiting and to accurately test the conductivity of the liquid.

If the bulb does not glow when the circuit is completed, the first thing to check is the tightness of all connections. Loose connections can prevent the circuit from being completed, causing the bulb to not glow.

Understanding Electrical Safety
Handling electrical appliances with wet hands or while standing on a wet floor poses significant risks due to the properties of water and its interaction with electricity. Here’s a detailed explanation:
Why Water is Dangerous
- Water as a Conductor: Water, especially when it contains impurities (like minerals and salts), is a good conductor of electricity. This means that when your hands are wet, they can easily transmit electric current from an appliance to your body.
- Increased Risk of Electric Shock: If you touch an electrical appliance with wet hands, the moisture can create a pathway for the electric current to flow through your body. This may lead to electric shock, which can cause serious injury or even be fatal.
Impact of Wet Floors
- Ground Conductivity: Standing on a wet floor also increases the risk of electric shock. Wet surfaces can conduct electricity, which means that if an appliance malfunctions or has a fault, the current could easily travel through the water on the floor and reach you.
- Loss of Control: Wet conditions can make it difficult to maintain grip and balance, increasing the likelihood of accidents or falls when handling electrical devices.
Precautions to Take
- Avoid Wet Conditions: Always ensure your hands and the surrounding area are dry before using electrical appliances.
- Use Insulated Equipment: Whenever possible, use appliances designed for wet conditions, which have safety features to reduce the risk of electric shock.
In summary, handling electrical appliances with wet hands or on wet surfaces is dangerous due to the conductive nature of water, significantly increasing the risk of electric shock. Always prioritize safety to prevent accidents.

Understanding the Use of LED in Detecting Weak Currents
When it comes to detecting weak currents, the choice of the indicator plays a crucial role. Using an LED instead of a traditional bulb is an effective method for several reasons.
Why Choose LED?
- Lower Power Requirement:
- LEDs require much less current to illuminate compared to incandescent bulbs. This makes them sensitive to low current levels, allowing them to light up even with weak currents.
- Higher Sensitivity:
- Due to their design, LEDs can respond to lower voltage and current. They effectively indicate the presence of a current that might not be sufficient to activate a conventional bulb.
Drawbacks of Other Options:
- Thicker Wire:
- Using thicker wire does not enhance the ability to detect weak currents; it merely reduces resistance for stronger currents.
- Increased Battery Voltage:
- Raising the voltage could potentially damage sensitive components, including LEDs, and does not address the detection of weak currents.
- Larger Compass Needle:
- While a larger compass needle can indicate magnetic fields, it is not designed for direct current measurement and may not provide clear visibility of weak current levels.
Conclusion
Using an LED as a current tester is advantageous due to its efficiency and sensitivity in low current scenarios. This makes it an ideal choice for detecting weak currents in various applications.

When electric current passes through a bulb, the filament of the bulb heats up to a high temperature. This is called heating effect of current. The temperature is so high that it starts glowing. 

However, if the current through a circuit is weak, the filament does not get heated up sufficiently and it does not glow. 

Explanation:
Electroplating is a process of depositing a thin layer of metal on an object by using electricity. It is based on the chemical effect of electricity. The process involves the following steps:

1. Preparation of the object: The object to be plated is first cleaned and polished to remove any impurities or dirt.

2. Preparation of the electrolyte: An electrolyte is a solution of a salt of the metal to be deposited. It is prepared by dissolving the salt in water.

3. Setting up the electroplating cell: The electroplating cell consists of the object to be plated, the electrolyte solution, and a source of direct current (DC) electricity. The object to be plated is connected to the negative terminal of the DC power source, while the metal to be deposited is connected to the positive terminal.

4. Plating process: When the DC current is passed through the electrolyte solution, the metal ions in the solution are attracted to the object to be plated. These metal ions then react with the object's surface, forming a thin layer of metal on it.

5. Finishing: Once the plating process is complete, the object is removed from the electrolyte solution and washed to remove any excess electrolyte or impurities.

Conclusion:
The chemical effect of electricity is what makes electroplating possible. It involves the use of an electrolyte solution, a source of direct current (DC) electricity, and the attraction of metal ions to the object to be plated. The result is a thin layer of metal deposited on the object, which can improve its appearance, durability, and resistance to corrosion.

This activity uses the magnetic effect of current. When the tester’s free ends are dipped into a conducting liquid like lemon juice, current flows, producing a magnetic field that causes the compass needle to deflect, even if the current is weak.

Understanding the Electrolysis Process
In electrolysis, an electric current is passed through a solution or molten substance, causing it to decompose into its constituent elements. The electrodes play a crucial role in this process.
Electrodes and Their Functions
- Anode: The positive electrode where oxidation occurs.
- Cathode: The negative electrode where reduction occurs.
Gas Formation at the Cathode
When a battery is connected to an electrolyte solution, during electrolysis:
- At the Cathode: Reduction takes place. If the electrolyte is a solution of water or an acid, hydrogen ions (H+) from the solution gain electrons and are reduced to form hydrogen gas (H2).
- Chemical Reaction: The reaction at the cathode can be simplified as:
- 2H+ + 2e- → H2 (gas)
Why Hydrogen Gas is Formed
- Electrolyte Type: In most common electrolytes (like water with a small amount of acid), hydrogen ions are readily available.
- Electrode Reaction: The cathode attracts positive ions, leading to the production of hydrogen gas through the reduction of hydrogen ions.
Conclusion
Thus, when a battery powers the electrolysis process, the gas formed at the electrode connected to the negative terminal (cathode) is indeed hydrogen gas (option B).

Why Use Tin for Food Storage Cans?
Tin cans are a common choice for food storage due to their protective properties. The process of electroplating tin onto iron serves specific purposes that enhance the longevity and safety of canned foods.
Less Reactivity of Tin
- Corrosion Resistance: Tin is significantly less reactive than iron. This characteristic helps prevent the can from rusting and reacting with acidic foods, which could compromise food safety and flavor.
- Food Safety: By using tin, the likelihood of harmful reactions between the can material and the food is minimized. This is particularly important for preserving the nutritional quality of the food.
Strength and Durability
- Structural Integrity: While iron provides strength to the can, tin acts as a barrier. The combination allows for a durable container that withstands handling and storage conditions.
Cost Considerations
- Cost-Effectiveness: Although tin may not be cheaper than iron, its use in small amounts as a coating allows manufacturers to keep costs down while providing a safe storage solution.
Conclusion
In summary, the choice of tin electroplated onto iron for food storage cans is primarily due to the lower reactivity of tin. This ensures that the food remains safe, fresh, and free from undesirable reactions, making it an optimal choice for food preservation.

Understanding Mineral Salts in Natural Water
The presence of mineral salts in natural water significantly influences its properties, particularly conductivity.
1. What are Mineral Salts?
- Mineral salts are inorganic compounds that dissolve in water to form ions.
- Common examples include sodium, potassium, calcium, magnesium, chloride, sulfate, and bicarbonate.
2. Contribution to Conductivity
- Conductivity refers to the ability of water to conduct electric current.
- When mineral salts dissolve in water, they dissociate into positively and negatively charged ions.
3. Mechanism of Conductivity
- The free movement of these ions allows the water to carry electric current effectively.
- Higher concentrations of dissolved salts result in increased ion availability, thus enhancing conductivity.
4. Comparison with Other Options
- Increased Viscosity (Option A): Mineral salts generally do not increase viscosity; in fact, they can lead to a slight decrease in viscosity due to the presence of ions.
- Decreased Temperature (Option C): The presence of mineral salts does not inherently decrease temperature; in some cases, it may slightly increase it due to exothermic dissolution processes.
- Lowered pH (Option D): While certain mineral salts can affect pH, not all salts lower pH. The overall impact on pH depends on the specific ions present.
5. Conclusion
- The primary contribution of mineral salts in natural water is to enhance its conductivity, making option 'B' the correct answer. This conductivity is essential for various ecological processes and the overall health of aquatic environments.

False. When salt is dissolved in distilled water, it creates an electrolyte solution and does conduct electricity. The ions in the salt dissolve in water and become mobile, allowing the solution to conduct electricity.

Understanding Conductivity in Liquids
When testing the conductivity of liquids like lemon juice or vinegar using a simple circuit with a tester and a bulb, the key concept is the presence of ions in the liquid.
What is Electrical Conductivity?
- Electrical conductivity refers to the ability of a substance to allow the flow of electric current through it.
- Liquids that contain ions (charged particles) can conduct electricity well.
Lemon Juice and Vinegar as Conductors
- Lemon juice and vinegar are acidic solutions, which means they contain a significant amount of ions (H+ ions).
- These ions facilitate the movement of electric charge, allowing current to flow.
Process of the Experiment
- When you pour lemon juice or vinegar into the cap and dip the tester's bulb into the liquid, you are completing a circuit.
- If the liquid is a good conductor, it allows electricity to flow through it.
Why the Bulb Glows Brightly
- In this scenario, the bulb glows brightly because the electric current is flowing freely through the lemon juice or vinegar.
- The high concentration of ions means that there is little resistance to the flow of electricity, resulting in a bright glow.
Conclusion
- Therefore, when the liquid is a good conductor like lemon juice or vinegar, the bulb will glow brightly, indicating effective conductivity.
- This experiment illustrates the principle of how certain liquids can conduct electricity based on their ionic content.

Distilled water which is a pure form of water, is neither acidic nor basic in nature. So distilled water does not dissociate into ions. Since, conduction of electricity requires free ions so, distilled water does not conduct electricity.