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

As distilled water contains no minerals it is an insulator. when the contains minerals, it will have iron in it ( which is a conductor of electricity )

All of the above

When electricity is passed through tap water, the gas which evolves is a combination of hydrogen (H2) and oxygen (O2). This can be explained by the process of electrolysis.

Explanation:

1. Electrolysis:
- Electrolysis is a chemical process that uses an electric current to drive a non-spontaneous chemical reaction.
- It involves the decomposition of a compound into its constituent elements using electrical energy.

2. Tap Water Composition:
- Tap water is a mixture of various substances, including water (H2O) and dissolved minerals and gases.
- It contains impurities such as dissolved salts and gases, which make it a good conductor of electricity.

3. Electrolysis of Tap Water:
- When electricity is passed through tap water, electrolysis occurs at the electrodes.
- The positive electrode (anode) attracts negatively charged ions (anions), while the negative electrode (cathode) attracts positively charged ions (cations).

4. Anode Reactions:
- At the anode, the negatively charged ions (anions) give up their electrons and undergo oxidation.
- In tap water, the most common anions are chloride ions (Cl-) and hydroxide ions (OH-).
- The chloride ions (Cl-) undergo oxidation and form chlorine gas (Cl2) according to the following reaction: 2Cl- → Cl2 + 2e-

5. Cathode Reactions:
- At the cathode, the positively charged ions (cations) gain electrons and undergo reduction.
- In tap water, the most common cations are hydrogen ions (H+) and metal ions (e.g., sodium or potassium).
- The hydrogen ions (H+) undergo reduction and form hydrogen gas (H2) according to the following reaction: 2H+ + 2e- → H2

6. Overall Reaction:
- The overall reaction during the electrolysis of tap water can be represented as follows:
2H2O → 2H2 + O2

Conclusion:
When electricity is passed through tap water during electrolysis, the gas evolved is a combination of hydrogen (H2) and oxygen (O2). This is because the hydrogen ions (H+) are reduced at the cathode, forming hydrogen gas (H2), while the chloride ions (Cl-) are oxidized at the anode, forming chlorine gas (Cl2). However, since chlorine gas is highly reactive and can be hazardous, it is typically not observed in significant quantities during the electrolysis of tap water.

Explanation:

Electroplating is a process in which a metal object is coated with a layer of another metal using electricity. The metal object on which electroplating is to be done is called the cathode.

The cathode is the correct answer.

Reason:

1. In electroplating, the metal object to be plated is connected to the negative terminal of the power supply. This is done to ensure that the metal object attracts the positively charged metal ions from the electrolyte solution.

2. The cathode is the electrode where reduction occurs. Reduction is the process of gaining electrons, and in electroplating, it is the process of depositing a layer of metal onto the object.

3. The anode, on the other hand, is connected to the positive terminal of the power supply. It is the electrode where oxidation occurs. Oxidation is the process of losing electrons, and in electroplating, it is the process of metal ions dissolving from the anode into the electrolyte solution.

4. The electrolyte solution contains metal ions of the metal to be plated. These metal ions are attracted to the cathode (the metal object) and are reduced to form a layer of metal on its surface.

5. The solution acts as a medium for the transfer of metal ions between the anode and the cathode.

6. Therefore, the metal object on which electroplating is to be done should be the cathode, as it is the electrode where reduction occurs and the metal ions are deposited.

In summary, during electroplating, the metal object on which electroplating is to be done should be the cathode.

Introduction:
When a solute salt is added to water, it affects the electrical conductivity of the solution. Electrical conductivity is a measure of a material's ability to conduct an electric current. In the case of a salt-water solution, the presence of dissolved ions allows the solution to conduct electricity.

Explanation:
1. What happens when salt is added to water?
- When salt (solute) is added to water (solvent), it dissolves and dissociates into its constituent ions, such as sodium (Na+) and chloride (Cl-) ions.
- The water molecules surround and separate these ions, preventing them from recombining and forming salt crystals.

2. How does the presence of dissolved ions affect electrical conductivity?
- Electrical conductivity depends on the presence of charged particles that can move freely. In a salt-water solution, the dissolved ions can move freely in the solution.
- When an electric potential is applied across the solution, the ions are attracted or repelled by the electric field, resulting in the flow of electric current.
- The more ions present in the solution, the higher the electrical conductivity.

3. Why does adding salt to water increase its electrical conductivity?
- When salt is added to water, it increases the concentration of dissolved ions in the solution.
- The increased concentration of ions provides more charged particles that can move freely, enhancing the ability of the solution to conduct electricity.
- Therefore, adding solute salt to water increases its electrical conductivity.

Conclusion:
When solute salt is added to water, the electrical conductivity of the solution increases. This is due to the presence of dissolved ions, which can move freely and carry electric current. The increased concentration of ions in the solution enhances its ability to conduct electricity.

Answer:

To protect iron from rusting and make it shine, electroplating with chromium is the most suitable option. Here's why:

1. Rust Protection:
- Iron is highly susceptible to rusting when exposed to moisture and oxygen in the air.
- Chromium is a corrosion-resistant metal that forms a protective layer on the surface of iron when electroplated.
- This layer of chromium oxide acts as a barrier, preventing the iron underneath from coming into contact with moisture and oxygen, thus inhibiting rust formation.

2. Durability and Longevity:
- Chromium is known for its excellent durability and resistance to wear and tear.
- When electroplated onto iron, chromium forms a hard and durable surface, providing long-lasting protection against rust and corrosion.
- This ensures that the iron remains protected and maintains its shine for an extended period of time.

3. Aesthetic Appeal:
- Chromium electroplating gives iron a lustrous and shiny appearance, enhancing its aesthetic appeal.
- The reflective surface of chromium adds a visually appealing and elegant touch to iron objects, making them more attractive.

4. Versatility:
- Chromium electroplating can be done on various iron objects, including household items, automotive parts, and industrial equipment.
- It can be applied to both small and large iron surfaces, making it a versatile option for protecting and enhancing the appearance of iron objects of different sizes.

5. Easy Maintenance:
- Chromium electroplated iron surfaces are relatively easy to clean and maintain.
- The smooth and non-porous nature of chromium makes it resistant to staining and allows for effortless removal of dirt and grime, keeping the iron surface looking shiny and clean.

In summary, electroplating iron with chromium not only provides effective protection against rust but also adds a shiny and aesthetically appealing finish. It is a durable and versatile option that enhances the longevity and visual appeal of iron objects.

Understanding Electrolysis
Electrolysis is a chemical process that involves the movement of ions in an electrolyte solution, driven by an electric current. This process is fundamental in various applications, such as metal plating.
Battery Terminals
A battery has two terminals: the positive terminal (anode) and the negative terminal (cathode). The behavior of ions during electrolysis is influenced by these terminals.
Why Copper Deposits on the Negative Terminal
- Electron Flow: When an electric current is passed through the electrolyte, electrons flow from the negative terminal (cathode) to the positive terminal (anode).
- Reduction Reaction: At the cathode, where the electrons are supplied, copper ions (Cu²⁺) in the solution gain electrons. This reduction reaction causes the copper ions to convert into solid copper metal.
- Deposition Process: The solid copper metal then deposits onto the plate connected to the negative terminal. This is why you observe copper build-up on the cathode.
Conclusion
In summary, copper gets deposited on the plate connected to the negative terminal of the battery due to the reduction reaction facilitated by the flow of electrons. This fundamental principle of electrolysis highlights the significance of terminal polarity in the deposition of metals.

Electrolysis
Electrolysis is the process of using an electric current to drive a non-spontaneous chemical reaction. It involves the decomposition of an electrolyte, which is a substance that conducts electricity when dissolved in a solvent or melted. The electrolyte is typically an ionic compound, such as a salt or an acid.

Process of Electrolysis
During electrolysis, an electric current is passed through an electrolyte using electrodes. The electrodes are usually made of a conducting material, such as metal, and are connected to a power source. There are two types of electrodes: the cathode and the anode.

Cathode
The cathode is the electrode where reduction occurs. Reduction is the gain of electrons, and it happens at the cathode because it is connected to the negative terminal of the power source. As a result, positively charged ions in the electrolyte are attracted to the cathode and gain electrons, becoming neutral species or even elements.

Anode
The anode, on the other hand, is the electrode where oxidation occurs. Oxidation is the loss of electrons, and it happens at the anode because it is connected to the positive terminal of the power source. As a result, negatively charged ions in the electrolyte are attracted to the anode and lose electrons, becoming neutral species or even elements.

Decomposition of the Electrolyte
The decomposition of the electrolyte occurs when the positively charged ions migrate towards the cathode and the negatively charged ions migrate towards the anode. At the electrodes, they undergo chemical reactions and are transformed into new substances. This process is known as electrolysis.

Applications of Electrolysis
Electrolysis has various practical applications. For example:
- Electrorefining of metals: Electrolysis is used to purify and refine metals, such as copper and aluminum.
- Electroplating: Electrolysis is used to coat objects with a layer of metal, improving their appearance and durability.
- Electrochemical cells: Electrolysis is used in batteries and fuel cells to generate electrical energy from chemical reactions.

In conclusion, the decomposition of an electrolyte on passing electric current is called electrolysis. It involves the migration of ions towards the electrodes, where they undergo chemical reactions and are transformed into new substances. Electrolysis has various applications in industries and technology.

Understanding LED Leads
LEDs, or Light Emitting Diodes, have two leads that indicate their polarity, which is crucial for proper operation.

Lead Lengths and Their Significance
- The **shorter lead** of an LED is typically the **negative terminal**, also known as the **cathode**.
- The **longer lead** is the **positive terminal**, or the **anode**.

Why the Polarity Matters
- LEDs are diodes, meaning they only allow current to flow in one direction.
- If connected incorrectly (i.e., reversing the leads), the LED will not light up and could potentially be damaged.

Identifying LED Connections
- **Negative Terminal (Cathode)**:
- Shorter lead.
- Connected to the negative side of the power source.
- **Positive Terminal (Anode)**:
- Longer lead.
- Connected to the positive side of the power source.

Conclusion
Understanding the correct orientation of LED leads is essential for proper functionality. The answer to the original question is **option 'B'**, as the shorter lead is indeed attached to the **negative terminal** and the longer lead to the **positive terminal**. Proper connection ensures that the LED operates efficiently and safely.

Flow of Electric Current and Conventional Current

Introduction:
When it comes to the flow of electric current, there are two different concepts: the flow of electric charge and the flow of conventional current. The flow of electric charge refers to the movement of negatively charged particles called electrons, while the flow of conventional current refers to the movement of positively charged particles called protons.

Understanding Electric Charge:
- Electrons are negatively charged particles that orbit around the nucleus of an atom.
- In a neutral atom, the number of electrons is equal to the number of protons in the nucleus, resulting in a balanced charge.
- When an atom gains or loses electrons, it becomes positively or negatively charged, respectively.

Flow of Electric Charge:
- Electric current is the flow of electric charge through a conductor.
- In a closed circuit, electrons flow from a negatively charged body to a positively charged body.
- This flow of electrons from a negatively charged body to a positively charged body is known as the flow of electric charge.
- In this scenario, the negatively charged body is the source of electrons, while the positively charged body is the destination.

Understanding Conventional Current:
- Conventional current is a concept that simplifies the understanding of electric circuits.
- It assumes that electric current flows from the positive terminal of a battery to the negative terminal.
- This convention was established before the discovery of the electron and the understanding of atomic structure.

Flow of Conventional Current:
- In a closed circuit, conventional current flows from a positively charged body to a negatively charged body.
- This flow of conventional current from a positively charged body to a negatively charged body is opposite to the flow of electrons.
- In this scenario, the positively charged body is the source of conventional current, while the negatively charged body is the destination.

Conclusion:
The flow of electric current, as opposed to conventional current, is from a negatively charged body to a positively charged body. While conventional current assumes the opposite direction, it is important to note that the flow of electric charge, which involves the actual movement of electrons, is from negative to positive.

Introduction to Chromium in Electroplating
Chromium is widely utilized in the electroplating industry due to its unique properties that enhance the performance and durability of coated materials. Two primary reasons make chromium a preferred choice for electroplating.
Corrosion Resistance
- Chromium is highly resistant to corrosion, which protects the underlying material from environmental factors such as moisture, salt, and chemicals.
- This resistance prolongs the lifespan of products, making them more reliable and cost-effective in the long run.
Scratch Resistance
- The electroplating process with chromium creates a hard and durable surface that is resistant to scratches and wear.
- This quality is particularly important for items that undergo frequent handling or are exposed to harsh conditions, ensuring they maintain their aesthetic appeal and functional integrity.
Conclusion
In summary, chromium's exceptional properties of corrosion resistance and scratch resistance make it an ideal choice for electroplating. These characteristics enhance the durability and longevity of products, leading to increased customer satisfaction and reduced maintenance costs. As a result, the correct answer is option 'C': both (a) and (b).

Understanding Conductors of Electricity
Electricity requires a medium to flow, and materials are classified based on their ability to conduct electric current. Let's explore the options provided:
1. Air
- Air is generally considered a poor conductor.
- It can become ionized and conduct electricity under extreme conditions, such as during a lightning strike, but under normal circumstances, it acts as an insulator.
2. Wood
- Wood is also a poor conductor of electricity.
- It is primarily an insulator due to its dry composition, which makes it difficult for electric current to pass through.
3. Rubber
- Rubber is widely known as an excellent insulator.
- It is used to coat electrical wires and other components to prevent accidental electric shocks and short circuits.
4. None of These
- Given the properties of air, wood, and rubber, none of these materials effectively conduct electricity.
- Therefore, the correct answer is 'D', indicating that none of the options listed are good conductors.
Conclusion
- The materials mentioned (air, wood, rubber) do not facilitate the flow of electric current under normal conditions.
- Understanding the properties of conductors and insulators is essential for safety in electrical applications.

Understanding Electric Current Flow
To create an electric current, two essential components are required: a complete circuit and an electromotive force (emf). Let's explore why electromotive force is crucial.
What is Electromotive Force (emf)?
- Electromotive force, commonly referred to as emf, is the energy provided per charge by a power source (like a battery or generator).
- It is measured in volts and serves as the driving force that pushes electrons through a circuit.
Role of Electromotive Force in Current Flow
- Initiates Electron Movement: Emf creates a potential difference between two points in a circuit, prompting electrons to move from the negative terminal to the positive terminal.
- Maintains Current Flow: Without a sufficient emf, electrons would not have the energy needed to overcome resistance in the circuit, and current would cease.
Why Not Other Options?
- Electrodes: While electrodes are essential for connecting the circuit to the power source, they do not create the current themselves.
- Electrons: Electrons are indeed the charge carriers, but they require a force (emf) to move and create a current.
- Metal: Metals can conduct electricity, but they do not generate emf. They merely provide a pathway for the current.
Conclusion
In summary, while a circuit allows for current flow, it is the electromotive force that initiates and maintains the movement of electrons, thereby enabling electric current to flow effectively. Thus, the correct answer is option 'C', electromotive force (emf).

Understanding Positive Charge
When we talk about electric charge, it's essential to grasp how it works at a fundamental level. A positively charged body has a specific relationship with electrons.
What are Electrons?
- Electrons are negatively charged subatomic particles.
- They are found in the outer regions of atoms.
Charge Imbalance
- In a neutral atom, the number of protons (positively charged) equals the number of electrons (negatively charged).
- A positively charged body implies that there are fewer electrons than protons.
Deficit of Electrons
- The term "deficit" indicates that there is a lack or shortage.
- Therefore, a positively charged body has a deficit of electrons because it has lost some electrons, leading to a net positive charge.
Why is this Important?
- Understanding this concept is crucial in fields such as physics and chemistry.
- It helps explain how electric charge influences interactions between different materials.
Conclusion
- The correct answer to the question about a positively charged body is "a) deficit."
- This means that the body has fewer electrons than protons, resulting in a positive charge.
This foundational knowledge helps in grasping more complex concepts in electricity and magnetism as you progress in your studies.

Understanding the Tester
A tester is primarily used to assess the electrical conductivity of liquids. This is crucial in various applications, especially in laboratories and industrial settings.
Key Functions of a Tester
- Electrical Conductivity Measurement:
- The tester measures how well a liquid can conduct electricity. This property is important in determining the purity and concentration of solutions.
- Conductivity and Ion Presence:
- Liquids that contain dissolved ions (like saltwater) are good conductors, while pure water has low conductivity. The tester helps identify these differences.
Applications of Conductivity Testing
- Water Quality Testing:
- In environmental science, testers are used to monitor water quality, ensuring that it meets safety standards.
- Industrial Processes:
- Industries use testers to maintain optimal conditions for chemical reactions, where conductivity can indicate the right concentration of solutions.
Limitations of Other Options
- Magnetic Effect of EMF:
- Testers are not designed to measure magnetic fields or the effects of electromotive force (EMF).
- Battery/Cell Power:
- While some testers may check voltage, they do not measure the overall power or capacity of batteries directly.
- Electroplating:
- Testers do not assess the electroplating process; this requires specialized equipment to monitor deposition rates and metal quality.
Conclusion
In summary, the primary function of a tester is to measure the electrical conductivity of liquids, making option 'A' the correct answer. Understanding this functionality is essential for various scientific and industrial applications.

Understanding Electric Current
Electric current is the flow of electric charge. In most conductive materials, this charge is carried by electrons. However, some substances, particularly insulators or pure substances, do not conduct current through electrons.
Why Distilled Water Does Not Conduct Current
- Purity of Distilled Water: Distilled water is pure H2O, which lacks dissolved ions. Electric current typically requires charged particles to flow, which distilled water does not provide.
- Absence of Ions: For a substance to conduct electricity, it must have free-moving charged particles. In distilled water, there are no ions (like sodium or chloride) present to facilitate conductivity.
- Comparison with Conductors:
- Copper and Aluminium: These metals contain free electrons that move easily, allowing them to conduct electricity efficiently.
- Mercury: As a liquid metal, mercury also has free electrons that enable it to carry electric current.
Conclusion
In summary, while copper, aluminum, and mercury are excellent conductors due to the movement of electrons, distilled water stands out as an insulator because of its lack of free ions. This makes option 'C', distilled water, the correct answer to the question of which substance does not conduct current through electrons.