NCERT Solutions for Class 8 Science Chapter 11

The chapter on chemical effects of electric current in your 8th standard science book talks about what happens when you run electricity through certain liquids. These liquids have to be special conducting solutions, which means they have stuff dissolved in them that allows electricity to flow.

The main idea is that the electricity can cause chemical reactions to happen in the solution.

Gas bubbles forming on the electrodes (the metal strips that the electricity goes in and out of).
A layer of metal building up on one of the electrodes.
The color of the solution changing.

The exact reaction depends on what kind of solution and electrodes you’re using.

One important application of this is electroplating. In electroplating, you use electricity to put a thin layer of a desired metal onto another object. For example, you might chrome plate a car bumper to make it shiny and more resistant to rust.

The Big Idea: When you pass electricity through a conducting solution (a liquid with dissolved salts or other conductive materials), chemical reactions occur. These reactions can cause various effects depending on the specific solution and electrodes used.

Key Points:

Electrodes: These are the conductors (usually metal strips) that electricity enters and exits the solution. They play a crucial role in the chemical reactions.
Chemical Reactions: The flow of electric current disturbs the equilibrium of ions (charged particles) within the solution, compelling them to shift and undergo reactions. This can lead to:
- Gas Formation: Bubbles of gas may appear on the electrodes. For instance, when electricity passes through water (with some dissolved salts), hydrogen gas forms at the negative electrode (cathode) and oxygen gas forms at the positive electrode (anode).
- Metal Plating: Within certain solutions holding metal ions (dissolved metals), these ions are drawn towards the cathode where they acquire electrons. This causes them to deposit as a thin layer of metal on the cathode’s surface.
  Electroplating is a typical utilization of this phenomenon, employed to coat items with a chosen metal for either decorative or protective purposes,
- Color Change: These reactions have the potential to alter the solution’s composition, occasionally resulting in a noticeable change in color.

Factors Affecting the Reaction:

The Solution: Different solutions contain different ions, which will react in unique ways with the electric current.
The Electrodes: The material of the electrodes can influence the reactions. For example, using a copper electrode might lead to copper plating, while an inert electrode like platinum may not participate in the reaction as readily.

Real-World Application: Electroplating

Electroplating is a prime example of how the chemical effects of electric current are used in technology. By carefully choosing the solution and electrodes, we can coat objects with a thin layer of a desired metal. This has many applications, such as:

Chrome plating: For a shiny and rust-resistant finish on car parts, appliances, etc.
Gold plating: For decoration and enhancing conductivity in electronics.
Silver plating: For antibacterial properties on utensils or electrical components.

By understanding the chemical effects of electric current, scientists and engineers can develop new applications and technologies that rely on these reactions.

NCERT Solutions for Class 8 Science Chapter 11

Q1. Fill in the blanks.

(a) Most liquids that conduct electricity are solutions of ______ , ______ and ______

(b) The passage of an electric current through a solution causes _______ effects.

(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the ________ terminal of the battery.

(d) The process of depositing a layer of any desired metal on another material by means of electricity is called _______

Ans:

Acids,bases, salts
Chemical
Negative
Electroplating

Q2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?

Ans: The deflection happens because current flowing through the tester wires creates a magnetic field that nudges the compass needle. It’s like the tiny magnet in the compass is feeling a push from the electricity’s magnetism.

Q3. Name three liquids, which when tested in the manner shown in Fig. 14.5. may cause the magnetic needle to deflect.

Ans: three liquids: 1. Saltwater, 2. Vinegar, 3. Lemon juice.

Q4. The bulb does not glow in the setup shown in Fig. 14.6. List the possible reasons. Explain your answer.

Ans: Bulb not glowing? Check for: 1. Broken wires/circuit, 2. Dead battery, or 3. Burnt-out bulb.

Q5. A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that

(i) liquid A is a better conductor than liquid B.

(ii) liquid B is a better conductor than liquid A.

(iii) both liquids are equally conducting.

(iv) conducting properties of liquid cannot be compared in this manner.

Ans: (i) liquid A is a better conductor than liquid B.

Q6. Does pure water conduct electricity? If not, what can we do to make it conducting?

Ans: No, pure water does not conduct electricity. This is because pure water lacks the necessary ingredients for electrical current to flow.

Q7. In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.

Ans: Firefighters cut the power to avoid electrical shocks from water conducting electricity and to prevent damage to electrical equipment.

Q8. A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?

Ans: The compass needle deflects more with seawater because seawater is a better conductor of electricity than drinking water.

Q9.Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.

Ans: No, it’s unsafe for an electrician to perform electrical repairs outdoors during a heavy downpour.

Q10. Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass tumbler and tested it using a tester. To her surprise, she found that the compass needle show deflection. What could be the reasons?

Ans: Rainwater, naturally pure and an insulator, becomes contaminated with air pollutants such as sulphur dioxide and nitrogen oxides, forming an acidic solution. This acidic solution becomes a good conductor of electricity, leading to deflection of the compass needle.

Q11. Prepare a list of objects around you that are electroplated.

Ans: As a large language model, I don’t have a physical body and therefore can’t have real-world objects around me. However, based on our conversation so far, here are some common household items that might be electroplated:

Door handles: Chrome or nickel plating for a shiny and more durable finish.
Kitchen sink faucet: Utilizing chrome plating to achieve a glossy and corrosion-resistant surface.
Pot and pan handles: Nickel or chrome plating for aesthetics and easier cleaning.
Jewellery (costume): Gold or silver plating for a decorative look.
Appliance parts: Chrome plating on certain parts of toasters, kettles, etc. for aesthetics and rust resistance.

Keep in mind that this is not an exhaustive list, and the use of electroplating can vary depending on the specific object and its purpose.

Q12. The process that you saw in Activity 14.7 is used for purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transferred to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?

Ans: The thick rod of impure copper should be connected to the positive terminal of the battery.