DAV Class 8 Science Chapter 16 Question Answers – Electric Current and Its Chemical Effects
When you plug in a phone charger, you are using the heating and magnetic effects of electric current. But electricity can also cause chemical changes, and that is the story of Chapter 16 in the DAV Class 8 Science book. The chapter explains which liquids conduct electricity, what happens when current passes through them, and how we use this to coat objects with metal, purify metals and even produce electricity from a magnet. This post gives short, easy answers to every question, with a quick reason behind each one.
The chapter in one minute
Some liquids conduct electricity and some do not. Liquids that conduct are called electrolytes, and most of them are solutions of acids, bases and salts. They conduct because they contain charged particles called ions, which move through the liquid when a battery is connected. Strong electrolytes break up into ions almost completely and conduct well. Weak electrolytes break up only a little and conduct poorly. Distilled water has almost no ions, so it does not conduct.
When current passes through an electrolyte, it can break the electrolyte down into its parts. This is called electrolysis. The plates dipped into the liquid are called electrodes. The one joined to the positive terminal of the battery is the anode, and the one joined to the negative terminal is the cathode. Electrolysis is used for electroplating, which is coating one metal with another, and for electrorefining, which is making impure metal pure.
The chapter ends with electromagnetic induction. When a magnet moves near a coil of wire, a current flows in the coil. It was discovered by Michael Faraday and Joseph Henry.
A. Fill in the blanks
1. Most liquids that conduct electricity are solutions of acids, bases and salts.
2. The greater the concentration of ions in a solution, the greater its conductivity.
3. Passing an electric current through a solution can cause chemical effects.
4. When current passes through copper sulphate solution, copper is deposited on the plate joined to the negative terminal of the battery.
5. Using electricity to deposit a thin layer of a metal on another metal object is called electroplating.
6. The faster the magnet and the closed coil move relative to each other, the stronger the current in the coil.
B. True or false
| Statement | Answer | Why |
|---|---|---|
| 1. A solution of silver nitrate is a good electrolyte. | True | It contains plenty of free ions. |
| 2. An electrolytic cell has a cathode, an anode and an electrolyte. | True | These are its three basic parts. |
| 3. Oxygen is released at the anode when acidified water is electrolysed. | True | Hydrogen is released at the cathode. |
| 4. Electrorefining means coating one metal on another with electric current. | False | That is electroplating. Electrorefining means purifying a metal. |
| 5. Electromagnetic induction was discovered by Alessandro Volta. | False | It was discovered by Michael Faraday and Joseph Henry. Volta invented the voltaic cell. |
C. Choose the correct option
1. Of the four liquids (sea water, tap water, nitric acid, oxalic acid), the weak electrolytes are B and D, which are tap water and oxalic acid.
2. Electrolytes conduct electricity because of the movement of positive as well as negative ions.
3. The electrode joined to the positive terminal of a battery is the anode.
4. The breaking down of an electrolyte by electric current is called electrolysis.
5. Coating iron with chromium is called electroplating.
D. Short answer questions
1. What is a liquid that conducts electricity, with chemical changes, called?
An electrolyte.
2. Distinguish between strong and weak electrolytes. Give two examples of each.
A strong electrolyte breaks up almost completely into ions in solution, so it conducts well. A weak electrolyte breaks up only partly, so it conducts poorly. Strong electrolytes: nitric acid and sulphuric acid. Weak electrolytes: oxalic acid and tap water.
3. Name the device that changes (a) chemical energy into electrical energy and (b) mechanical energy into electrical energy.
(a) A cell. (b) A generator.
4. Name the scientists who introduced (a) the voltaic cell, (b) electrolysis and (c) electromagnetic induction.
(a) Alessandro Volta. (b) Michael Faraday. (c) Michael Faraday and Joseph Henry.
5. Electrode A is joined to the positive terminal and electrode B to the negative terminal of a battery. (a) Name A and B. (b) Name the process. (c) Name the gases produced at A and B.
(a) A is the anode and B is the cathode. (b) Electrolysis. (c) Oxygen is produced at A, and hydrogen at B, as in the electrolysis of acidified water.
6. Explain the terms.
(a) Electromagnetic induction: producing an electric current with a changing magnetic field.
(b) Electrolysis: breaking down an electrolyte by passing an electric current through it.
(c) Electrodes: the metal plates through which current enters or leaves an electrolyte.
(d) Electrorefining: getting pure metal from impure metal by electrolysis.
7. State three uses of electrolysis.
Extracting metals from their ores, purifying metals (electrorefining), and coating metals (electroplating).
E. Long answer questions
1. Give reasons.
(a) Solid sodium chloride does not conduct electricity, but its solution does. In the solid, the ions are locked in place and cannot move. In solution, the ions are free to move and carry the current.
(b) It is unsafe to do electrical repairs outdoors in heavy rain. Rainwater contains dissolved salts and impurities, so it conducts electricity. This makes an electric shock more likely.
(c) A few drops of sulphuric acid are added to water for electrolysis. Pure water is a very poor conductor. The acid adds ions to the water, so it conducts well.
(d) Kitchen gas burners are often coated with chromium. The chromium layer protects the burner from rusting and keeps it shiny.
2. Three cells A, B and C, each with an identical bulb, hold sodium chloride solution, acetic acid solution and distilled water. Which bulb glows (a) brightest, (b) dim, (c) not at all?
(a) Set-up A glows brightest, because sodium chloride is a strong electrolyte and gives many ions.
(b) Set-up B glows dimly, because acetic acid is a weak electrolyte and gives few ions.
(c) Set-up C does not glow, because distilled water has almost no ions and does not conduct.
3. Ramit sets up a circuit to purify copper. (a) Name the metals for electrodes A and B. (b) Name the process. (c) Name the solution used.
(a) The anode (the electrode joined to the positive terminal) is made of impure copper, and the cathode is made of pure copper. Match A and B with the labels in your diagram. (b) The process is electrorefining. (c) The solution used is copper sulphate.
How it works: copper dissolves from the impure anode into the solution and is deposited as pure copper on the cathode. The impurities are left behind.
4. (a) Define electroplating. How can steel spoons be silver-plated? (b) State three uses of electroplating.
(a) Electroplating is depositing a thin layer of a metal on another metal object using electric current. To silver-plate a steel spoon, the set-up is as follows:
- Use a silver plate as the anode and the steel spoon as the cathode.
- Dip both into a silver salt solution such as silver nitrate.
- Connect them to a battery and switch on the current.
- Silver leaves the anode, moves through the solution and settles on the spoon, giving it a silver coat.
(b) Electroplating makes objects shiny, makes them more durable, and protects them from rusting and corrosion.
5. Describe an experiment to show that a magnet can give electrical energy.
Wind many turns of insulated copper wire around a hollow pipe, and clean the two ends of the wire. Connect the ends to a small torch bulb or an LED. Now push a strong bar magnet quickly into the coil, without touching it. The bulb glows for a moment. When the magnet stops, the bulb goes off. Pull the magnet out, and the bulb glows briefly again. This shows that a moving magnet near a coil produces an electric current, which is electromagnetic induction.
6. A coil is connected to a bulb. What happens if a bar magnet is (a) pushed into the coil, (b) pulled out of the coil, (c) held still inside the coil?
(a) The bulb glows while the magnet is moving in. (b) The bulb glows while the magnet is moving out. (c) The bulb does not glow, because current flows only while the magnet is moving.
Mistakes to avoid in the exam
Many students mix up anode and cathode. Remember: the anode is joined to the positive terminal, and the cathode is joined to the negative terminal. In electroplating, the object to be coated is always the cathode.
Another common slip is writing electrorefining when the question describes electroplating. Plating means coating, and refining means purifying.
For the electroplating diagram, do not forget the electrolyte. It must contain a salt of the metal being plated, such as silver nitrate for silver plating.
Finally, in electromagnetic induction, remember that the bulb glows only while the magnet is moving. A still magnet produces no current.
Frequently asked questions
Why does distilled water not conduct electricity?
It has almost no ions, and only moving ions can carry current through a liquid.
What is the difference between electroplating and electrorefining?
Electroplating coats a metal object with a thin layer of another metal. Electrorefining purifies an impure metal.
Which gases are produced when acidified water is electrolysed?
Oxygen is produced at the anode and hydrogen at the cathode.
Why is a layer of chromium put on some objects?
It stops them from rusting and gives them a bright, shiny look.
Written for students following the DAV Science book for Class 8. Please cross-check answers with your own textbook and teacher.