NCERT Class 8 Science Chapter 14 - Chemical Effects of Electric Current Notes

Chapter 14 - Chemical Effects of Electric Current Notes

1. Conduction of Electricity
Similar to many solids, some liquids also conduct electricity.
Conductor: Substances which allow electricity to pass through them are called conductors; such as silver, gold, acidic solution, salt solution, etc.
Insulator: Substances which do not allow electric current to pass through them are called insulators; such as plastic, rubber, etc.

2. What is a circuit tester?
If the bulb lights up, it means the material allows electric current to flow through the closed circuit.
The brighter the bulb, the better the conductor of electricity is the material.

3. Use of a circuit tester
To test whether the material is a good electrical conductor.

4. How to use a circuit tester?
Step 1: Set up the circuit tester as shown above.
Step 2: Place the object between the open ends of the wire or crocodile clips.

5. Good conductor and Bad Conductor:
Substances which allow electric current to pass through them easily are called good conductors of electricity such as silver, gold, aluminium, etc.
Some substances allows electric current to pass through them but in very little amount. Therefore, such substances called bad conductors of electricity, rather than being called as insulator.
In fact, most of the substances allow electric current to pass through them under certain conditions, so instead of using terms conductors and insulators, good conductors and bad conductors are used.

6. Electrical conductors are materials that allow electric current to flow through.
Examples: salt water, metals such as copper, aluminium and non-metals like graphite (a form of carbon).

7. Electrical insulators are materials that do not allow electric current to flow through.
Examples: plastics, wood, rubber, glass.

8. Testing of conduction of electricity through liquid:
Liquids which contain salt dissolved in them; conduct electricity. The conduction of electricity through a liquid can be tested using two electrodes and a bulb. But the conduction of electricity through the liquids which are poor conductor cannot be tested. But conduction of electricity through such liquids can be tested using LED or magnetic compass needle
For this, you need a compass needle, an empty matchbox, two electric cells, some wires, a plastic bottle cap and the liquid which is to be tested. Keep the compass needle inside the empty tray of matchbox and wrap a couple of rubber bands around it. Connect the wires to the battery and insert two wires in the liquid which is kept in the plastic bottle cap. You will observe that when the current flows through the wire, there is deflection in the compass needle. This shows that the given liquid conducts electricity.
In fact, magnetic compass needle can detect even feeble current. LED can also be used in place of compass needle. 

9. Salts dissolved in water are responsible for conduction of electricity through water. Tap water, acid solutions, basic solutions, etc. are good conductors of electricity. Distilled water is poor conductor of electricity because in no salt is present in distilled water. 
• Place the two aluminum plates into the liquid tested.
• If the bulb lights up, a closed circuit is formed, allowing current to flow through it.
• The liquid is a conductor of electricity.

10. Some liquids are good conductors of electricity. Example tap water, salt solution, hydrochloric acid, sodium hydroxide etc. (solutions of acids, bases and salts are conductors of electricity)
Some liquids are poor conductor of electricity. Example: distilled water, sugar solution, kerosene, spirit etc.

11. Chemical Effects of Electric Current
When electric current is passed through a conducting solution, some chemical reaction takes place.
Example:
(a) When electric current is passed through water, water dissociates into hydrogen and oxygen. Hydrogen is deposited over negative pole and oxygen is deposited over positive pole. Deposition of hydrogen and oxygen at different poles is visible in the form of bubbles.
(b) When electric current is passed through the solution of a metal salt, such as solution of copper sulphate, metal gets deposited at the negative pole, because metal is positively charged.
(c) Sometimes, the colour of solution also changes when electric current passes through it.
The above examples are some of the chemical effects of electric current. The chemical reaction depends upon the type of solution through which electric current is passed.

12. Electroplating
Electroplating is one of the chemical effects of electric current. Electroplating is a chemical process using which a metal is coated with a layer of another desired metal. Electroplating is done to make the metals shiny. Electroplating is done over the articles made of iron to make the iron shiny and to prevent iron from getting rusted.
Example: Wheel rims, handle of cycle, etc. are made shiny by the method of electroplating.

13. Process of electroplating
In the process of electroplating, metal salt solution is taken in a container. Salt of metal is chosen which is to be coated over another metal. Metal which is to be coated is dipped in the solution and connected with negative pole. Metal for desired coating is connected with positive pole.
When electric current is passed through the solution, metal from anode is dissolved in the salt solution and deposited over the negative pole (cathode). Thus, coating of desired metal is obtained on another metal.

14. Use of electroplating
(a) Water pipes, which are made of iron, are coated with a layer of zinc metal by the process of electroplating. Zinc is less reactive than iron and thus prevents the iron pipe from getting rusted.
(b) Rims of wheel of cycle, cars, etc. are electroplated with chromium metal. Layer of chromium metal give them shiny appearance and prevents from rust.
(c) Ornaments made of silver or other cheap metals are electroplated with gold to give them appearance like gold.
(d) Tin cans used for storing food are made of iron electroplating with a coat of tin. Tin is less reactive than iron, and prevent foods packed in them from getting spoiled.

15.Electrodes are the terminals through which current is passed into a liquid.

16. Electrolysis is the breaking up of a compound from its solution on passing electric current through the electrolyte.

17. An ion is a molecule or an atom which is positively or negatively charged.

18. Cathode is the electrode connected to the negative terminal of the battery.

19. Anode is the electrode connected to the positive terminal of the battery.

20. Tester
A tester is a piece of electrical equipment used to check the presence of electric current. It is usually a conductor with a LED/bulb to indicate that the current is present in the circuit.

21. The full form of LED is Light Emitting Diode. The LED bulb has two leads. The lead connected to the positive terminal of battery is called anode and the lead connected to the negative terminal is cathode. The slightly longer lead than the other is always connected to the positive terminal and the shorter lead is connected to the negative terminal.

22. In 1800, a British chemist, William Nicholson had shown that if electrodes were immersed in water, and a current was passed, bubbles oxygen and hydrogen were produced. Oxygen bubbles formed on the electrode connected to the positive terminal of the battery and hydrogen bubble formed on the other electrode.

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