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Chemical Effects of Electric Current - Class 8 : Notes


Electric Current:
The flow of electrons in any material is termed as an electric current.

Flow of Electrons in the Matter

Good Conductors of Electricity:
The materials which allow the current to pass through them are known as good conductors. Examples are copper, aluminium, etc.

Good Conductors Metals


Poor Conductors of Electricity:

The materials which do not allow the current to pass through them, are known as poor conductors. They are also called as insulator. Examples are glass, plastic, etc.

Poor Conductors

Tester:
It is a device to test if a particular material allows electric current to pass through it or not.

Tester


Conductivity in Liquids:

(i) A tester can be used to check if a liquid is conducting or non-conducting.
(ii) To check if the liquid is conducting or not, connect the liquid between the two ends of tester by completing the connection of the circuit properly. If bulb in the tester glows, it means the liquid is conducting. But, if does not glow then it means liquid is non-conducting.

Testing conduction of electricity in liquid

(iii) Most liquids that conduct electricity are solutions of acids, bases and salts.

 

Heating effect of current:
The heating effect of current is responsible for the glowing of the bulb.

To test whether substance is conducting or not using heating effect:
When current passes through the bulb, the filament gets heated to a high temperature and as a result bulb starts glowing. But, if current is very small then the filament will not get heated to a high temperature and so will not glow.

Heating Effect of Electric Current for Glowing Bulb

LED (Light Emitting Diodes):
LED’s can be used to detect weak currents, since; their filament does not require much temperature to glow.
They have two terminals called anode and cathode. The length of anode lead is slightly longer than the cathode lead and is always connected to the positive terminal of the battery. On the hand, cathode lead is shorter and is connected to the negative terminal of the battery.

LED

 

Magnetic effect of current:
The magnetic effect of current is responsible for the deflection in magnetic compass when current passes nearby it. It can detect weak currents.


To test whether substance is conducting or not using magnetic effect:

For a closed circuit, when current passes nearby a magnetic needle and if the deflection is observed in the needle then it means the substance is conducting; otherwise it is non-conducting.

Magnetic Compass

Tester By using Magnetic Compass:
(i) Take the tray from inside a blank matchbox.
(ii) Wrap an electric wire a few times around the tray.
(iii) Insert a small compass needle inside it.
(iv) Now connect one free end of the wire to the terminal of a battery. Leave the other end free.
(v) Take another piece of wire and connect it to the other terminal of the battery

Tester by using the magnetic compass

Join the free ends of two wires momentarily. The compass needle should show deflection. Your tester with two free ends of the wire is ready.
Touch the both ends of tester to any substance to check whether the substance is conducting the electricity or not. If the deflection is observed in the needle then it means the substance is conducting; otherwise it is non-conducting.


Tap Water:
The water obtained from various sources like rivers, wells, taps, etc. is not in its purest form. It contains many impurities in it. These impurities include different salts too. As a result of these salts, the tap water becomes a good conductor of electricity.

Distilled Water:
It is the purest form of water and does not contain any kind of impurities in it. Hence, it is a poor conductor of electricity.

Distill Water

We can check the conductivity of tap water or distill water via the magnetic tester. 


Chemical Effects of Electric Current:
Electrodes:
These are conducting materials through which current enters or leaves a substance.

Types of Electrodes:
(i) Positive Electrode: It is the electrode which is connected to the positive terminal of the battery. It is also known as Anode.
(ii) Negative Electrode: It is the electrode which is connected to the negative terminal of the battery. It is also known as Cathode.

There can be various chemical effects observed on passing electric current depending on the type of solution and electrodes:

1. Formation of Gas Bubbles:
A British Chemist named William Nicholson performed an experiment showing that if current is passed through water, then, bubbles of oxygen and hydrogen were produced. The oxygen bubbles will be present on positive electrode and hydrogen bubbles on the negative electrode. The passage of an electric current through a conducting solution causes chemical reactions. As a result, bubbles of a gas may be formed on the electrodes

William Nicholson
Passing current through water

2. Deposits of metal may be visible on the electrodes.
3. The color of solution might change.

 

Electroplating:
The process using which layer of some metal is deposited on another material by using electricity is known as electroplating.

Experiment: We need to take copper sulphate and two copper plates of same size. Take 250 mL of distilled water in a clean and dry beaker. Dissolve two teaspoonfuls of copper sulphate in it. Add a few drops of dilute sulphuric acid to copper sulphate solution to make it more conducting. Connect the copper plates to the terminals of a battery and immerse them in copper sulphate solution.

 

Connect the circuit as shown in the figure.
When current is allowed to pass through the copper sulphate solution, then the solution will separate into the copper and sulphate. The separated free copper of the solution will get deposited on the negative electrode. And the same amount of copper will get dissolved in the solution from the positive electrode. Hence, we can say that copper from positive electrode got transferred to the negative electrode. This kind of transfer is known as electroplating.

Applications of Electroplating:
(i) It is used in industries for coating different metals on other metal objects. For example, chromium which is a lustrous, corrosion free, scratch resistant, etc. but being costly is deposited on materials like car parts, taps, burners, etc. to lower the manufacturing cost.

(ii) Silver and gold are deposited on cheaper materials by jewelers to lower the jewellery cost, but, keeping the appearance intact.

(iii) Iron cans are electroplated with tin used for storing food, as iron gets easily rusted and so protects the food from spoiling.


(iv) Zinc is deposited on iron used for the construction of bridges, vehicles, etc. to protect it from rust and corrosion.

 

 

 

 

 

 

 



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