SUBJECT : SCIENCE

(CLASS:VIII)

CHAPTER-14:

THE CHEMICAL EFFECT OF ELECTRIC

CURRENT

DATE:

25 NOV 2020

SUBJECT : SCIENCE

(CLASS:VIII)

CHAPTER-14:

THE CHEMICAL EFFECT OF ELECTRIC

CURRENT

TOPIC-1:

CONDUCTION OF ELECTRICITY

THROUGH LIQUID

OBJECTIVES:

Upon completion of the topic, you will be able to

1. DEFINE CONDUCTORS AND INSULATORS.

2. TEST WHETHER A GIVEN LIQUID IS CONDUCTOR OR NOT WITH

PROPER CIRCUIT DIAGRAM.

3. DEFINE ELECTRIC CHARGE, IONS, ELECTROLYTES

4. WRITE THE TYPES OF CHARGE AND TYPES OF IONS

CONDUCTION OF ELECTRICITY THROUGH LIQUID: TESTING

TESTING OF CONDUCTIVITY OF LIQUID:To test whether a given sample of liquid is conductor or insulator the arrangement(experimental set up) is shown in the above figure.On inserting the two free ends of the conducting wire(tester) into the sample of liquid we need to observe whether the bulb glows or not. If it glows , it means liquid is a conductor of electricity. If the bulb does not glow, it means the liquid is insulators.

TESTING OF CONDUCTIVITY OF DISTILLED WATER AND

IMPURE WATER:

To test whether distilled water conduct electricity or not, the

arrangement is shown in FIG-A. On inserting the two free ends of

conducting wire into the distilled water, the bulb does not glow. It

proves that distilled water does not conduct electricity.

On the other hand to test the conductivity of impure water(water

containing dissolved salts) the arrangement is shown in FIG-B.

On inserting the two free ends of conducting wire into the impure

water, the bulb is found to glow. It proves that impure water

(water containing dissolved salts) conduct electricity.

FIG:A FIG:B

1. ELECTRIC CHARGE: Electric charge is the basic property of matter which is

responsible for electrical phenomena.

2. TYPES OF CHARGE: It has two types: (a) positive charge (example-proton)

and (b) Negative Charge (example-electron)

3. UNIT OF ELECTRIC CHARGES: Coulomb(C), milli coulomb, micro coulomb

4. IONS: An atom or group of atoms that carries a charge is called an ions.

5. TYPES OF IONS: It has two types. (a) positive ions and (b) Negative ions

6. ELECTROLYTES: Chemical compounds that conduct electricity when in

solution or in the molten state are called electrolytes. Example- salt solutions

like copper Sulphate solution , many Acids and bases etc

7. CONDUCTIVITY: The degree to which electrolytes conduct electricity is

known as conductivity. A good conductor has high conductivity and a poor

conductor has low conductivity.

8. NOTE:

a. Liquids that have free electrons in them conduct electricity.

b. When chemical substances that can form ions (such as salts) are added in a

liquid its conductivity increases.

c. Tap water is a good conductor of electricity and thus we should avoid

handling switches and electrical appliances with wet hands.

HOME-WORK

1. Define the following

(a) conductors

(b) Insulators

(c) electric charge

(d) Ions

(e) Electrolytes

2. You are given three liquids A , B and C. B conducts electricity

but A and C do not conduct electricity. How will you test it?

3. Write difference between conductors and insulators

4. How does electricity flow through liquids?

5. Why does electricity pass through tap water?

6. State whether the following liquids are conductor or insulator

(a) kerosene

(b) Vinegar

(c) Lemon Juice

(d) Alcohol

(e) Dilute hydrochloric acid

(f) Dilute Sodium Hydroxide solution

HOME-WORK

7. Why is LED used rather than a bulb to demonstrate conduction

in liquids?

8. Why do some liquids conduct electricity whereas others do not?

9. Why is distilled water a poor conductor of electricity?

10. Why should we not operate electrical items with wet hands?

DATE:

01 DEC 2020

SUBJECT : SCIENCE

(CLASS:VIII)

CHAPTER-14:

THE CHEMICAL EFFECT OF ELECTRIC

CURRENT

TOPIC-2:

ELECTROLYSIS

&

ELECTROLYTIC CELL

OBJECTIVES:

Upon completion of the topic, you will be able to

1. DEFINE ELECTROLYSIS.

2. DEFINE ELECTRODES

3. EXPLAIN ELECTROLYSIS OF WATER

4. DEFINE ELECTROLYTIC CELL

5. WRITE THE USES OF ELECTROLYSIS

6. DRAW THE DIAGRAM OF ELECTROLYTIC CELL

ELECTROLYSIS

Electrolytes:The chemical compounds that conduct electricity when in solution or in the molten state are called electrolytes.Electrolysis :The process by which an electrolyte decomposes or break down into its constituent components when electricity is passed through it is called electrolysis.

ELECTROLYSIS OF WATER

To demonstrate the electrolysis of water the experimental set up(arrangement) is shown in the following Diagram

ELECTROLYSIS OF WATER

EXPLANATION:

When electric current is passed through the circuit and thus

through water, then water molecules break down into its constituent

ions or elements. Thus on breaking of water molecules, hydrogen

and oxygen gas are produced. We observe bubbles formation near

both the carbon rods. These gases are collected in two different

test tubes that are placed over the carbon rods. When the gas

collected in the test tube over negative electrode is tested with a

burning splinter, the splinter goes out with a ‘pop’ indicating that the

gas is hydrogen. When the glowing splinter is introduced into the

other test tube, it bursts into flame, indicating that the gas is

oxygen. This test shows that water has chemically decomposed to

its constituent gases oxygen and hydrogen.

ELECTROLYTIC CELL

Electrolytic cell: A device consisting of an electrolyte in which two electrodes are immersed is called an electrolytic cell.

USES OF ELECTROLYSIS

1. Electrolysis is used to coat one metal with another.

2. It is used to extract and purify metals of high reactivity from

their compounds(e.g. Aluminium, sodium, potassium)

3. It is used for purification metals like copper and gold.

4. Non-metals such as chlorine and oxygen gas are produced by

electrolysis.

5. It is used to clean metals also

HOME -WORK

1. Define Electrolysis

2. Define electrolytic cell

3. Define (a) cations (b) anions (c) cathode and (d)anode

4. What should be connected to the electrodes for electrolysis of an

electrolyte to take place?

5. Draw a neat labeled diagram of an electrolytic cell.

6. Write any four uses of electrolysis.

7. With diagram explain the electrolysis process of water.

8. Where is hydrogen formed when water is electrolysed?

9. Where is oxygen formed when water is electrolysed?

10.Write one difference between primary cell and an electrolytic

cell.

11.Which metals are extracted by electrolysis?

DATE:

04 DEC 2020

SUBJECT : SCIENCE

(CLASS:VIII)

CHAPTER-14:

THE CHEMICAL EFFECT OF ELECTRIC

CURRENT

TOPIC-3:

ELECTROPLATING

OBJECTIVES:

Upon completion of the topic, you will be able to

1. DEFINE ELECTROPLATING.

2. DEFINE ELECTRODES

3. EXPLAIN THE PROCESS OF ELECTROPLATING

4. DEFINE ELECTROLYTIC CELL

5. DRAW EXPERIMENTAL SET UP FOR OCCURRENCE OF

ELECTROPLATING.

6. DRAW THE DIAGRAM OF ELECTROLYTIC CELL

ELECTROPLATING

Electroplating:

The process of coating of one metal with another metal by the

process of electrolysis is called electroplating.

Important point:

1. The part that has to be plated is made the cathode(the

negative electrode)

2. The metal that is to be deposited is made the anode(the

positive electrode)

3. The electrolyte is a solution that has ions of the metal that will

be deposited(commonly a salt of the metal is used)

4. In the process- the anode is worn away where as the cathode

becomes thicker.

5. The thickness of the deposited layer depends on:

a. the strength of electric current

b. time for which the current is passed

c. concentration of ions in the solution

ELECTROPLATING OF COPPER ON IRON KEY:

ELECTROPLATING OF COPPER ON IRON KEY:

EXPLANATION:

For electroplating of copper on iron key the arrangement is shown

in the above diagram. When electric current is passed through the

circuit, the metal atoms in the anode ( in this case, the copper

plate) are detached and are converted to positively charged

cations ( in this case copper ions) and enter the electrolytic

solution. At the same time copper sulphate molecule from the

electrolyte solution breaks down and copper positive ions and

sulphate negative ions are formed. Copper positive ions from the

solution move towards the cathode (iron key) and get deposited

over it and forms layer of copper over iron key. On the other hand

sulphate negative ions combine with copper positive ions (that

comes to the solution from copper plate) and form copper

sulphate molecule. The process continues as long as the current

passes through solution. The anode is worn away in the process,

whereas the cathode becomes thicker. In the process

concentration of solution remains unchanged. This is how a layer

of copper metal is given over the iron key.

FACTORS THAT DECIDE THE AMOUNT OF METAL DEPOSITED DURING

ELECTROPLATING:

1. Strength of the electric current

2. Time for which the current is passed

3. Concentration of ions in the solution

USES OF ELECTROPLATING

1. Bathroom taps, door handles, kitchen items are electroplated

with chromium to make these shiny and attractive

2. It is done to make the items attractive and expensive

3. Jewellery made of cheaper metals like silver is electroplated

with gold. This makes the Jewellery look more expensive

4. It is done to save the materials from corrosion or rusting

5. Plating with silver or copper is done to increase electrical

conductivity.

NOTE: Generally coating of costly metal is given over cheaper metal.

HOME-WORK

1. What do you understand by electroplating?

2. Describe an experiment to coat a key with copper

3. What are the factors that decide the amount of metal

deposited during electroplating?

4. Write any four uses of electroplating.

5. Why is silver jewellery coated with gold?

DATE:

07 DEC 2020

SUBJECT : SCIENCE

(CLASS:VIII)

CHAPTER-14:

THE CHEMICAL EFFECT OF ELECTRIC CURRENT

TOPIC-4:

EXTRACTION AND PURIFICATION OF METALS

(ELECTROLYTIC REFINING)

OBJECTIVES:

Upon completion of the topic, you will be able to

1. DEFINE EXTRACTION OF METAL.

2. DEFINE & EXPLAIN PURIFICATION OF METAL

3. DEFINE AND EXPLAIN ELECTROLYTIC REFINING

4. DEFINE ELECTROLYTIC CELL

5. WRITE THE IMPORTANCE OF ELECTROLYTIC REFINING

6. EXPLAIN PURIFICATION OF COPPER BY ELECTROLYTIC REFINING

PROCESS

ELECTROLYTIC REFINING OF METAL(COPPER)

DEFINITION:ELECTROLYTIC REFINING: The process of purifying a metal by electrolysis is called electrolytic refining

ELECTROLYTIC REFINING

ELECTROLYTIC REFINING: EXPLANATION

Settle down below the anode as anode mud. Thesoluble impurities stay in the solution. The anodebecomes thinner and the cathode becomes thicker.

HOME-WORK

1. Define (a) electrolytic refining (b) anode mud2. What happens to impurities in the process of electrolytic

refining?3. With a neat, labeled diagram, explain how copper is

electrolytically refined4. What do you understand by the terms “extraction and

purification” of metals?

QUESTIONS: MCQ-TYPE QUESTIONS

1. An electric current can produce

(a) heating effect only.

(b) chemical effect only.

(c) magnetic effect only.

(d) chemical, heating, and magnetic effects.

2. When an electric current is passed through a conducting

solution, there is a change of colour of the solution. This indicates

(a) the chemical effect of current.

(b) the heating effect of current.

(c) the magnetic effect of current.

(d) the lightning effect of current

3. Which one of the following solutions will not conduct electricity?(a) lemon juice(b) vinegar(c) tap water(d) vegetable oil4. Which of the following metals is used in electroplating to make objects appear shining?(a) iron(b) copper(c) chromium(d) aluminium

QUESTIONS

5. Plastic wire isa. An insulatorb. A conductorc. Both of thesed. None of these

6. Electroplating is based ona. Magnetic effect of electricityb. Chemical effect of electricityc. Heating effect of electricityd. Physical effect of electricity

7. Adding common salt to distilled water makes ita. Good conductorb. Insulatorc. Nod. Both a and b

8. An electrolyte isa. a metalb. a solutionc. a liquid that conducts currentd. all of above

9. Distilled water isa. Poor conductorb. Good conductorc. Both a and bd. None

QUESTIONS

10. Flow of electron is calleda. Electroplatingb. Electric currentc. Electrodesd. Electrolyte

11. Electroplating preventsa. Currentb. Chemical effectc. Rustingd. All

12. Ram and Sita performed experiments taking similar bulbs and cells but two different solutions A and B as shown in the figure.

They found that the bulb in the set up A glows more brightly as compared to that of the set up B. You would conclude that(a) higher current is flowing through the circuit in set up A(b) higher current is flowing through the circuit in set up B(c) equal current is flowing through both the circuits(d) the current flowing through the circuits in the two set up cannot be compared in this manner

QUESTION

13. Which one of the following solutions will not conduct electricity?(a) Lemon juice (b) Vinegar(c) Tap water (d) Vegetable oil14. The process of depositing a layer of any desired metal on another material by means of electricity is called ___________.(a) Electric plating(b) Electroplating(c) Electric depositing(d) None of the above 15. When electrodes are immersed in water and electricity passed, the bubbles formed on the negative terminal is actually _______ gas.(a) Hydrogen(b) Carbon dioxide(c) Oxygen(d) Nitrogen16. Tap water is a good conductor of electricity while distilled water is not because(a) Tap water contain salts(b) Distilled water do not contain salt(c) Only (a) is correct(d) Both (a) & (b) is correct

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