Class 8 Science Chemical Effects of Electric Current Exam Notes

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Class 8 Science Chemical Effects of Electric Current Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Covers following topics:

  1. Static Electricity
  2. Electrical Charges
  3. Types of charged bodies
  4. Electrical conductivity
  5. Chemical effect of electric current
  6. Application of electrolysis
  7. Magnetic effect of current
  8. Danger of electricity
  9. Safety measures in using electricity

Introduction

When amber is rubbed with fur, it acquires the force to attract small pieces of matter. Electron came into existence from the word amber as amber in Greek is called electron.

Static electricity

  1. Static electricity is produced when electrical charges build up on the surface of a material hence it is called static electricity as the charges are not in motion.
  2. Common examples of static electricity in action are :
  3. Sometimes when you touch the door knob you may get a mild shock. The electric shock arises due to the accumulation of static charges on the knob.
  4. You enter your room after coming back from your school and as soon as you pull of your woolen cap, all of a sudden all your hair stand on end. It happens because of static electricity caused due to accumulation of charges on your hair.
  5. During winter when you take off woolen or synthetic clothes in a dark room you might see some sparkling light with a crackling sound. The sparkles are also produced because of static electricity.

Electrical Charges

As protons, neutrons and electrons have characteristic properties, hence, they are very different from each other. One of the properties of these constituents of atoms is an electrical charges.

Class 8 Science Chemical Effects of Electric Current Exam Notes

Protons are said to have a "positive" (+) charge, electrons have a "negative" (–) charge while neutrons are neutral, i.e., have no charge.

 Interaction between electrical charges Two static charges of opposite types attract each other and two static charges of the same type repel each other.

→TYPES OF CHARGED BODIES

Since there are two types of charges (+ve and –ve), there are two types of charged bodies :

♦Positively charged bodies

♦Negatively charged bodies.

When an electron moves away (due to any reason) from an atom, it carries its negative charge with it. The number of electrons also becomes less than the number of protons in the atom. It results in the body having excess positive charge. Thus on loosing an electron, a body is said to get positively charged.
Vice versa, when an electron moves towards an atom it brings its negative charge with it. The number of protons becomes less than the number of electrons in the atom. It results in the body having excess of negative charge. Thus on gaining electrons, a body is said to get negatively charged.
Thus, we conclude that a body gets positively charged if, its protons are more and negatively charged if the
electrons are more.

 ELECTRICAL CONDUCTIVITY

Electrical conductivity is a measure of the ability of a substance to carry electric current. Substances that are good conductors of electricity have high electrical conductivity as compared to substances that are poor electrical conductors (also called insulators). Same liquids, but not all, are also good conductors of electricity.

(a) Conductivity of Water:

Pure water is a poor conductor of electricity. But the water that we use in our houses is not pure water. Generally, water (tap water, pond water, well water, etc.) contains a lot of impurities, most of which are usually dissolved salts. The presence of even a small amount of impurity makes water a good conductor of electricity. Touching an electrical appliance with wet hands could, therefore, be dangerous.

(b) Conductivity of Other Liquids (Lemon Juice:

A lemon is a citrus fruit, an excellent source of Vitamin C and makes a cool and refreshing summer drink. But did you know we can generate electricity using a lemon ? Lemon juice contains an acid called citric acid. To test the electrical conductivity of lemon juice, we arrange four to five lemons using strips of copper and zinc, a few alligator clips and connecting. The electricity produced, although not very strong, is enough to light an LED (light emitting diode).

NOTE: An LED means a light emitting diode. It is similar to a bulb but it runs on very low voltages and requires very small amount of current to glow as compared to a bulb. The coloured lights that you see in retail stores, stages, bridges and even in the keypads of mobile phones are all LED's.

Class 8 Science Chemical Effects of Electric Current Exam Notes

CHEMICAL EFFECT OF ELECTRIC CURRENT

Electrical energy is a very useful form of energy. It can be converted into chemical energy of certain types of substances. This is what we call as the chemical effects of current.

When an electric current is passed through water containing sulphuric acid, the water breaks up into its components hydrogen and oxygen. Therefore an electric current can cause a chemical change. This effect of electric current is used in electroplating i.e. coating a thin layer of a metal on another metal. The metal which is to be electroplated is made cathode and the metal to be deposited is made anode while the soluble salt of the same metal serves as the electrolyte. When a current is passed, a thin film of metal is deposited on the metal, which becomes electroplated.

Ex.. Let we are having a spoon which is to be electroplated with silver. The spoon is made the cathode and the piece of silver is made the anode. AgNO3 salt is used as electrolyte. When a current is passed through the electrolyte the atoms of silver gets deposited on the spoon.

Class 8 Science Chemical Effects of Electric Current Exam Notes
(i) General terms associated with the passage of current through solutions:

(A) Electrolyte:

A solution of a chemical compound which conducts electric current and at the same time undergoes a chemical change is called electrolyte.

Examples:

(i) Aqueous solutions of all acids, such as HCl, HNO3, H2SO4 etc. (ii) Aqueous solutions of all alkalis, such as NaOH, KOH etc.

(iii) Aqueous solutions of salts, such as common salt, copper sulphate, sodium nitrate, zinc chloride, etc.

(B) Non-electrolyte:

A solution of a chemical compound which does not conduct electric current and hence does not undergo any chemical change is called non-electrolyte.

Example :

Petrol, kerosene oil, diesel oil, vegetable oil, chloroform, carbon tetrachloride, alcohol, ether, benzene, distilled water etc.

(C) Electrolysis:

The process due to which a solution of a chemical compound conducts electric current and at the same time undergoes a chemical change is called electrolysis.

(D) Electrodes:

The metal wires/plates/rods through which the current enters or leaves an electrolyte are called electrodes.

(E) Cathode:

The electrode connected to the negative terminal of a cell/battery is called cathode.

(F)Anode:

The electrode connected to the positive terminal of a cell/battery is called anode.

(G) Ions :

The electrically charged atoms/group of atoms formed when a chemical compound is dissolved in water are called ions.

(H) Cations:

The positively charged ions formed, when a chemical compound dissolves in water are called cations. During electrolysis, the cations are discharged at cathode by taking electric charges from it.

(I) Anions :

The negatively charged ions formed, when a chemical ~ compound dissolves in water are called anions. During electrolysis, the anions are discharged at anode by losing electric charges to it.

(J) Voltameter :

An apparatus in which electrolysis is carried out, which consist of a vessel, two electrodes and electrolyte is called voltameter.

◊ ELECTROPLATING :

The process of depositing a thin layer of any superior metal over an object of a cheaper metal, with the help of electric current is called electroplating.
For example, deposition of silver on brass or copper objects and that of copper, nickel, chromium etc., on objects made of iron is done by electroplating.
Electroplating is done with the following purpose/objectives.

♦ For decoration purposes : Silver or gold plating of brassware such as flower vase.

♦ For preventing corrosion : Iron objects, such as, bathroom fittings etc., are electroplated with chromium.

 PROCESS OF ELECTROPLATING :

The process of electroplating involves the followings steps :

♦ Clean and wash the object to be electroplated thoroughly.

♦ The object to be electroplated is made cathode.

♦ A sheet of pure metal (to be electroplated) is made anode.

♦ The electroplating tank is filled with the solution of a salt of the metal to be electroplated.

♦ Connect the cathode to the negative (–) terminal and the anode to the positive (+) terminal of the battery.

♦ Pass the currnt for a certain time to deposite a thin layer of the metal.

The process of electroplating is illustrated below.

 ELECTROPLATING OF COPPER :

The experimental set up for electroplating copper on any object, such as a metallic spoon is shown in fig. given below :

 Class 8 Science Chemical Effects of Electric Current Exam Notes

Here,

♦ The object to be electroplated, say iron spoon is made cathode (–ve electrode).

♦ A thin sheet of pure copper is made anode (+ve electrode).

♦ An acidified solution of copper sulphate (CuSO4) is used as the electrolytic solution.

When electricity is passed through the solution, Cu—2+— ions move towards the cathode and get reduced to copper metal. This copper metal gets electroplated on the objects, i.e., metallic spoon.

The sulphate ions (SO42–) move towards anode. Here SO42–ions do not get oxidised. Instead, the copper metal of the anode gets oxidised to Cu2+ions. These copper ions (Cu2+) go into the solution. As a result of this reaction at the anode, concentration of Cu2+ ions in the solution is maintained.

At cathode :

 Class 8 Science Chemical Effects of Electric Current Exam Notes

→ APPLICATION OF ELECTROLYSIS

The principle of electrolysis is employed in the following processes :

♦ Manufacture of industrial chemicals

♦ Extraction of metals

♦ Refining of metals

♦ Electroplating

 MANUFACTURE OF INDUSTRIAL CHEMICALS

Many chemicals which are used in industry in large quantities are prepared by electrolytic method. Some of these are :

(i) Sodium hydroxide, NaOH (caustic soda) and Chlorine gas, Cl2(g) are prepared by the electrolysis of brine (20% common salt solution).

(ii) Hydrogen gas, H2(g) is prepared by the electrolysis of acidified water, or 20% , NaOH solution. Oxygen is obtained as by-product.

 EXTRACTION OF METALS–ELECTROMETALLURGY :

More electropositive metals, such as sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), aluminium (Al), etc. cannot be obtained by carbon reduction process. These metals can be obtained by the electrolysis of their molten chlorides, hydroxides or oxides.

For example,

♦ Sodium and potassium are obtained by the electrolysis of their molten chlorides and hydroxides.

♦ Calcuium and magnesium are obtained by the electrolysis of their molten chlorides

♦ Aluminium is obtained by the electrolysis of its molten oxide (in the presence of some other compounds).

 REFINING OF METALS :

The metals obtained by chemical reduction methods generally contain many impurities. Such metals can be refined very easily by electrolytic method. The method of purifying metals by using electricity is called electro refining. Metals, such as zinc, copper, silver, nickel, gold, aluminium, etc are refined by electrical
method.

→ MAGNETIC EFFECT OF CURRENT

We know that a moving magnet can generate an electric current. Also we can say that electric current can produce a magnetic effect. A coil, through which when an electric current flows behave like a magnet with its two poles.

Ex. Electric bell shows magnetic effect of current.

→  DANGER OF ELECTRICITY

(i) If the current happens to pass through the heart, it cause the heart muscles to contract and generally death occurs.

(ii) A strong electric shock can give the body a big shock that can damage the body cells. Such a shock occurs on touching a live electric wire suddenly.

(iii) Handling electrical appliances in wet places is very dangerous.

(iv) Electricity could turn dangerous due to loose connections in switches, improper wiring, overloading (i.e. passing excess current above the rated capacity), improper earthing.

→  SAFETY MEASURES IN USING ELECTRICITY

(i) The wires used in the circuit should be of good quality and with good insulation.

(ii) Defective and damaged plugs, sockets and switches must be immediately replaced.

(iii) All connections in plugs, switches and sockets must be made of a proper insulating material. (iv) Extension cords must not be overloaded and must be regularly tested.

(v) Switches and plugs should not be touched with wet hands.

(vi) The main switch should be immediately switched off in case of fire or short circuit. (vii)Always use a safety fuse of proper rating and material in an electric circuit.

(viii)All appliances must be properly earthed.

(ix) Rubber soled shoes should be worn while repairing electric circuit. This protect the body from) electric shocks.

(x) The inside of socket are made of conducting material and therefore must not be touched.

EXERCISE # 1

A→ Very Short Answer Type Questions

Q.1 What are conductors ?

Sol.1 Materials which allow electric current to flow through them are called conductors.

Q.2 What are insulators ?

Sol.2 Materials which do not allow electric current to flow through them are called insulators.

Q.3 Why is distilled water conductor or an insulator ?

 Sol.3 Distilled water is an insulator because there are no salts dissolved in it.

Q.4 What are electrodes ?

 Sol.4 Electrodes are the terminals through which current is passed into a liquid.

Q.5 What is an electrolyte ?

 Sol.5 An electrolyte is a liquid which is a good conductor of current.

Q.6 What happens when an electric current is passed through a conducting solution ?

 Sol.6 A chemical reaction takes place when electric current is passed through a conducting solution.

Q.7 Define electroplating.

 Sol.7 Electroplating is the process of depositing a layer of any desired metal on another metallic object by means of electricity.

Q.8 Apart from chemical effect, which other effect electric current produce ?

 Sol.8 Electric current can produce magnetic effect also.

Q.9 Name one non-metal which a good conductor of electricity ?

 Sol.9 Graphite

Q.10 What is electrolysis ?

 Sol.10 Electrolysis is the breaking up of a compound from its solution on passing electric current through the electrolyte.

Q.11 How can distilled water be made a good conductor of electricity ?

Sol.11 Distilled water can be made a good conductor of electricity by adding acid, base or salt to it.

Q.12 In what proportion the two products from electrolysis of water are obtained ?

 Sol.12 By volume of hydrogen gas is twice the volume of oxygen gas.

B→Short Answer Type Questions

Q.13 Name three liquids, which when tested in the manner shown in figure, may cause the magnetic needle to deflect.

Class 8 Science Chemical Effects of Electric Current Exam Notes

 Sol.13 The compass needle will show deflection with tap water, lemon juice and sodium chloride solution.

Q.14 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.

Sol.14 Firemen shut off the main electrical supply for the area because water is a good conductor of electricity and the firemen can get electrocuted.

Q.15 Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour ? Explain.

Sol.15 No, it is not advisable for wiremen to carry out electrical repairs during heavy downpour because water is a good conductor of electricity and the person can get shock.

Q.16 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 ?

Sol.16 The sea water contains more salts dissolved in it as compared to the tap water. So, the deflection of the compass needle is more.

Q.17 Why should you not touch electrical appliances with wet hands ?

Sol.17 Water is a solution of salts and so it is a good conductor of electricity. Therefore, we can be electrocuted. 

Q.18 How are bridges and automobiles prevented from rusting ?

Sol.18 Bridges and automobiles are prevented from rusting by plating them with zinc. 

Q.19 What is the disadvantage of electroplating done in the factories ?

Sol.19 The disposal of the used conducting solution is hazardous and can cause environmental pollution.

There are specific disposal guidelines for environmental protection.

Q.20 Why is chromium used for electroplating ?

 Sol.20 Chromium is used for electroplating because it has a shiny appearance, does not corrode and is scratch resistant.

Q.21 Why the iron cans are electroplated with tin?

Sol.21 Tin is less reactive than iron. Thus food, stored in iron cans, is prevented from being spoilt by the iron by electroplating it by tin.

Q.22 After the electroplating of a spoon with silver, it was found that the anode has become thin why ?

Sol.22 During electroplating, the silver metal from the anode is deposited on the cathode. Therefore, it becomes thin.

Q.23 The bulb does not glow in the setup shown in figure. List the possible reasons. Explain your answer.

Class 8 Science Chemical Effects of Electric Current Exam Notes

 Sol.23 It cannot be said for sure that liquid does not conduct electricity because :

 

(a) may be the cells are weak.

(b) may be the current is so weak that it does not heat the filament of the bulb, so that it can glow.

Q.24 The liquid is a conductor and the circuit is also complete, but the bulb does not glow. Why ?

Sol.24 The bulb does not glow even though the circuit is complete because the current is too small. The filament of the bulb is heated due to current and then it glows.

Q.25 Explain the process of electroplating of copper.

Sol.25 When electric current is passed through copper sulphate solution, the copper sulphate breaks up into copper and sulphate ions. The free copper gets drawn to the plate connected to the negative terminal of the battery and gets deposited on that plate. From the other plate an equal amount of copper gets dissolved in the solution. The loss of copper from solution is compensated and the process goes on.

Q.26 During electrolysis of water, why does hydrogen collect on cathode and oxygen collect on anode ?

Sol.26 When electric current is passed in water hydrogen ions (H+) move towards cathode and get collected over it. Hydroxyl ions (OH–) move towards the anode and oxygen is collected on it.

EXERCISE # 2

→Single Correct Answer type Questions

Q.1 The smallest particle of matter is ..........

(A) an electron

(B) a proton

(C) a neutron

(D) a atom

Q.2 The neutron carries a charged of .........

(A) 1.66 × 10–19 Coulomb

(B) –1.66 × 10–-19 Coulomb

(C) no charge

(D) none of these

Q.3 Static electricity ..............

(A) does not flow

(B) flows in the circuit

(C) sometimes flows

(D) none of these

Q.4 The lightning occurs due to ...........

(A) static electricity

(B) dynamic electricity

(C) thunder

(D) none of these

Q.5 Plastic and rubber are ...........

(A) charged bodies

(B) conductors

(C) insulators

(D) none of these

Q.6 Which of the following is a good conductor of electricity ?

(A) wood

(B) steel spoon

(C) dry air

(D) chalk

Q.7 Which of the following is a bad conductor of electricity ?

(A) gold

(B) mercury

(C) silver

(D) plastics

Q.8 Electric charge can flow only through

(A) insulators

(B) conductors

(C) both (A) and (B)

(D) None of these

Q.9 Two objects rubbed against each other

(A) will lose electrons

(B) will repel each other

(C) will attract each other

(D) may attract or repel each other

Q.10 The gold leaf electroscope can be used to

(A) detect charge only

(B) detect or measure charge only

(C) detect, measure and find the nature of charge

(D) none of these

Q.11 Which of the following can be charged with static electricity ?

(A) metal

(B) alloy

(C) insulator

(D) semiconductor

Q.12 When two bodies are rubbed against each other, they acquire 

(A) equal and like charges 

(B) equal and unlike charges 

(C) unequal and like charges 

(D) unequal and unlike charges 

Q.13 The two objects rubbed against each other 

(A) will lose electrons 

(B) will gain electrons 

(C)one will lose and the other will gain electron 

(D) none of these 

Q.14 A positively charged ion is called 

(A) atom

(B) anion

(C) cation

(D) neutral ion

Q.15 The instrument needed to measure the current flowing through a circuit is ?

(A) voltmeter

(B) voltameter

(C) galvanometer

(D) ammeter

Q.16 The process in which any electrolyte gets decomposed when electricity is passed through it, is called

(A) electrolysis

(B) decomposition

(C) dissociation

(D) splitting

Q.17 The process of depositing a thin layer of any superior metal over an object of a cheaper metal with the help of electricity is called

(A) electrorefining

(B) electrometallurgy

(C) electroplating

(D) electrowinning

Q.18 Which of the following is the most essential for an electric circuit to work ?

(A) A switch

(B) An electric current

(C) A switch board

(D) none of these

Q.19 The connecting wires used in an electric circuit are usually made of

(A) gold

(B) silver

(C) copper

(D) plastic

Q.20 Pure or distilled water is a/an

(A) conductor

(B) insulator

(C) partial conductor

(D) none of these

Q.21 Water supplied in households and normal water is a/an

(A) conductor

(B) insulator

(C) partial conductor

(D) none of these

Q.22 Which of the following liquids is a conductor of electricity ?

(A) Kerosene

(B) Cooking oil

(C) Alcohol

(D) Vinegar

Q.23 In an electrolyic cell, the electrode which is connected to the positive terminal of a battery is called

(A) Anode

(B) Cathode

(C) Antinode

(D) None of these

Q.24 Electrostatics deals with the study of the

(A) Charges at rest

(B) Charges in motion

(C) Charges both at rest and in motion

(D) Charges neither at rest nor in motion

Q.25 The surest test for a body to be a charged body or not is

(A) Attraction

(B) Repulsion

(C) Both attraction and repulsion

(D) Neither attraction nor repulsion

Q.26 A body possessing an equal number of positive and negative charges is

(A) Neutral

(B) Negatively charged

(C) Positively charged

(D) None of these

Q.27 In induction, the charge possessed by the charging body and charged body are

(A) Different

(B) Same

(C) Can be same or different

(D) Both the bodies become neutral

Q.28 In induction, the transfer of charges takes place by

(A) Touching

(B) Rubbing

(C) From a distance

(D) Either by rubbing or touching

Q.29 The S.I. unit of current is –

(A) Ampere

(B) Volt

(C) Ohm

(D) Mho

Q.30 Which of the following is a conductor ?

(A) Salt solution

(B) Sugar solution

(C) Mica

(D) Plastic, PVC

Q.31 The charge carried by cation is

(A) positive

(B) negative

(C) positive or negative

(D) None of these

Q.32 During purification of metals, the refined metal is obtained at the

(A) cathode

(B) anode

(C) surface of electrolyte

(D) None of these

Q.33 During electroplating, the pure metal is deposited at the

(A) Cathode

(B) Anode

(C) Both cathode and anode

(D) At bottom of the electrolytic cell

Q.34 In an electrolytic cell, the electrode that is connected to the positive terminal of the battery is called

(A) Cation

(B) Cathode

(C) Anion

(D) Anode

Q.35 The process by which a chemical change takes place in a substance when electric current is passed through it is called

(A) Electrolysis

(B) Electroplating

(C) Electrodes

(D) Thermionic conduction

Q.36 Adding a soluble metallic salt to water

(A) Increases its electrical conductivity

(B) decreases its electrical conductivity

(C) never produces any change in the conductivity

(D) none of these

Q.37 Electroplating is a method of

(A) making plates using electricity

(B) plating a metal with another metal

(C) coating any object with an electrically conducting plate

(D) coating a metal with another metal by passing an electric current

Q.38 An electrolyte is

(A) a light electric cell

(B) a liquid that conducts electricity

(C) a metal

(D) none of these

Q.39 When electric current is passed through acidulated water, the gases produced are

(A) hydrogen and oxygen

(B) hydrogen and ozone

(C) oxygen and hydrogen perioxide

(D) None of these

Q.40 The object to be electroplated is made

(A) cathode

(B) anode

(C) cathode or anode

(D) None of these

Q.41 The method of purifying metals by passing electricity is called

(A) electrolysis

(B) electroplating

(C) electrorefining

(D) None of these

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