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Subject:

science-chemistry

Class:

class VIII

Key Concepts

ions, electroplating, electrolysis, insulator, cathode, anode, electrolyte, conductor

essential questions

1. What happens when electric current is passed through a substance and how does it bring in chemical change in the substance?

Remembering:

  1. Which of the following is an insulator? (a) Human body (b) Graphite  (c) Pure water   (d) Copper
  2. The extraction of metals from their ores by electrolysis is called: (a) electroplating (b) electrometallurgy  (c) electro refining  (d) none
  3. The refining or purification of metals by passing electric current is called: a) electroplating (b) electrolysis (c) electro refining  (d) none
  4. Carbon rods, used as electrodes are made up of: (a) graphite (b) wood (b) iron  (d) diamond
  5. Which of these are the applications of electrolysis? (a) Metal Extraction (b) Electroplating (c) Electro refining  (d) All these
  6. Which of these is plated on the iron to prevent it from chemically reacting with food? (a) Gold (b) Silver  (c) Chromium  (d) Tin
  7. The process of producing chemical decomposition of a compound by passing electricity through the compound is……………. a) electrolysis b) electroplating c) electrolyte  d) None of the above
  8. Which of the following is bad conductor of electricity? a) Distilled water b) alcohol  c) Salt water  d) tap water
  9. Plastic wire is : a) An insulator  b) A conductor  c) Both of these
  10. Electroplating is based on a) Magnetic effect of electricity  b) Chemical effect of electricity  c) heating effect of electricity  d) Physical effect of electricity
  11. Adding common salt to distilled water makes it: a) Good conductor b) insulator  c) None  d) both a and b
  12. An electrolyte is___.  a) metal  b) a solution c) a liquid that conducts current  d) all of above
  13. Copper wire is a ___.  a) good conductor  b) Poor conductor  c) both a and b
  14. Poor conductors are____ a) Plastics  b) clothes  c) wood   d) all of these
  15. Distilled water is___ : a) Poor conductor  b) good conductor  c) Both a & b   d) none
  16. Flow of electron is called____ a) electroplating  b) electric current  c) electrodes  d) electrolyte
  17. Electroplating prevents :  a) current  b) chemical effect  c) rusting   d) all
  18. The filament used in electric heater is of high resistance? a) true b) false
  19. Fill in the blanks:  1. Electroplating is based on the ______effect of electric current.
  1. Wood is a—— conductor of electricity.
  2. Pencil lead is a_____
  3. The flow of electrons is called——.
  4. Tap water is a_____ conductor of electricity.
  5. Tap water conducts electricity because it has ______in it.
  6. The process used to purify a metal is called ______.

 

 

understanding:

  1. In electroplating, the object to be electroplated is made: a) anode (b) cathode (c) none  (d) both
  2. Out of the following, which metal is used to electroplate iron to make it shining and rust resistant? (a) Chromium  (b) Gold (c) Zinc  (d) Copper
  3. In an electrolytic cell, the electrode which is connected to the positive terminal of the battery is called: a) cation (b) anion  (c) cathode (d) anode
  4. What is resistance of a conductor?  a) When there are more atoms and molecules in conductor than free electrons, so as to restrict movement of electrons on getting current    b) When there are more free electrons than molecules and there is no restriction in movement of electrons on getting current  c) When conductor has equal amount of free electrons and atoms and molecules
  5. When electric current is passed through copper solution, the copper deposits on the electrode connected to  a) positive terminal of battery  b) negative terminal of battery  c) any terminal can be chosen
  6. An electric lamp glows due to__: a) Chemical effect  b) magnetic effect  c) heating effect  d) None
  7. What is resistance of a conductor? a) when there are more atoms and molecules in conductor than free electrons so as to restrict movement of electrons on getting current  b) When there are more free electrons than molecules and there is no restriction in movement of electrons on getting current  c) When conductor has equal amount of free electrons and atoms and molecules
  8. Which of the following statement is incorrect a) Anode is an electrode connected to positive terminal of battery  b) Pure water is poor conductor of electricity  c) Electrolysis is used in refining of impure metals  d) ions can have only a positive charge
  9. Which of the following is correct: a) Water can be used for extinguishing fires caused due to electrical faults  b) carbon is non metal so it cannot be used in electrolytic cell  c) A liquid conducts electricity because of presence of ions.  d) Pure water forms ions to conduct electricity
  10. Which of the following cells uses electric current to produce chemical reaction: a) dry cell  b) solar cell  c) electrolytic cell  d) Both dry cell and electrolytic cell
  11. State whether True or False:
    1. Chromium corrodes easily.
    2. The sugar solution is an electrolyte.
    3. Cations are negatively charged particles.
    4. Electrolytes produce ions when dissolved in water.
    5. A cycle rim is made of iron that is electroplated with chromium.
    6. Touching an electrical appliance with wet hands can be very dangerous.
    7. Tin cans are the tin electroplated on iron

Application:

  1. Tin cans are used to preserve food items. Why not iron cans used without Tin coating? a) Tin is less reactive than Iron b) Tin is more reactive than Iron c) Tin is cheaper than Iron d) Tin is lighter than Iron
  2. Why is mica sheets used in an iron instruments? A) mica is good conductor of electricity. B) mica is a bad conductor of electricity and heat   c) mica is a bad conductor of electricity but good conductor of heat. D) Mica is very light and bad conductor of electricity
  3. Which property of the electricity is responsible for use of Fuse wire in household wiring? A) Chemical effect b) heating effect  c) Magnetic effect  d) All of above
  4. When current is passed through calculated water, the Hydrogen and Oxygen separates. The hydrogen gets accumulated on the negative electrode. Why does it not deposit on positive electrode?  a) Hydrogen is positive charge   b) Hydrogen is negatively charged   c) That is because , it is the characteristic of acidulated water  d) None of the above
  5. Electrolysis of a solution of sodium chloride produces: a) Hydrogen gas at anode and chloride at cathode  b) Hydrogen gas at cathode, chlorine at anode  and sodium hydroxide  c) Hydrogen gas at anode and sodium hydroxide at cathode  d) chlorine gas at anode and sodium hydroxide at cathode
  6. In which of the following solution will the bulb glow: a) Kerosene   b) sugar solution   c) distilled water   d) sodium chloride
  7. In which one of the following situation does a conventional electric current flow due north: a) Protons in a beam are moving due North  b) Water molecule is moving due north  c) Electron in a beam are moving due south  d) none

Analysis:

  1. In the given figure electrolyte, anode, cathode respectively are:                                                 a) A, B,C    b) B, A, C     c) B, C, A     d) A,C, B
  2. The diagram shows a beaker containing a solution of zinc sulphate and two carbon electrodes. A battery is placed next to it. In order that the electrode P be plated with zinc. a) P must be connected to S and Q to R  b) P must be connected to Q and S to R  c) P must be connected to R and Q to S  d) P and Q be connected to R

5. In the experiment, if A is plated with copper, the anode would be         a) A   b) B  c) C d) data insufficient

6. When electricity is passed through calcium nitrate solution in water, what is likely to be formed at the cathode and why? a) Hydrogen because it is very reactive than calcium   b) Calcium because it is very reactive than hydrogen  c) Hydrogen because it is less reactive than calcium  d) calcium because it is less reactive than hydrogen

7. An electric current is passed through a conducting solution.Statement1:  Deposits of metal may be seen on electrodes  Statement 2: Solution may get heated  Statement 3: Bubbles of gas may be formed on the electrode  Following are some of the observation:   a) Only Statement 2 is true    b) Only Statement 1 and Statement 2 are true c) Only statement 1 and statement 3 are true    d) All of them are true

8. In the given filter paper contains starch soaked in potassium, Iodine solution is connected to the battery as shown. When we write on wet paper, it has blue black color.    Statement 1: Iodine is formed at the  anode   Statement 2: Iodine turn blue black in the presence of starch        a) Only 1 is correct  b) Only 2 is correct  c) Both 1 and 2 are correct  d) Neither 1 nor 2 are correct

9. There are two different sets of solution P and Q. The bulb is up P glows more brightly as compared to that of Q. What are possible causes: 1) The connection of circuit Q maybe loose  2) The liquid Q may have small conductivity  3) Liquid in P is equivalent to a battery while liquid in Q is equivalent to a cell of the battery of liquid P.                    a) Only 1 and 2   b) Only 2 and 3  c) Only 1  d) only 2

10. Four substances were tested for their electrical conductivity. The results are shown in the table:

  Substance            Bulb              Substance at cathode                  Substance at anode

P                            lights up                   Aluminium                                          Nothing

Q                                lights up                    Nothing                                           Nothing

R                       does not light up              Hydrogen                                        carbon dioxide

S                          lights up                           Hydrogen                                        carbon dioxide

Which solution could be alumina and magnesium nitrate?    a) P and Q     b)  R and Q    c) P and S     d) P is the alumina but none is magnesium nitrate

11.  The diagram shows a plotting compass placed above a copper wire when there is no current flowing in the wire. What happened to compass needle when a large direct current flows from point A to point B?                      a) compass needle points towards right   b) compass needle points towards left   c) compass needle points upwards  d) compass needle points downward

Evaluation:

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