AP 8th Class Physics 8th Lesson Questions and Answers Chemical Effects of Electric Current

AP State Board new syllabus 8th Class Physics Guide 8th Lesson Synthetic Fibres and Plastics Textbook Exercise Questions and Answers.

AP 8th Class Physical Science 8th Lesson Questions and Answers Synthetic Fibres and Plastics

8th Class Physics 8th Lesson Synthetic Fibres and Plastics Questions and Answers (Exercise)

1. Fill in the blanks.

a) Most liquids that conduct electricity are solutions of …………… and ……………
b) The passage of an electric current through a solution causes …………… effects.
c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the …………… terminal of the battery.
d) The process of depositing a layer of any desired metal on another material by means of electricity is called ……………
Answer:
a) Acids, bases, salts.
b) Chemical
c) Negative
d) Electroplating

Question 2.
When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?
Answer:

• When the free ends of a tester are dipped into a solution, the connecting wire gets the magnetic property while passing a current through it.
• This magnetism influences the needle of the compass.
• Hence, the magnetic needle shows deflection.

Question 3.
Name three liquids, which when tested in the manner shown in Fig., may cause the magnetic needle to deflect.
Answer:

• Copper sulphate solution
• Salt solution
• Dill. hydrochrolic acid

Question 4.
The bulb does not glow in the setup shown in Fig. List the possible reasons. Explain your answer.
Answer:

• The liquid may be a poor conductor. Poor conductors like sugar solutions do not allow to conduct of electricity. Hence circuit is open.
• Batteries may be discharged or not in working condition. It causes no electrical supply in the circuit.

• The bulb may be damaged. It causes to open circuit.
• There may be breaks in the connecting wires. It causes to open circuit.
• Circuit may be switched off. It also causes to open circuit.

Question 5.
A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that
i) liquid A is a better conductor than liquid B.
ii) liquid B is a better conductor than liquid A.
iii) both liquids are equally conducting.
iv) conducting properties of liquid cannot be compared in this manner.
Answer:
i) liquid A is a better conductor than liquid B.

Question 6.
Does pure water conduct electricity? If not, what can we do to make it conducting?
Answer:

• Pure water does not conduct electricity.
• By dissolving a salt or an acid or a base we can make it conduct.

Question 7.
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.
Answer:

• Water conducts electricity.
• The firemen may get an electric shock if sprinkling water touches the electrical wires or panel boards.
• So, they shut off the main electrical supply for the area.

Question 8.
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?
Answer:

• Sea water contains more salts than the drinking water.
• Salty sea water solution can pass more current than the drinking water.
• So, the compass needle deflects more in the case of seawater.

Question 9.
Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
Answer:

• It is not safe for the electrician to carry out electrical repairs outdoors during heavy downpour.
• Due to pollution, oxides of the nitrogen, sulphur in the atmosphere, dissolve into the rain water and cause to acid rains.
• So, the rain water conducts electricity as it is acidic in nature.
• The acid rain water will cause electric shock if an electrician carry out electrical repairs outdoors during heavy downpour.
• There may be breaks in the connecting wires. It causes to open circuit.
• Circuit may be switched off. It also causes to open circuit.

Question 10.
Paheli had heard that rainwater is as good as distilled water. So, she collected some rainwater in a clean glass tumbler and tested it using a tester. To her surprise, she found that the compass needle showed deflection. What could be the reasons?
Answer:

• The rainwater could be acidic in nature as it mixed with oxides of sulphur, nitrogen etc.
• The acidic nature of water conducts electricity.
• So, the compass needle showed deflection.

Question 11.
Prepare a list of objects around you that are electroplated.
Answer:
The list of objects that are electroplated :

• cycle handlebar (chromium coating on iron)
• ornaments (gold coating on copper)
• key chains (chromium or nickel coating on iron)
• tin cans (tin coating on iron)
• medals (gold or silver coating on iron
  rods in the bus (chromium coating on iron)
• door handles (copper coating on iron)
• nickel coating on iron nails bolts etc.
• zinc coating on railings of bridges
• vessels (copper coating on aluminum).

Question 12.
The process that you saw in Activity 7 is used for the purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transferred to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?
Answer:

• The thin pure copper plate should be attached to the negative terminal of the battery and the impure copper rod to the positive terminal.
• Then the copper metal separates from the impure copper rod and deposits on the pure copper rod which is connected to the negative terminal.

Extended Learning – Activities and Projects

Question 1.
Test the conduction of electricity through various fruits and vegetables. Display your result in a tabular form.
Answer:

Name of the fruit/vegetable	Good conductor/poor conductor
orange	Good conductor
banana	Poor conductor
papaya	Poor conductor
apple	Good conductor
peach	Good conductor
grapes	Good conductor
pineapple	Poor conductor
lemon	Good conductor
onion	Poor conductor
cabbage	Poor conductor
tomato	Good conductor
carrot	Good conductor
raddish	Good conductor
cauliflower	Poor conductor
potato	Good conductor

Question 2.
Repeat Activity 7 with a zinc plate in place of the copper plate connected to the negative terminal of the battery. Now replace zinc plate with some other metallic object and again repeat the activity. Which metal gets deposited over which other metal? Discuss your findings with your friends.
Answer:

• Layer of pure copper deposits on zinc plate.
• If we use iron, tin, lead plates instead of copper plate, then also layer of copper deposits over.
• If we use silver or gold copper layer does not form.
• Reason : Metals like zinc, tin, lead, iron are more reactive than copper whereas silver and gold are low reactive than copper.

Question 3.
Find out if there is a commercial electroplating unit in your town. What objects are electroplated there and for what purpose ? (The process of electroplating in a commercial unit is much more complex than what we did in Activity 7). Find out how they dispose off the chemicals they discard.
Answer:

• There is a unit of chromium plating situated in our town. Company people are called as chrome plating.
• Chrome plating is a technique of electroplating a thin layer of chromium onto a metal object.
• The product of chrome plating is called chrome. The chromed layer can be decorative, provide corrosion resistance, ease cleaning procedures, or increase surface hardness.
• The products are bolts, nuts, rods etc.

Disposal of waste chemicals :

• Plating waste water contains heavy metals, oil and grease and suspended solids at levels that might be considered hazardous to the environment and could pose risks to public health.
• The plating process involves, alkaline cleaning, acid pickling, plating and rinsing.

Question 4.
Imagine that you are an ‘entrepreneur’ and have been provided a loan by a bank to set up a small electroplating unit. What object would you like to electroplate and for what purpose ? (Look up the meaning of ‘entrepreneur’ in a dictionary)
Answer:

• Now day gold plating jewellery has more demand.
• So, I will establish a gold plating unit.

Question 5.
Find out the health concerns associated with chromium electroplating. How are people trying to resolve them?
Answer:
Health problems : Chromium can cause irritate the nose, throat and lungs. It can damage the mucous membranes of the nasal passages also and lead to ulcers. Repeated skin contact with chromium dust can lead to dermatitis with edema. The following precautions are being taken by them to resolve the problems.

• The workers wear the proper masks while they working.
• They wash their hands before meals with soap.
• They also practice breathing exercises.
• They drink plenty of water.

Question 6.
You can make a fun pen for yourself. Take a conducting metal plate and spread a moist paste of potassium iodide and starch. Connect the plate to a battery as shown in Fig. Now using the free end of the wire, write a few letters on the paste. What do you see?
Answer:

• This happens due to electrolysis of potassium iodide solution which produces lodine.
• Iodine on reaction with starch produce ink of blue color.

8th Class PS 8th Lesson Questions and Answers (Lab Activities)

Activity – 1

How do you check the tester before using whether it is working or not?
Answer:

• Make a tester by using cell, connecting wires and bulb etc.
• Join the free ends of the tester together for a moment.
• This completes the circuit of the tester and the bulb should glow.
• However, if the bulb does not glow, it means that the tester is not working.

• Check that all the connections are tight. If they are, then replace the bulb with another bulb.
• Now test if the tester is working or not.
• If it is still not working then replace the cells with fresh cells.

Activity – 2

How do you check whether liquids conduct electricity or not?
Answer:

• Collect a few small plastic or rubber caps of discarded bottles and clean them.
• Pour one teaspoon of lemon juice or vinegar in one cap.
• Bring your tester over this cap and let the ends of the tester dip into lemon juice or vinegar.
• Take care that the ends are not more than 1cm apart but at the same time do not touch each other.

• The bulb of the tester glows.
• This means lemon juice or vinegar conducts electricity.
• When the liquid between the two ends of the tester allows the electric current to pass, the circuit of the tester becomes complete.
• The current flows in the circuit and the electricity in lemon juice or vinegar bulb glows.
• When the liquid does not allow the electric current to pass, the circuit of the tester is not complete and the bulb does not glow.

(Note: Sometimes if the current through a circuit is too weak, the filament does not get heated sufficiently and it does not glow.)

Activity – 3

Can we make a tester using the magnetic effect of currents ?
(OR)
How can you make an alternate tester by using the magnetic effect of currents ?
(OR)
Write the procedure of testing various liquids for the conductivity of electricity.
Answer:

• Take the tray from inside a discarded matchbox.
• Wrap an electric wire a few times around the tray.
• Place a small compass needle inside it.
• Now connect one free end of the wire to the terminal of a battery. Leave the other end free.

• Take another piece of wire and connect it to the other terminal of the battery.
• Join the free ends of two wires momentarily.
• The compass needle should show deflection.
• Now the tester with two free ends of the wire is ready.
• Now repeat the activity for testing vinegar and lemon juice using this tester.
• We can find a deflection in the compass needle the moment you dip the free ends of the tester in lemon juice as well as vinegar.
• Take out the ends of the tester from the lemon juice, dip them in water and then wipe them dry.
• Repeat the activity with other liquids such as tap water, vegetable oil, milk, honey.
• Remember to wash and wipe dry the ends of tester after testing each liquid.
• In each case observe whether the magnetic needle shows deflection or not. Record your observations in Table.

S.No.	Material	Compass Needle Shows Deflection Yes / No	Good Conductor / Poor Conductor
1.	Lemon Juice	Yes	Good Conductor
2.	Vinegar	Yes	Good Conductor
3.	Tap Water	Yes	Good Conductor
4.	Vegetable oil	No	Poor Conductor
5.	Milk	Yes	Good Conductor
6.	Honey	No.	Poor Conductor
7.
8.
9.
10.

Activity – 4

How do you prove that distilled water does not conduct electricity ?
(OR)
Does the salt water allow electricity? How do you prove it?
Answer:

• Take about two teaspoonfuls of distilled water in a clean and dry plastic or rubber cap of a bottle.
• Use the tester to test whether distilled water conducts electricity or not.
• Distilled water does not conduct electricity.
• Now dissolve a pinch of common salt in distilled water.
• Again test.
• The water mixed with salt conducts electricity.
• When salt is dissolved in distilled water, we obtain the salt solution. This is a conductor of electricity.
• Conclusion : Distilled water does not conduct electricity.

Activity – 5

How do you verify that the dissolving of substances like acids, bases and salts in the distilled water, makes it a good conductor?
Answer:

• Take three clean plastic or rubber caps of bottles.
• Pour about two teaspoonfuls of distilled water in each of them.
• Add a few drops of lemon juice or dilute hydrochloric acid to distilled water in one cap.
• Now in the second cap containing distilled water, add a few drops of a base such as caustic soda or potassium iodide.
• Add a little sugar to the distilled water in the third cap and dissolve it.
• Test which solutions conduct electricity and which do not.
• We can observe that electricity conducts through the solutions of lemon juice, dilute hydrochloric acid,caustic soda and potassium iodide but not through the sugar solution.
• Conclusion : Most liquids that conduct electricity are solutions of acids, bases and salts.

Activity – 6

How do you demonstrate the chemical effect of electricity
Answer:

• Take out carbon rods carefully from two discarded cells.
• Clean their metal caps with sand paper.
• Wrap copper wires around the metal caps of the carbon rods and join them to a battery.
• We call these two rods electrodes.

• Instead of carbon rods, you may take two iron nails about 6 cm long.
• Pour a cupful of water into a glass/plastic bowl.
• Add a teaspoonful of salt or a few drops of lemon juice to the water to make it more conducting.
• Now immerse the electrodes in this solution.
• Make sure that the metal caps of the carbon rods are outside the water.
• Wait for 3-4 minutes.
• Observe the electrodes carefully.
• You can notice gas bubbles near the electrodes.
• The change taking place in the solution is a chemical change.
• 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.
• Deposits of metal may be seen on electrodes.
• Changes in the colour of solutions may occur.
• The reaction would depend on what solution and electrodes are used.
• These are some of the chemical effects of the electric current.

Activity – 7

Write the procedure of electroplating by using copper sulphate.
(OR)
Explain electroplating through an activity.
Answer:

• Take copper sulphate and two copper plates of size around 10cm × 4cm
• Take 250m 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 the copper sulphate solution to make it more conducting.

• Clean copper plates with sand paper.
• Now rinse them with water and dry them.
• Connect the copper plates to the terminals of a battery and immerse them in copper sulphate solution.
• Allow the current to pass for about 15 minutes.
• Now remove the electrodes from the solution and look at them carefully.
• Do you find any difference in any one of them?
• Do you find a coating over it?
• What colour is the coating ?
• Note down the terminal of the battery with which this electrode is connected.
• When an electric current is passed through the copper sulphate solution, copper sulphate dissociates into copper and sulphate.
• The free copper gets drawn to the electrode connected to the negative terminal of the battery and gets deposited on it.

• From the other electrode, a copper plate, an equal amount of copper gets dissolved in the solution.
• Thus, the loss of copper from the solution is restored and the process continues. This means that copper gets transferred from one electrode to the other.