Slm continued pg13

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After doing the electroplating activity, Mysteri interchangedthe electrodes and repeatedthe activity. What do you thinkshe would observe this time? Fig.-8 : Some electroplated objects

When electric current is passedthrough the copper sulphate solution,copper sulphate dissociates into copper and sulphate. The free coppergets drawn to the electrode connectedto the negative terminal of the batteryand gets deposited on it. But whatabout the loss of copper from thesolution?From the other electrode, a copperplate, an equal amount of copper getsdissolved in the solution. Thus, theloss of copper from the solution isrestored and the process continues.This means that copper getstransferred from one electrode to the other.

FACT: In the electroplating factories thedisposal of the used conductingsolution is a major concern. It is apolluting waste and there are specificdisposal guidelines to protect theenvironment.

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Panacea could get only onecopper plate. So heperformed Activity.7 byconnecting a carbon rod inplace of the copper platewhich was connected tothe negative terminal ofthe battery. He succeededin obtaining a coating ofcopper on carbon rod.

The process of depositing a layer ofany desired metal on another materialby means of electricity is calledelectroplating. It is one of the mostcommon applications of chemical effects of electric current.Electroplating is a very usefulprocess. It is widely used in industryfor coating metal objects with a thinlayer of a different metal (Fig.14.8). The layer of metal deposited has somedesired property, which the metal of the object lacks. For example, chromium plating is done on many objects such as car parts, bath taps, kitchen gas , burners, bicycle handlebars, wheel rimsand many others. Chromium has a shiny appearance. It does not corrode. It resists scratches. However, chromium is expensive and itmay not be economical to make thewhole object out of chromium. So the

object is made from a cheaper metal and only a coating of chromium over it is deposited. Jewellery makers electroplatesilver and gold on less expensive metals.These ornaments have the appearanceof silver or gold but are much lessexpensive.Tin cans, used for storing food, aremade by electroplating tin onto iron. Tinis less reactive than iron. Thus, fooddoes not come into contact with iron andis protected from getting spoilt.Iron is used in bridges andautomobiles to provide strength.However, iron tends to corrode and rust.So, a coating of zinc is deposited on ironto protect it from corrosion andformation of rust.

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CHEMICAL EFFECTSOF CURRENT WHAT MAKES WATER?

Neera was reading a book on science experiments. It said that water was made up of two gases-hydrogenand oxygen. Neera got herself a glass of water and looked at it carefully. 'How is that possible'? WonderedNeera. Water is a liquid but hydrogen and oxygen are gases.Next day she asked her science teacher about it. The science teacher asked her to collect a few thingsfrom home. The teacher told her that they will be doing an experiment to understand this better. You toojoin in to unravel the mystery about water.

YOU WILL NEED:Two used cells (torch cells), a 9V battery, two metre copper wire, plastic container, hammer, plaster ofparis, two empty test tubes of equal size.

WHAT TO DO:1. Remove the carbon rods from the used cells. (Ask an adult to help you with this)2. Make holes at the bottom of the plastic container to fit the carbon rods.3. Use plaster of paris paste (by mixing it in water) to block leakage of water where the carbon rodsare fixed.4. Fill the bowl upto 3/4th with water.5. Connect the wire to the ends of the carbon rods below the plastic container as shown.

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6. Fill two test tubes with water.7. Carefully invert them over the carbon rods, one on each rod. This must be done in such a way thatwater does not escape out of the test tube and no air enters the test tube.8. Now connect the two carbon rods to the 9V battery.9. Watch carefully at the two rods. What do you notice?

WHAT I HAVE LEARNT:

When electric current passes through water, it decomposes to hydrogen and oxygen. Hydrogencollects at negative terminal and oxygen at positive terminal. The volume of these two gases formedsuggests that the water is formed by hydrogen and oxygen combined together in 2: 1 ratio.

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WHAT YOU HAVE LEARNT :

Some liquids are good conductors of electricity and some are poor conductors.

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

The passage of an electric current through a conducting liquid causes chemical reactions.

The resulting effects are called chemical effects of currents. The process of depositing a layer of any desired metal on

another material, by means of electricity, is called electroplating.

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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 getsdeposited on the plate connected to the terminal of thebattery.(d) The process of depositing a layer of any desired metal on anothermaterial by means of electricity is called .

2. When the free ends of a tester are dipped into a solution, the magneticneedle shows deflection. Can you explain the reason?

3. Name three liquids, whichwhen tested in the mannershown in Fig.9, maycause the magnetic needleto deflect.

EXERCISES

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.

4. The bulb does not glow in the setup shown in Fig.10. List the possiblereasons. Explain your answer.

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 forliquid 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.

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

FIG-9.

FIG - 10.

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conducting?

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.

8. A child staying in a coastal region tests the drinking water and also theseawater with his tester. He finds that the compass needle deflects morein the case of seawater. Can you explain the reason?

9. Is it safe for the electrician to carry out electrical repairs outdoors duringheavy downpour? Explain.

10. Mystery had heard that rainwater is as good as distilled water. So shecollected some rainwater in a clean glass tumbler and tested it using atester. To her surprise she found that the compass needle showed deflection. What could be the reasons?

11. Prepare a list of objects around you that are electroplated.

12. The process that you saw in Activity-7 is used for purification of copper.A thin plate of pure copper and a thick rod of impure copper are used aselectrodes. Copper from impure rod is sought to be transfered to the thincopper plate. Which electrode should be attached to the positive terminalof the battery and why?

EXERCISES

Extended Learning — Activities and Projects

Test the conduction of electricity through various fruits andvegetables. Display your result in a tabular form.2. Repeat Activity 14.7 with a zinc plate in place of the copper plateconnected to the negative terminal of the battery. Now replace zincplate with some other metallic object and again repeat the activity.Which metal gets deposited over which other metal? Discuss yourfindings with your friends.3. Find out if there is a commercial electroplating unit in your town.What objects are electroplated there and for what purpose? (Theprocess of electroplating in a commercial unit is much more complexthan what we did in Activity-7). Find out how they dispose off thechemicals they discard.4. Imagine that you are an ‘entrepreneur’ and have been provided aloan by a bank to set up a small electroplating unit. What objectwould you like to electroplate and for what purpose? (Look up the

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electronics.howstuffworks.com/led.htm

BIJLI 2

BIJLI 3

https://www.google.co .

LEDs (Light Emitting Diodes)are available in many colours such as red, green, yellow, blue, white and are increasingly being used for many applications, for example in traffic signal lights.LEDs are increasingly being used for lighting. Acluster of white LEDs grouped together forms aLED light source. LED light sources consume lesselectricity and have longer lifetime than light

EXERCISES

Test the conduction of electricity through various fruits andvegetables. Display your result in a tabular form.2. Repeat Activity 14.7 with a zinc plate in place of the copper plateconnected to the negative terminal of the battery. Now replace zincplate with some other metallic object and again repeat the activity.Which metal gets deposited over which other metal? Discuss yourfindings with your friends.3. Find out if there is a commercial electroplating unit in your town.What objects are electroplated there and for what purpose? (Theprocess of electroplating in a commercial unit is much more complexthan what we did in Activity-7). Find out how they dispose off thechemicals they discard.4. Imagine that you are an ‘entrepreneur’ and have been provided aloan by a bank to set up a small electroplating unit. What objectwould you like to electroplate and for what purpose? (Look up the

FOR FURTHER READING ON THIS TOPIC VISIT…

DIDI YOU KNOW??

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bulbs and fluorescent tubes. But LED lightsources are expensive, so CFLs are currently thebest choice. However, CFLs contain mercurywhich is toxic. Therefore, used or broken CFLsneed to be disposed off safely. Once thetechnological advances reduce the cost of LEDs,they will become the preferred lighting source.