CHEMICAL EFFECTS OF ELECTRIC

CURRENT

CHEMICAL EFFECTS OF ELECTRIC

CURRENT

Your elders might have cautionedyou against touching an electricalappliance with wet hands. But

do you know why it is dangerous totouch an electrical appliance with wethands?

We have learnt earlier that thematerials, which allow electric currentto pass through them, are goodconductors of electricity. On the otherhand, materials, which do not allowelectric current to pass through themeasily, are poor conductors of electricity.

In Class VI, we made a tester(Fig.14.1) to test whether a particularmaterial allows the electric current topass through it or not. Do you recallhow the tester helped us in decidingthat?

We found that metals such as copperand aluminium conduct electricitywhereas materials such as rubber,

plastic and wood do not conductelectricity. However, so far we have usedour tester to test materials which werein solid state. But what about liquids?Do liquids also conduct electricity? Letus find out.

Paheli and Boojho want toremind you that one

should not experimentwith the electric supply

from the mains or agenerator or an inverter.Use only electric cells for

all the activities suggestedhere.

Fig.14.1 : A tester

14.1 Do Liquids ConductElectricity?

To test whether a liquid allows electriccurrent to pass through it or not, wecan use the same tester (Fig. 14.1) which

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we made in Class VI. However, replacethe cell by a battery. Also, before usingthe tester we should check whether it isworking or not.

Activity 14.1

Join the free ends of the testertogether for a moment. Thiscompletes the circuit of the testerand the bulb should glow. However,if the bulb does not glow, it meansthat the tester is not working. Canyou think of the possible reasons?Is it possible that the connectionsare loose? Or, the bulb is fused? Or,your cells are used up? Check thatall the connections are tight. If theyare, then replace the bulb withanother bulb. Now test if the testeris working or not. If it is still notworking then replace the cells withfresh cells.

Now that our tester is working, letus use it to test the various liquids.

(Caution: While checking your tester,do not join its free ends for more thana few seconds. Otherwise the cells ofthe battery will drain very quickly.)

Activity 14.2

Collect a few small plastic or rubbercaps of discarded bottles and cleanthem. Pour one teaspoon of lemonjuice or vinegar in one cap. Bringyour tester over this cap and let theends of the tester dip into lemon juiceor vinegar as shown in Fig.14.2. Takecare that the ends are not more than1 cm apart but at the same time donot touch each other. Does the bulb

Fig. 14.2 : Testing conduction of electricity inlemon juice or vinegar

of the tester glow? Does lemon juiceor vinegar conduct electricity? Howwould you classify lemon juice orvinegar— a good conductor or a poorconductor?

When the liquid between the twoends of the tester allows the electriccurrent to pass, the circuit of the testerbecomes complete. The current flows inthe circuit and the bulb glows. Whenthe liquid does not allow the electriccurrent to pass, the circuit of the testeris not complete and the bulb does notglow.

In some situations even though theliquid is conducting, the bulb may notglow. It may have happened in Activity14.2. What can be the reason?

Do you remember why the bulbglows when the electric current passesthrough it? Due to the heating effectof current, the filament of the bulb getsheated to a high temperature and itstarts glowing. However, if the currentthrough a circuit is too weak, thefilament does not get heated

CHEMICAL EFFECTS OF ELECTRIC CURRENT 175

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Activity 14.3

Take the tray from inside a discardedmatchbox. Wrap an electric wire a fewtimes around the tray. Place a smallcompass needle inside it. Nowconnect one free end of the wire tothe terminal of a battery. Leave theother end free. Take another piece ofwire and connect it to the otherterminal of the battery (Fig. 14.4).

Fig 14.4 : Another tester

sufficiently and it does not glow. Andwhy is the current in the circuit weak?Well, though a material may conductelectricity, it may not conduct it aseasily as a metal. As a result, thecircuit of the tester may be completeand yet the current through it may betoo weak to make the bulb glow. Canwe make another tester which candetect a weak current?

You may use an LED (Fig. 14.3) inplace of the electric bulb in the testerof Fig. 14.2. LED glows even when aweak electric current flows throughit.

There are two wires (called leads)attached to an LED. One lead isslightly longer than the other.Remember that while connecting toa circuit, the longer lead is alwaysconnected to the positive terminal ofthe battery and the shorter lead isconnected to the negative terminal ofthe battery.

Fig. 14.3 : LEDs

We can use another effect of an electriccurrent to make another kind of tester.Do you recall that electric currentproduces a magnetic effect? Whathappens to a compass needle kept nearbywhen current flows in a wire? Even ifthe current is small, the deflection of themagnetic needle can be seen. Can wemake a tester using the magnetic effectof currents? Let us find out.

Join the free ends of two wiresmomentarily. The compass needleshould show deflection. Your testerwith two free ends of the wire is ready.

Now repeat Activity 14.2 using thistester. Do you find a deflection in thecompass needle the moment you dipthe free ends of the tester in lemonjuice?

Take out the ends of the testerfrom the lemon juice, dip them inwater and then wipe them dry.Repeat the activity with other liquidssuch as tap water, vegetable oil, milk,honey. (Remember to wash and wipedry the ends of tester after testing eachliquid). In each case observe whetherthe magnetic needle shows deflectionor not. Record your observations inTable 14.1.

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Table 14.1 : Good/Poor Conducting Liquids

S.No Material Compass Needle Shows Good Conductor/Deflection Yes/No Poor Conductor

1. Lemon juice Yes Good Conductor

2. Vinegar

3. Tap Water

4. Vegetable oil

5. Milk

6. Honey

7.

8.

9.

10.

When the free ends of thetester do not touch each

other, there is an air gapbetween them. Paheli knows

that air is a poor conductor ofelectricity. But she has also readthat during lightning, an electriccurrent passes through air. Shewonders if air is indeed a poorconductor under all conditions.This makes Boojho ask whether

other materials classified aspoor conductors also allowelectricity to pass under

certain conditions.

From Table 14.1, we find that someliquids are good conductors of electricityand some are poor conductors.

Actually, under certain conditionsmost materials can conduct. That is whyit is preferable to classify materials asgood conductors and poor conductorsinstead of classifying as conductors andinsulators.

We have tested the conduction ofelectricity through tap water. Let us nowtest the conduction of electricity throughdistilled water.

Activity 14.4

Take about two teaspoonfuls ofdistilled water in a clean and dryplastic or rubber cap of a bottle. (Youmay obtain distilled water from yourschool science lab. You may also getdistilled water from a medical storeor a doctor or a nurse). Use the testerto test whether distilled waterconducts electricity or not. What doyou find? Does distilled waterconduct electricity? Now dissolve apinch of common salt in distilledwater. Again test. What do youconclude this time?

When salt is dissolved in distilledwater, we obtain salt solution. This is aconductor of electricity.

The water that we get from sourcessuch as taps, hand pumps, wells andponds is not pure. It may contain several

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salts dissolved in it. Small amounts ofmineral salts are naturally present in it.This water is thus a good conductor ofelectricity. On the other hand, distilledwater is free of salts and is a poorconductor.

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

When an electric current flowsthrough a conducting solution, does itproduce an effect on the solution?

14.2 Chemical Effects ofElectric Current

In Class VII, we have learnt some effectsof electric current. Can you list theseeffects? What effect does the currentproduce when it flows through aconducting solution? Let us find out.

Activity 14.6

Take out carbon rods carefully fromtwo discarded cells. Clean their metalcaps with sand paper. Wrap copperwires around the metal caps of thecarbon rods and join them to abattery (Fig. 14.5). We call these tworods electrodes. (Instead of carbon

Small amounts of mineral saltspresent naturally in water arebeneficial for human health.

However, these salts make waterconducting. So, we should never

handle electrical applianceswith wet hands or whilestanding on a wet floor.

We have found that common salt, whendissolved in distilled water, makes it a goodconductor. What are the other substanceswhich, when dissolved in distilled water,make it conducting? Let us find out.

Caution: Do the next activity under thesupervision of your teacher/parent orsome elderly person, because the use ofacid is involved in it.

Activity 14.5

Take three clean plastic or rubber capsof bottles. Pour about two teaspoonfulsof distilled water in each of them. Adda few drops of lemon juice or dilutehydrochloric acid to distilled water inone cap. Now in the second capcontaining distilled water, add a fewdrops of a base such as caustic sodaor potassium iodide. Add a little sugarto the distilled water in the third capand dissolve it. Test which solutionsconduct electricity and which do not.What results do you obtain? Fig.14.5 : Passing current through water

Metal Cap

Carbonrod

Water

Carbonrod

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Boojho decided to testwhether some fruits andvegetables also conductelectricity or not. He cut

a potato into two halves andinserted the copper wires of a testerinto it. Just then his mother calledhim and he forgot to take out thewires of the tester inserted into thepotato. When he came back afterhalf an hour, he noticed that therewas a greenish blue spot on thepotato around one wire whereasthere was no such spot around theother wire (Fig. 14.6).

rods, you may take two iron nailsabout 6 cm long ). Pour a cupful ofwater in a glass/plastic bowl. Add ateaspoonful of salt or a few drops oflemon juice to water to make it moreconducting. Now immerse theelectrodes in this solution. Make surethat the metal caps of the carbon rodsare outside the water. Wait for 3-4minutes. Observe the electrodescarefully. Do you notice any gasbubbles near the electrodes? Can wecall the change taking place in thesolution a chemical change? Recallthe definition of a chemical changethat you learnt in Class VII.

In 1800, a British chemist, WilliamNicholson (1753–1815), had shown

that if electrodeswere immersedin water, and acurrent was passed,bubbles of oxygenand hydrogen wereproduced. Oxygenbubbles formedon the electrode

connected to the positive terminal ofthe battery and hydrogen bubblesformed on the other electrode.

The passage of an electric currentthrough a conducting solution causeschemical reactions. As a result, bubblesof a gas may be formed on the electrodes.Deposits of metal may be seen onelectrodes. Changes of colour ofsolutions may occur. The reaction woulddepend on what solution and electrodesare used. These are some of the chemicaleffects of the electric current.

Fig. 14.6 : Testing potato

He was surprised with thisobservation and along with Pahelirepeated this activity many times.They found that it was always the wireconnected to the positive terminal,which had greenish blue spot aroundit. They felt that this discovery wasvery useful because it could be usedfor identifying the positive terminalof a cell or a battery concealed in abox. They decided to report theirfinding to a children’s magazine.

Remember that Boojho set out totest whether potato conducted

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Allow the current to pass forabout 15 minutes. Now remove theelectrodes from the solution andlook at them carefully. Do you findany difference in any one of them?Do you find a coating over it? Whatcolour is the coating? Note down theterminal of the battery with whichthis electrode is connected.

electricity or not. What he found wasthat current produced a chemicaleffect in the potato. To him this wasvery exciting. In fact, this is howscience sometimes works. You arelooking for something and youdiscover something else. Manyimportant discoveries have beenmade in this manner.

14.3 Electroplating

Recall that a brand new bicycle has shinyhandlebar and wheel rims. However, ifthese are accidentally scratched, theshiny coating comes off revealing a notso shiny surface beneath. You mighthave also seen women using ornaments,which appear to be made of gold.However, with repeated use, the goldcoating wears off, revealing silver or someother metal beneath.

In both these cases, a metal has acoating of another metal. Do you wonderhow a layer of one metal can bedeposited on top of another? Well, letus try doing it ourselves.

Activity 14.7

We will need copper sulphate andtwo copper plates of size around 10cm × 4 cm. Take 250 mL of distilledwater in a clean and dry beaker.Dissolve two teaspoonfuls of coppersulphate in it. Add a few drops ofdilute sulphuric acid to coppersulphate solution to make it moreconducting. Clean copper plates withsand paper. Now rinse them withwater and dry them. Connect thecopper plates to the terminals of abattery and immerse them in coppersulphate solution (Fig. 14.7).

When electric current is passedthrough the copper sulphate solution,copper sulphate dissociates intocopper 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, the

After doing the electroplatingactivity, Paheli interchangedthe electrodes and repeated

the activity. What do you thinkshe would observe this time?

Fig.14.7 : A simple circuit showing electroplating

Copper plate

Coppersulphatesolution

Copper plate

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loss of copper from the solution isrestored and the process keeps going.This means that copper getstransferred from one electrode to theother.

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.

Fig. 14.8 : Some electroplated objects

silver 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.

Boojho could get only onecopper plate. So he

performed Activity 14.7 byconnecting a carbon rod inplace of the copper platewhich was connected tothe negative terminal of

the battery. He succeededin obtaining a coating of

copper 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 effectsof 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). Thelayer of metal deposited has somedesired property, which the metal of theobject lacks. For example, chromiumplating is done on many objects suchas car parts, bath taps, kitchen gasburners, 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 theobject is made from a cheaper metal andonly a coating of chromium over it isdeposited. Jewellery makers electroplate

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KEYWORDS

ELECTRODE

ELECTROPLATING

GOOD CONDUCTOR

LED

POOR CONDUCTOR

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.

Exercises

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 ofdepositing a layer ofany desired metal onanother material bymeans of electricity iscalled .

2. When the free ends of a testerare dipped into a solution,the magnetic needle showsdeflection. Can you explainthe reason?

3. Name three liquids, whichwhen tested in the mannershown in Fig.14.9, maycause the magnetic needleto deflect.

Fig. 14.9

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4. The bulb does not glow in the setup shown in Fig.14.10. List the possiblereasons. Explain your answer.

Fig. 14.10

5. A tester is used to check the conduction of electricity through two liquids,labeled 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 itconducting?

7. In case of a fire, before the firemen use the water hoses, they shut off themain 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. Paheli 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 showeddeflection. What could be the reasons?

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

12. The process that you saw in Activity 14.7 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 battery and why?

E X

E R

C I

S E

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Extended Learning — Activities and Projects

1. Test the conduction of electricity through various fruits andvegetables. Display your result in a tabular form.

2. Repeat the Activity 14.7 with a zinc plate in place of the copperplate connected to the negative terminal of the battery. Now replacezinc plate with some other metallic object and again repeat theactivity. Which metal gets deposited over which other metal? Discussyour findings 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 14.7). 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 objectyou would like to electroplate and for what purpose? (Look up themeaning of ‘entrepreneur’ in a dictionary).

5. Find out the health concerns associated with chromiumelectroplating. How are people trying to resolve them?

6. You can make a fun pen for yourself. Take a conducting metal plateand spread a moist paste of Potassium Iodide and starch. Connectthe plate to a battery as shown in Fig. 14.11. Now using the freeend of the wire, write a few letters on the paste. What do you see?

Fig. 14.11

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Did You Know?

LEDs (Light Emitting Diodes) are available inmany colours such as red, green, yellow, blue,white and are increasingly being used for manyapplications, 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 lightbulbs 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.

For more information on this topic visit:� www.tutorvista.com/content/physics/physics-iv/thermal-

chemical-currents/chemical-effects-current.php� www.physchem.co.za/Redox/Electrolysis.htm� electronics.howstuffworks.com/led.htm

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