Chemical effects of electric current part 2

Chemical effects of electric current - II

To Test Whether a Liquid Conducts Electricity or Not

We will test two liquids for the conduction Of electricity, dilute hydrochloric acid solution and sugar solution, one by one.

Take a small beaker. Fix two iron nails on a rubber cork about 1 cm apart and place this cork in the beaker . The two iron nails will act as the two electrodes. Connect the two nails to the two terminals Of a battery by including a torch bulb and a switch in the circuit. Pour a solution Of dilute hydrochloric acid in the beaker carefully. NOW pass electric current through HCL solution by closing the switch.

Observation : As soon as we switch on the current, the bulb Starts glowing .The bulb can glow only if the hydrochloric acid solution taken in the beaker conducts electricity (making the circuit complete). so, the glowing Of bulb in this case tells us that hydrochloric acid solution conducts electricity. 

To test if sugar solution is an elctrolyte

Let us now take sugar solution in the beaker and switch on the electric current by closing the switch. We Will observe that the bulb does not glow in this case . This shows that sugar solution does not conduct electricity.

Thus sugar solution , milk , honey etc are non-electrolytes.

Weak Conductors of Electricity

When an electric current is passed through vinegar (or lemon juice) taken in a

beaker, then the bulb glows very dimly (even when a large battery is applied) .The very dim glowing Of bulb indicates that though vinegar and lemon juice conduct electricity but they are weak conductors Of electricity. 

·      If a large battery is not used to pass current through vinegar solution (or lemon juice), then the bulb may not glow at all.

·      Ordinary water (Or tap water) is a very weak conductor Of electricity. 

Multiple Choice

Which one of the following solutions will not conduct electricity?

(a) lemon juice

(b) vinegar

(c) tap water

(d) vegetable oil

• a

• b

• c

• d

Detection of Weak Current Flowing Through a Liquid

(i) by using a LED (Light-Emitting Diode)

·     LED is a semiconductor device which glows even when a very weak current passes through it.

·     There are two wires (called leads) attached to an LED . One lead is slightly longer than the other.

·     While connecting LED in the circuit ,the longer lead is always connected to the positive terminal Of the battery and the shorter lead is connected to the negative terminal Of the battery.

·     Three LEDs are normally connected.    

(ii) by using a compass (surrounded by turns Of circuit wire).

The matchbox tray containing the compass inside it and having wound up wire around and it is connected in place Of torch bulb in the liquid to be tested for conductivity. Even if a weak electric current flows through the liquid in the circuit the  magnetic needle Of compass will show deflection because  even a weak electric Current flowing through a wire produces a magnetic field around it.

·     When liquids such as vinegar, lemon juice, tap water, rainwater and sea water are included in the circuit, a deflection in the magnetic needle Of compass is observed. This shows that these liquids are conductors Of electricity.

·     When liquids like distilled water, milk, vegetable Oil and honey are included in the circuit, no deflection is observed in the magnetic needle Of compass. This shows that these liquids are poor conductors Of electricity (or non-conductors Of electricity).

Multiple Choice

LED stands for

• light emitting diode

• light efficient diode

• length emitting diode

• light emitting device

Multiple Choice

Conduction of electricity in liquids occurs through electrons.

• True

• False

Multiple Choice

Sugar solution is a good conductor of electricity.

• True

• False

CHEMICAL EFFECTS OF ELECTRIC CURRENT

When electric current is passed through acidified water by using carbon electrodes, then a chemical reaction takes place to form hydrogen gas and oxygen gas.

Water is a chemical compound, and hydrogen and oxygen are elements.

The breaking up Of water into hydrogen and oxygen is actually a 'chemical decomposition' reaction caused by passing an electric current through acidified water (which is a conducting liquid).

The chemical decomposition produced by passing an electric current through a conducting liquid is called electrolysis.

Example : The decomposition Of acidified water into hydrogen and oxygen by passing an electric current (or electricity) is an example Of electrolysis. 

Demonstration of the Chemical Effect of Electric Current

In the year 1800, a British chemist, William Nicholson, had shown that if electric current is passed through acidified water, then bubbles Of oxygen gas and hydrogen gas are produced at the two electrodes immersed in it.

(i)  Oxygen gas is formed at the positive electrode (anode) which is connected to the positive terminal Of the battery, and

(ii) Hydrogen gas is formed at the negative electrode (cathode) which is connected to the negative terminal Of the battery.

Important questions

*     Why do the liquids that conduct electricity are solutions Of acids, bases and salt?

            This is because the solutions Of acids, bases and salts contain charged particles (called ions)     which conduct electricity through these solutions.

*     Why is Distilled water is a poor conductor (Or non-conductor) Of electricity? 

This is because it does not contain any dissolved salts in it (which can provide it ions to conduct electricity).

 

*     Why should we never operate an electric switch or touch any working electrical appliance with wet hands?

It is dangerous to operate an electric switch or touch a working electrical appliance with wet hands because the tap water present on wet hands is a conductor Of electricity due to which it may conduct electric current from the electric switch to Our hand and give us an electric shock.

*     Why is electrical conductivity of sea water more than that of tap water ?

Drinking water contains small amounts Of dissolved salts in it but sea water contains a large amount Of dissolved salts in it. Since the sea water contains more salts, therefore, the electrical conductivity Of sea water is much more than that Of drinking water.

 

*     Rainwater is said to be pure water , then why does it conduct electricity ?

But when rainwater falls to the earth through the atmosphere, it dissolves an acidic gas carbon dioxide from the air and forms a weak acid called carbonic acid. The rainwater may also dissolve Other acidic gases such as sulphur dioxide and nitrogen oxides (which are present in polluted air) to form small amounts Of Other acids such as sulphuric acid and nitric acid. Due to the presence Of small amounts Of acids in it, rainwater becomes a conductor Of electricity.

*     Why is electrical conductivity of sea water more than that of tap water ?

Drinking water contains small amounts Of dissolved salts in it but sea water contains a large amount Of dissolved salts in it. Since the sea water contains more salts, therefore, the electrical conductivity Of sea water is much more than that Of drinking water.

 

*     Rainwater is said to be pure water , then why does it conduct electricity ?

But when rainwater falls to the earth through the atmosphere, it dissolves an acidic gas carbon dioxide from the air and forms a weak acid called carbonic acid. The rainwater may also dissolve Other acidic gases such as sulphur dioxide and nitrogen oxides (which are present in polluted air) to form small amounts Of Other acids such as sulphuric acid and nitric acid. Due to the presence Of small amounts Of acids in it, rainwater becomes a conductor Of electricity.

WHY DOES POTATO CHANGE ITS COLOUR WHEN ELECTRIC CURRENT IS PASSED THROUGH IT ?

• The electric current is causing electrolysis of the water in the potato, taking the water apart into hydrogen and oxygen. ... The negative wire is producing oxygen, which combines with the copper of the wire, forming a green copper oxide. That is what colors the potato green.

• The fact that a greenish-blue spot on potato surface is always formed around the electrode connected to the positive terminal Of a battery, it can be used to identify the positive terminal Of a battery 

Multiple Choice

The electrode thats connected to the positive terminal of the battery is called a

• anode

• cathode

• elctrode

Multiple Choice

When current is passed through a mixture of water and lemon juice, bubbles rise from one of the electrodes. The bubbles are rising from the solution because

• of the heating effect of current
• of the chemical effect of current
• current produces magnetic field around the electrodes
• current always produces air bubbles when passed through a liquid

Multiple Choice

An electric current can produce

• heating effect only.

• chemical effect only.

• magnetic effect only.

• chemical, heating, and magnetic effects.

Multiple Choice

When an electric current is passed through a conducting solution, there is a change in the colour of the solution. This indicates:

(a) the chemical effect of current.

(b) the heating effect of current.

(c) the magnetic effect of current.

(d) the lightning effect of current.

• a

• b

• c

• d

Multiple Choice

Name the effect of current responsible for the glow of bulb in an electric circuit.

• magnetic effect of current

• chemical effect of current

• heating effect of current

Multiple Choice

the phenomenon of decomposition of an electrolyte when electricity is passed through is known as

• conduction

• coating

• electrolysis

• electrorefining