NCERT - Chemical Effects of Electric Current Class 8 Chapter Notes

Science Class 8th Notes From Ch. 14 – Chemical Effects of Electric Current

The materials which allow electric current to pass through them easily are called good conductors of electricity. Examples: Copper, aluminium etc.

The materials which do not allow electric current to pass through them easily are called poor conductors of electricity (or non-conductors of electricity). Examples: Rubber, plastic and wood etc.

The liquids that conduct electricity are solutions of acids, bases and salts in water. For example, a solution of sulphuric acid, hydrochloric acid or any other acid in water conducts electricity. Vinegar contains acetic acid and lemon juice contains citric acid. Vinegar and lemon juice also conduct electricity because they are solutions of acids. Similarly a solution of sodium hydroxide, potassium hydroxide or any other soluble base in water conducts electricity. And, a solution of copper sulphate, common salt (sodium chloride), or any other salt in water also conducts electricity.

The important differences in conduction of electricity by solids such as metals and liquids such as solutions of acids, bases and salts are given:

1. In solids like metals, electricity is carried by electrons but in liquids, electricity is carried by ions (positively charged ions and negatively charged ions)
2. When electricity is passed through a solid, then no chemical change takes place but when electricity or electric current is passed through a liquid, then a chemical change takes place

The liquids which conduct electricity are called conducting liquids or conducting solutions. The chemical changes which takes place in conducting liquids on passing electric current through them are called chemical effects of electric current.

A liquid or solution of a substance which can conduct electricity is called an electrolyte. In other words, a conducting liquid is called an electrolyte. Copper sulphate solution conducts electricity, therefore, copper sulphate solution is an electrolyte. The solutions of acids, bases and salts in water are electrolytes.

Electrolytes are of two types: strong electrolytes and weak electrolytes. A strong electrolyte is a liquid or solution which conducts electricity very well. A strong electrolyte is a very good conductor of electricity because it contains a lot of ions in it. Some of the examples of strong electrolytes are: Sulphuric acid solution, Potassium hydroxide solution, Common salt solution, copper sulphate solution and Silver nitrate solution.

A weak electrolyte is a liquid or solution which conducts electricity to a lesser extent. A weak electrolyte is a weak conductor of electricity because it contains lesser number of ions. Some of the examples of weak electrolytes are: Vinegar (acetic acid solution), Lemon juice (citric acid solution), Carbonic acid solution, Ammonium chloride solution, Ordinary water (tap water) and Rain water.

A solid electrical conductor through which an electric current enters or leaves something like a dry cell or an electrolytic cell, is called an electrode. Electrodes are carbon rods or metal rods depending upon where they are being used. Electrodes are of two types: anode and cathode. The positively charges electrode is called anode and the negatively charged electrode is called cathode.

An arrangement having two electrodes kept in a conducting liquid or electrolyte in a vessel is called an electrolytic cell.

Sugar solution, Vinegar, glucose solution, distilled water, alcohol solution, milk, vegetable oil and honey are all poor conductors or non-conductors of electricity. They are non-electrolytes.

The weak electric current flowing through liquids (having low electrical conductivity) can be detected in two ways:

i). By using a LED (Light-Emitting Diode), and

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

LED is a semi-conductor (intermediate between a metal and an insulator) device which glows even when very weak current passes though 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 (in place of bulb), the longer lead is always connected to the positive terminal of the battery.

Distilled water becomes a good conductor of electricity on dissolving a little of salt in it. In other words, a salt solution is a good conductor of electricity.

Distilled water is a poor conductor or non-conductor of electricity because it does not contain any dissolved salts in it which can provide it ions to conduct electricity.

Distilled water or pure water can be made to conduct electricity in the following ways:

i). We can dissolve some common salt or any other salt in distilled water.

ii). We can add a little of acid such as dilute sulphuric acid, lemon juice or vinegar in distilled water.

iii). We can add a little of base such as sodium hydroxide or potassium hydroxide in distilled water.

Water of taps, handpumps, wells and ponds contain small amounts of several salts which are naturally present in it. So, water from all these sources is a conductor of electricity.

We should never operate an electric switch or touch any working electrical appliance with wet hands because the tap water present on wet hands is a conductor of electricity.

In case of a fire, before the firemen use big water hoses (flexible water pipes) to throw water on a burning house or building, they usually cut off the electricity supply to that area. This is done to prevent electrocution of firemen. Ordinary water is a conductor of electricity, so if the electric supply is not cut off and firemen come in contact with wet electric switches, electric wires and other electrical appliances, they may get electrocuted.

Sea water is much better conductor of electricity than drinking water as sea water has large amounts of dissolved salts in it.

Due to presence of small amounts of acids in it, rain water becomes a conductor of electricity.

The bulb in an experimental setup having battery, bulb, wires and liquid in beaker having two free ends of the wire dipped in liquid, will not glow due to the given reasons:

i). the connections of wires in the circuit may be loose.

ii). the bulb may be fused.

iii). the battery may be dead or all used up.

iv). the liquid may be a very weak conductor of electricity.

Electric current can bring about chemical changes, so it is said to have a chemical effect. For example, when electric current is passed through acidified water by using carbon electrodes, water is decomposed into hydrogen and oxygen by action of electric current.  The breaking up of water into hydrogen and oxygen is actually a “chemical decomposition” reaction caused by passing 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.

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

i). Oxygen gas is formed a 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.

The chemical reactions brought about by an electric current may produce one or more of the following effects:

i). Bubbles of gas (or gases) may be formed on the electrodes.

ii). Deposits of metals may form on electrodes.

iii). Changes in colour of solutions may occur.

The chemical effect produced by an electric current depends on the nature of conducting solution (through which it is passed), and on the nature of electrodes used for passing the electric current. Examples: Deposit of metal is formed when electric current is passed through copper sulphate solution (during electroplating), change of colour occurs when electric current is passed through cut potato.

The formation of a greenish-blue spot around the positive electrode, inserted in the surface of the cut potato shows that the chemical effect of current can bring change in the colour of a conducting solution. A fresh potato contains solution of many substances dissolved in water. Thus, an electric current produces a chemical effect in potato leading to a change in colour.

The fact that a greenish-blue spot on potato surface is always formed around the electrode connected to the positive terminal of a battery can be used to identify the positive terminal of a battery which is concealed in a box and whose terminals cannot be seen from outside.

The chemical effect of electric current is used in industries (or factories) for the following purposes:

i). Electroplating metals.

ii). Purification of metals.

iii). Production of certain metals from the ores.

iv). Production of chemical compounds, and

v). Decomposing chemical compounds.

The process of depositing a thin layer of a desired metal over a “metal object” with the help of electric current is called electroplating. The purpose of electroplating is to protect the metal objects from corrosion (or rusting) and to make the metal objects look more attractive.

The metal objects (or metal articles) are usually electroplated with chromium, tin, nickel, silver, gold or copper metals.

The following points should be remembered while electroplating:

i). The “metal object on which electroplating is to be done” is made the negative electrode (cathode): It is connected to the negative terminal of the battery.

ii). The metal whose layer is to be deposited is made the positive electrode (anode): It is connected to the positive terminal of the battery.

iii). A water soluble salt of the “metal to be deposited” is taken as the electrolyte (The electrolyte contains the metal to be deposited in the form of a soluble salt).

For electroplating an iron object with copper metal (or copper plating):

a). The iron object is made negative electrode (cathode). This means that the iron object is connected to the negative terminal of the battery.

b). A copper plate is made positive electrode (anode). This means that a copper plate is connected to the positive terminal of the battery.

c). Copper sulphate solution (CuSO4 solution) is taken as electrolyte (It contains copper metal in dissolved form as copper ions, Cu2+).

Carbon (in the form of graphite) is a non-metal which is a good conductor of electricity.

Uses of Electroplating

1. Electroplating is a very useful process and is widely sued in the industry for coating metal objects (or metal articles) with a thin layer of a desired metal. The metal “which is deposited” in the form of a thin layer has some properties which the metal of the object does not possess.
2. Chromium metal has shiny appearance and does not corrode (it does not rust). Chromium metal is however quite expensive and hence it is not economical to make whole object out of chromium. So, the object (or article) is made of cheaper metal (like iron or steel) and only a thin coating of chromium metal is deposited all over its surface by electroplating. For example, chromium plating is done on many objects made of iron metal (or steel) such as bicycle handlebars, bicycle bells, wheel rims, bathroom fittings (taps etc.), LPG stoves, motor cycle parts and many, many other objects.
3. Tin metal has shiny appearance, it does not corrode and it is non-poisonous. It is less reactive than iron. Tin cans used for storing food are made by electroplating tin metal on to iron. Due to tin plating over the surface of iron, the food does not come in contact with iron and is protected from getting spoilt. In fact, the less reactive and shiny metals like chromium, tin and nickel are electroplated on more reactive and dull looking metals like iron and steel to protect them from corrosion and give them an attractive finish.
4. Electroplating is used to give objects made of a cheap metal a coating of a more expensive metal to make them look more attractive.

The conducting solutions (or electrolytes) used in electroplating process are polluting wastes and hence should be disposed of in a proper way so as to protect the environment.

The chemical effect of electric current is used in the purification of impure metals (which are extracted from their naturally occurring compounds called ores). In the purification of an impure metal by using the chemical effect of current (or electrolysis):

a). A thick rod of impure metal is made positive electrode (or anode): It is connected to the positive terminal of the  battery.

b). A thin strip of pure metal is made negative electrode (or cathode): It is connected to the negative terminal of the battery.

c). A water soluble salt of the metal to be purified in taken as electrolyte

On passing electric current, the metal dissolves from the impure anode and goes into electrolyte solution. The metal present in dissolved form in electrolyte gets deposited in the cathode in the pure form. The metals like copper, zinc and aluminium, etc., are purified by the process of electrolysis by using the chemical effect of electric current.

The chemical effect of electric current (or electrolysis) is used in the production of various chemical compounds. For example, sodium hydroxide (or caustic soda) is produced by the electrolysis of an aqueous solution (water solution) of sodium chloride.

The chemical effect of electric current (or electrolysis) is used to decompose various chemical compounds into their elements. For example, water can be decomposed by passing electric current (or electrolysis) into two elements: hydrogen and oxygen.