Contents
Chromatography is a widely used technique in Chemistry Topics for separating and analyzing mixtures based on their different affinities to a stationary phase.
What is an Electrolyte ? And How Conduction of Electricity happen in Liquids ?
We have learnt in earlier classes that some solid materials allow electric current to pass through them whereas others do not allow electric current to pass through them. The materials which allow electric current to pass through them easily are called good conductors of electricity.
On the other hand, those materials which do not allow electric current to pass through them easily are called poor conductors of electricity (or non-conductors of electricity).
For example, the metals such as copper and aluminium allow electricity to pass through them easily (or conduct electricity), so they are good conductors of electricity. On the other hand, the materials such as rubber, plastic and wood do not allow electric current to pass through them (do not conduct electricity), so they are poor conductors of electricity (or non-conductors of electricity). In our earlier Classes, we have tested the electrical conductivity of solid materials (like metals, etc.). In this Class we will test the electrical conductivity of liquids.
Do Liquids Conduct Electricity
Just as some of the solids conduct electricity, in the same way, some of the liquids also conduct electricity. 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. There are, however, some important differences in the conduction of electricity by solids (such as metals) and liquids (such as solutions of acids, bases and salts). These differences are given below :
(i) In solids (like metals), electricity is carried by electrons but in liquids, electricity is carried by ions (positively charged ions and negatively charged ions). For example, in a solid like copper metal, electricity is carried by electrons but in a liquid like copper sulphate solution (CuSO4, solution), electricity is carried by copper ions (Cu2+) and sulphate ions (SO42-).
(ii) 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. For example, when electricity is passed through a copper wire, no chemical change takes place in it but when electricity (or electric current) is passed through acidified water, then a chemical change takes place in which water is decomposed into hydrogen and oxygen gases.
From the above discussion we conclude that some liquids conduct electric current (or electricity). The liquids which conduct electricity are called conducting liquids (or conducting solutions). When electric current (or electricity) is passed through conducting liquids, then chemical changes take place. The chemical changes which take place in conducting liquids on passing electric current through them are called chemical effects of electric current.
In this Chapter, we will study the chemical effects of electric current in detail. Before we go further, we should know the meaning of some new terms like electrolyte, electrodes (anode and cathode), and electrolytic cell which will help us in understanding the chemical effects of electric current. These are described below.
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 (A conducting liquid means a liquid which conducts electricity). A solution of copper sulphate salt in water is a liquid. Copper sulphate solution conducts electricity, therefore, copper sulphate solution is an electrolyte.
A conducting liquid or electrolyte contains ions (positively charged ions and negatively charged ions). The flow of these ions conducts electricity through the conducting liquid or 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, Hydrochloric acid solution, Nitric acid solution, Sodium hydroxide solution, Potassium hydroxide solution, Common salt solution (Sodium chloride 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 electrode which is connected to the positive terminal of the battery gets positively charged. The positively charged electrode is called anode. The electrode which is connected to the negative terminal of the battery gets negatively charged. 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. For example, if we keep two carbon electrodes in a beaker containing acidified water, it will be an electrolytic cell. An electrolytic cell is used for carrying out chemical reactions (or chemical changes) by passing an electric current through the conducting liquid (or electrolyte).
The conducting liquids (or electrolytes) are not as good conductors of electricity as metals are. So, in performing the activities on conduction of electricity through liquids, a single electric cell is not sufficient electric cells (or dry cells) joined together to study the conduction of electricity through liquids.
This battery will supply a larger amount of electric current. Thus, a battery (of cells) is used to pass an electric current through the electrolyte taken in the electrolytic cell during the study of chemical effects of electric current.
To Test Whether a Liquid Conducts Electricity or Not
We will now describe an activity to test whether a given liquid (or solution) conducts electricity or not. In other words, we will now describe an activity to test whether a given liquid allows electric current to pass through it or not. We will test two liquids for the conduction of electricity, dilute hydrochloric acid solution and sugar solution, one by one.
Activity 1
Take a small beaker. Fix two iron nails on a rubber cork about 1 cm apart and place this cork in the beaker as shown in Figure (a). 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 the hydrochloric acid solution by closing the switch. We will observe that as soon as we switch on the current, the bulb starts glowing [see Figure (a)], 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. In other words, hydrochloric acid solution is a conducting liquid (or electrolyte). Since the bulb glows brightly, we also conclude that hydrochloric acid solution is a very good conductor of electricity (or electric current).
If we repeat this activity by taking sulphuric acid solution, sodium hydroxide solution, common salt solution (sodium chloride solution), copper sulphate solution, vinegar or lemon juice in the beaker, the bulb glows again (though it may not glow equally bright in all the cases). This shows that all these solutions are conducting liquids (or electrolytes).
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 [see Figure (b)]. This shows that sugar solution does not conduct electricity. In other words, sugar solution is not a conducting liquid.
Sugar solution is a poor conductor (or non-conductor) of electricity. If we repeat this activity by taking glucose solution, distilled water, alcohol solution, milk, vegetable oil and honey in the beaker, one by one, we will find that the bulb does not glow at all.
This means that glucose solution, distilled water, alcohol solution, milk, vegetable oil and honey are all poor conductors (or non-conductors) of electricity. They are not conducting liquids. They are non-electrolytes.