Weak Conductors of Electricity and Detection of Weak Current Flowing through Liquids

Thermochemistry explores the heat changes that occur during chemical reactions, making it an integral part of Chemistry Topics.

Which out of Distilled water, Tap water, Sea water and River Water is the Best Conductor of Electricity ?

We have just studied that when an electric current is passed through hydrochloric acid solution, the bulb glows brightly indicating that the hydrochloric acid solution is a very good conductor of electricity [see Figure (a)]. This is not so in the case of vinegar (which is acetic acid solution) or lemon juice (which is citric acid solution).

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) [see Figure (b)].

The very dim glowing of bulb indicates that though vinegar and lemon juice conduct electricity but they are weak conductors of electricity. Please note that if a large battery is not used to pass current through vinegar solution (or lemon juice), then the bulb may not glow at all. And if we take ordinary water (tap water) in the beaker and pass electricity through it, then the bulb will not glow at all even if a large battery is applied.

This is because ordinary water (or tap water) is a very weak conductor of electricity. Thus, in some situations, even if the liquid is somewhat conducting, the bulb may not glow. There are, however, some methods (other than the glowing of bulb) to test the electrical conductivity of weak conducting liquids. These are described below.

Detection of Weak Current Flowing Through a Liquid

The current flowing in a circuit is usually detected by using a small electric bulb (such as a torch bulb). When an electric current flows through a bulb then due to the heating effect of current, the filament of bulb gets heated to a high temperature, becomes white hot and glows to produce light.

When the current flowing through a circuit is large, the heating effect is large due to which the bulb glows brightly. And when the current flowing through the circuit is small, the heating effect is also small, due to which the bulb glows dimly. If, however, the current flowing through the circuit is too weak, then the heating effect produced by current in the filament is too little, due to which the filament does not get heated sufficiently and hence the bulb does not glow at all.

The current in the circuit may be weak if the electrical conductivity of the liquid we are testing is very low. So, if we test a liquid having very low electrical conductivity by using a torch bulb in the circuit, then the bulb may not glow at all (due to too weak current flowing through it). So, a torch bulb cannot be used to detect weak electric current flowing through a liquid in a circuit.

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

1. by using a LED (Light-Emitting Diode), and
2. by using a compass (surrounded by turns of circuit wire).

We can use LED (light-emitting diode) in place of a torch bulb for detecting weak electric current passing through a liquid. LED is a semi-conductor device which glows even when a very weak current passes through it. There are two wires (called leads) attached to an LED (see Figure).

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 and the shorter lead is connected to the negative terminal of the battery. Please note that LED is connected in a circuit (for detecting an electric current flowing through it) where a torch bulb is normally connected.

We can also detect weak electric current flowing through a liquid by using a compass (by making use of the magnetic effect of current).
This can be done as follows : We take out the cardboard tray from the inside of a discarded matchbox. Place a small compass inside this cardboard tray. Wrap an electric wire a few times around the cardboard tray so as to make a type of coil of wire around the compass (as shown in Figure).

The matchbox tray containing the compass inside it and having wound up wire around it is connected in place of torch bulb in the circuit of 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.

This is because even a weak electric current flowing through a wire produces a magnetic field around it. And this magnetic field of electric current acts on the magnetic needle of the compass and deflects it from its usual north-south position. So, if a compass surrounded by electric wire of a circuit including a liquid in it shows deflection, it will mean that the liquid conducts electricity (or the liquid is a conductor of electricity).

In order to detect the electrical conductivity of a liquid, the two ends of wire (wound around the compass) are connected at a place in the circuit where the two terminals of a torch bulb are normally connected. We can test the electrical conductivity of liquids such as vinegar, lemon juice, tap water, rainwater, sea water, distilled water, milk, vegetable oil and honey with the help of a compass surrounded by wire of circuit.

(a) 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 vinegar, lemon juice, tap water, rainwater and sea water are conductors of electricity.

(b) 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 the liquids such as distilled water, milk, vegetable oil and honey are poor conductors of electricity (or non-conductors of electricity).

From the above discussion we conclude that some liquids are good conductors of electricity whereas other liquids are poor conductors of electricity (or non-conductors of electricity). Most of the liquids that conduct electricity are solutions of acids, bases and salts. This is because the solutions of acids, bases and salts contain charged particles (called ions) which conduct electricity through these solutions.

The Case of Distilled Water, Tap Water, Sea Water and Rainwater

If we take some distilled water in a beaker and pass electricity through it, we will find that there is no deflection in the magnetic needle of compass (included in the circuit in a suitable way). This shows that distilled water does not conduct electricity. In other words, distilled water is a poor conductor of electricity (or a non-conductor of electricity).

Let us now dissolve a pinch of common salt (sodium chloride) in distilled water. When salt is dissolved in distilled water, we get salt solution. If we test this salt solution by passing current through it, we will find that the magnetic needle of compass shows deflection.

Thus, distilled water becomes a good conductor of electricity on dissolving a little of salt in it. In other words, 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).

Thus, distilled water is pure water and it does not conduct electricity. We can make distilled water (or pure water) to conduct electricity in the following ways :

1. We can dissolve some common salt (or any other salt) in distilled water or pure water to make it a good conductor of electricity.
2. We can add a little of acid (such as dilute sulphuric acid, lemon juice or vinegar) in distilled water or pure water to make it a good conductor of electricity.
3. We can add a little of a base (such as sodium hydroxide or potassium hydroxide) in distilled water or pure water to make it a good conductor of electricity.

The water that we get from sources such as taps, handpumps, wells and ponds, etc., is not pure. The water of taps, handpumps, wells and ponds contains small amounts of several salts which are naturally present in it. So, water from all these sources is a conductor of electricity.

For example, tap water is a conductor of electricity because it contains small amounts of various salts dissolved in it. These salts come naturally to the tap water because tap water comes from the rivers, lakes or tube-wells (which contain dissolved salts). The small amounts of mineral salts present naturally in tap water are beneficial for human health but these salts make tap water a conductor of electricity.

We should 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 (or electrical appliance) to our hand and give us an electric shock. Similarly, we should never handle an electrical appliance while standing barefooted on a wet floor.

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 of that area. The electric supply is cut off to prevent electrocution of firemen who are busy in fire-fighting operations. Ordinary water is a conductor of electricity, so if the electricity supply is not cut off and firemen come in contact with wet electric switches, electric wires and other electrical appliances, they may get electrocuted.

Tap water is drinking 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.

So, if we test drinking water and sea water for their electrical conductivity by passing electric current through them, one by one, we will find that the deflection of magnetic needle of compass is much more in the case of sea water (than in the case of drinking water). This is because due to the presence of a large amount of dissolved salts in it, sea water is a much better conductor of electricity than drinking water (which contains only a small amount of dissolved salts in it).

Rainwater is said to be pure water. 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. So, if we pass electric current through a sample of rainwater, we will see a deflection in the magnetic needle of compass. This will show that rainwater is a conductor of electricity.

Since rainwater is a conductor of electricity, it is not safe for an electrician to carry out electrical repairs in the outdoor area during heavy downpour (heavy rain). The rainwater being a conductor of electricity may cause electrocution of the electrician. We will now answer a question taken from the NCERT science book.

Example Problem.
The bulb does not glow in the experimental set-up shown in Figure. List the possible reasons.
Answer:
It is possible that :

 

1. the connections of wires in the circuit may be loose.
2. the bulb may be fused.
3. the battery may be dead (all used up).
4. the liquid may be a very weak conductor of electricity.
5. the liquid may be a poor conductor (non-conductor) of electricity.