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Conduction through Liquids and Electrolytes- Physics Guide for Class 8

Conduction through Liquids and Electrolytes Class 8 Science Guide

Information about Conduction through Liquids and Electrolytes

Title

Conduction through Liquids and Electrolytes

Class

Class 8

Subject

Class 8 Physics

Topics Covered

  • Conduction through Liquids
  • Cause of Conductivity of liquids
  • Strong Electrolytes
  • Weak Electrolytes


  • Materials, which allow electricity to pass through them easily, are known as conductors of electricity.
    Most of the metals are good conductors of electricity.
  • Materials, which do not allow electricity to pass through them easily, are called poor conductors or insulators.
    Plastic, wood, bakellite are some of the better-known poor conductors of electricity.

Conduction through Liquids

Most of the well-known conductors are materials in the solid state.
But what about liquids? Do liquids conduct electricity? Can an electric current pass through a liquid?
To check this, let us perform the following activity. 

Activity 1
  • Connect a torch bulb (or LED) to a battery (of two cells of 1.5 V each) using (copper) connecting wires as shown in the diagram. This arrangement is known as a continuity tester.
  • Pour some lime juice in a clean beaker.
  • Now, dip the free (well-cleaned) ends of the two wires of the tester (kept away from each other by about 2-3 cm) in the lime juice.
  • Take care that the ends do not touch the bottom of the beaker.
  • Observe whether the bulb/LED glows or not.
  • Take out the ends of the wire from the lime juice and dry and clean them.
  • Now, dip them in a beaker containing ordinary tap water.
  • Again observe the bulb/LED.
  • Repeat your observation using distilled water. (This is available at medical stores or in the school chemistry laboratory).
We now realise that some liquids do conduct electricity: The above activity also shows that while some liquids are good conductors of electricity, there are many that are poor conductors of electricity.

The water that we get from different sources, like taps, hand pumps, wells and ponds, generally contains several salts dissolved in it. This water is thus a reasonably good conductor of electricity. We should, therefore, never handle electrical appliances with wet hands, or while standing on a wet floor. 

Cause of Conductivity of liquids

Let us first try to find out what happens when an electric current passes through a liquid. 
Activity 2 
  • Take some distilled water in a clean and dry beaker. We now know that distilled water does not conduct electricity.
  • Now, dissolve a small amount of common salt (sodium chloride) in this distilled water.
What do we observe with the continuity tester now?
The bulb/LED glows.
We conclude that a solution of common salt in water is a good conductor.

Why do salt, acid, or caustic soda make the water more conductive to electricity?
  • Common salt (NaCl) consists of (positively charged) sodium and (negatively charged) chlorine ions. 
  • The water not only breaks sodium chloride into (electrically charged) sodium and chlorine ions but also separates these ions.
  • It is the movement of these ions within the solution that makes distilled water conductive to electricity. 
  • We thus realise that the flow of electricity, in liquids, is due to the movement of ions in them.
The molecules of salts, acids and bases, on being dissolved in water, are known to break up into free (positive and negative) mobile ions. These free mobile ions allow the passage of electric current through the solution, making it conductive. We call such (conductive) solutions as electrolytes.

Electrolytes

We say that : A compound, that conducts electricity in molten (fused) or aqueous (solution) state, and which simultaneously undergoes decomposition (breaking into ions) when an electric current passes through it, is an electrolyte.

We classify electrolytes as strong electrolytes and weak electrolytes. This is done on the basis of the extent to which the electrolyte breaks into ions.

1. Strong electrolytes

  • These are electrolytes that get ionised completely to form free mobile ions in the solution. Hence, a large number of free mobile ions are available in them to conduct electricity.
  • Some of the examples of strong electrolytes are: sodium chloride, pure sulphuric acid and copper sulphate solution.

2. Weak electrolytes

  • These are electrolytes that get ionised only partially to form free mobile ions. Hence, only a small number of free mobile ions are available in them to conduct electricity.
  • Some of the examples of weak electrolytes are acetic acid, ammonium hydroxide and tap water.

Some Examples of Strong and Weak Electrolytes

Strong electrolyte

Weak electrolyte

Sea water

Ordinary tap water

Nitric acid

Oxalic acid

Copper sulphate solution

Carbonic acid

Sodium chloride

Ammonium hydroxide

Sulphuric acid

Citric acid


Some Points to remember

  • Air is a poor conductor of electricity. But during lightning, an electric current passes through air. Actually, under appropriate conditions, most materials can conduct. It is, therefore, preferable to classify materials as good conductors or poor conductors instead of classifying them as conductors and insulators. 
  • A non-electrolyte does not provide ions in its solution and, therefore, current does not flow through such solutions. Some examples of non-electrolytes are distilled water, alcohol, carbon disulphide and carbon tetrachloride. However, these can ionise various other substances when they are put in them.
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