Chemical Effects of Electric Current Class 8 Notes Science Chapter 11

Facts that Matter

• The materials which allow electric current to pass through them are called good conductors of electricity. For example, metals like copper and aluminium.
• The materials which do not allow electric current to pass through them easily are called poor conductors of electricity. For example, rubber, plastic and wood.  
• Due to the heating effect of current, the filament of the bulb of the tester gets heated to a high temperature and it starts glowing.   
• If the current through a circuit is too weak, the filament does not get heated sufficiently and it does not glow.   
• LED (Light Emitting Diodes) can be used in place of the electric bulb in the tester. LED glows even when a weak electric current flows through it.
• As el ectric current produces magnetic effect, a tester can be prepared by using a compass needle. The deflection of the magnetic needle can be seen, even when current is small.
• Distilled water is poor conductor of electricity. Water collected from taps, hand pumps, wells and ponds is not pure and contain dissolved salts so it is good conductor of electricity.
• Most liquids that conduct electricity are solutions of acids, bases and salts.   
• Chemical effect of the electric current: The passage of an electric current through a conducting solution causes chemical reaction. That may cause formation of bubbles of a gas on the electrodes, deposits of metal on electrodes, changes of colour of solutions etc.
• Electroplating: The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating. It is one of the most common application of chemical effects of electric current.
• Uses of Electroplating: Electroplating is widely used in industry for coating metal objects with a thin layer of a different metal. This may be done to
  — make objects appear shiny,
  — prevent corrosion, 
  — make objects resistant to scratches.
  — coat less reactive metal on more reactive metals to protect from getting spoilt,
  — electroplate less expensive metals with silver and gold to make ornaments.

For example:
— Chromium has a shiny appearance, does not corrode and is resistant to scratches. But, chromium is expensive and it is not economical to make the whole object out of chromium. So, the objects, such as taps, car parts, wheel rims etc. made from a cheaper metal and electroplated with chromium.
— Jewellery made of cheap metal is electroplated with silver or gold to appear as if made of silver or gold.
— Tin is less reactive than iron. So, iron cans used for food items are electroplated with tin to prevent spoilage from contact with iron.
— Iron used in bridges and automobiles is electroplated with a coating of zinc to protect it from corrosion and formation of rust.   

• The disposal of the used conducting solution of electroplating factories is a major problem. It is a polluting waste and there are specific disposal guidelines to protect the environment.