Class 8 Science Chapter 11 Important Question Answers - Chemical Effects of Electric Current

Q1: Which of the following liquids conduct electricity and which do not conduct electricity?
Lemon juice, Milk, Vinegar, Salt solution, Distilled water, Honey, Seawater, Rainwater.
Ans: 

• Conducting: Lemon juice, Vinegar, Salt solution, Seawater, Rainwater
• Non-Conducting: Milk, Distilled water, Honey
  Lemon Juice Conducting Electricity

Q2: What is the advantage of using LED over bulb in testing the electrical conductivity of liquids?
Ans: 

• When an electric current flows through a bulb then due to heating effect of the current, the filament of the bulb gets heated up to a high temperature; it starts glowing.
  LED is used for testing the conductivity of liquids
• Now for a liquids having low electrical conductivity, the current flowing through the circuit is very weak due to which the filament does not get heated sufficiently & hence the bulb does not glow. 
• Therefore LED is used in place of the bulb because LED glows even when weak electric current flows in the circuit.

Q3: Which effect of electric current is utilized for detecting the flow of current through a solution
(a) When a torch bulb is used?
(b) When a magnetic compass is used?

Ans: 
(a) Heating effect
(b) Magnetic effect.

Q4: Distilled water does not conduct electricity. What substances can be added to distilled water in small amounts to make it a good conductor of electricity?
Ans: Salt, acid & base.

• Pure water or distilled water does not conduct electricity. In distilled water there are no impurities, there are no ions, there are only neutral (no charge) water molecules and these neutral molecules do not have charge, but after adding salt/ acid/ base, it starts conducting due presence of ions.

Q5: In case of a fire, before the fireman uses the water hoses to throw water to douse the fire, they shut off the electricity supply for the area. Explain why this is done?
Ans:  

• Water usually contains impurities which makes it a good conductor of electricity. If the electrical supply is not shut off, then electricity may pass through water and harm the firemen. 
• This is the reason in case of a fire before the firemen use the water hoses; they shut off the main electrical supply for the area.

Q6: When the free ends of a conductivity tester (made by using a battery connected to a wire wound around a compass) are dipped into a solution, the magnetic needle shows deflection. Can you give the reason of this deflection?
Ans.

• Electric current flowing through the wire produces a magnetic field around it.
  Compass Needle Deflects
• And this magnetic field of electric current acts on the magnetic needle of compass which in turn deflects the needle.

Q7: What effects does electric current produce when flowing through a conducting solution?

Ans: When an electric current flows through the conducting solution, it causes a chemical reaction (or chemical change).

These chemical reactions may produce the following effects:

• Bubbles of gas or gases may be formed on the electrodes.
• Deposits of metals may form on electrodes.
• Change in colour of solution may occur.

Q8: When electric current is passed through acidified water then what is produced at
(a) Positive carbon electrode (anode)?
(b) Negative carbon electrode (cathode)?

Ans: 
(a) Oxygen gas
(b) Hydrogen gas.

Q9: Which effect of electric current is utilized when a thin layer of chromium metal is deposited on an iron tap? What is this process known as?

Ans: Chemical effect of current is utilized. The process is known as electroplating.

Q10: What is meant by electroplating? What is the purpose of electroplating? 

Ans: 

The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating. 

Electroplating is done:

• For protection against corrosion (or rusting).
• For decorative purposes.

Q11: Which properties of chromium metal make it suitable for electroplating it on car bumpers, bath taps & bicycle handle, etc., made of iron?

Ans: It has shiny appearance. It does not corrode easily. It resists scratches.

Q12: Which metal is electroplated on iron for making ‘cans’ used for storing food & Why?
Ans: 

• Tin. It has a shiny appearance, it does not corrode & it is non-poisonous. Tin is less reactive than 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.

Q13: On what factors the chemical effect produced by an electric current depends?

Ans: 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.

Q14: In the process of purification of copper metal, a thin plate of pure copper and a thick rod of impure copper are used as electrodes & a metal salt solution is used as an electrolyte
(a) Which electrode is connected to the positive terminal of the battery?
(b) Which electrode is connected to the negative terminal of the battery?
(c) Which metal salt solution is taken as electrolyte?

Ans: 
(a) Thick rod of impure copper. 
(b) Thin rod of pure copper. 
(c) Copper sulphate solution.

Q15: Write down the important points which should be remembered while electroplating?
Ans: 

• The metal on which electroplating is to be done should be cathode, i.e., the negative electrode.
  Electroplating
• The metal to be deposited should be anode, i.e., the positive electrode.
• A water-soluble salt of the ‘metal to be deposited’ is taken as the electrolyte.