Goa Board Class 8 Solutions for Science – Chemical Effects of Electric Current (English Medium)
Fill in the blanks.
(a) Most liquids that conduct electricity are solutions of ________, _______ and ________.
(b) The passage of an electric current through a solution causes _________ effects.
(c) If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the __________ terminal of the battery.
(d) The process of depositing a layer of any desired metal on another material by means of electricity is called _________.
(a) acids, bases and salts.
Concept insight: Solutions of acid, bases and salts conduct electricity because they produce ions when dissolved in water.
Concept insight: When an electric current passes through a solution, the solution decomposes into its positive and negative ions. This process of decomposition of the solution is a chemical effect of electricity.
Concept insight: When an electric current passes through a copper sulphate solution, the solution decomposes into positively charged copper ions and negatively charged sulphate ions. These positively charged copper ions get attracted towards the plate which is connected to the negative terminal of a battery.
Concept insight: By electroplating corrosion of metal objects can be prevented.
When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?
The deflection in the compass needle shows that current is flowing through the wound up wire and hence, through the circuit. This shows that the solution is a conducting solution because only then the current can flow through it and hence through the complete circuit. This current through the wound up wire produces a magnetic field around it which acts on the magnetic needle of the compass and deflects it.
Concept insight: A conducting liquid allows electric current to pass through it.
Name three liquids, which when tested in the manner shown in figure, may cause the magnetic needle to deflect.
Liquids like lemon juice, salt water and vinegar can conduct electricity. Hence, these liquids can cause the magnetic needle to deflect.
Concept insight: A conducting liquid or electrolyte contains positively and negatively charged ions. The flow of these ions conducts electricity.
The bulb does not glow in the setup shown in figure. List the possible reasons. Explain your answer.
The bulb may not glow because of the following reasons:
- Liquid in the beaker may be non-conducting. In such a case, the electric current would not be able to pass through the liquid. Hence, the circuit will not be complete.
- The conductivity of the liquid may be very low and so the current flowing through the circuit may be too weak to produce enough heat in the filament of the bulb so as to make it glow.
- The battery may be exhausted and does not have sufficient energy to generate electricity.
- The bulb may be fuse.
- Connections may be loose.
A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that
- Liquid A is a better conductor than liquid B.
- Liquid B is a better conductor than liquid A.
- Both liquids are equally conducting.
- Conducting properties of liquid cannot be compared in this manner.
- Liquid A is a better conductor than liquid B.
Concept insight: A strong conducting liquid contains a lot of ions in it, while a weak conducting solution contains lesser number of ions.
Does pure water conduct electricity? If not, what can we do to make it conducting?
No. Pure water does not conduct electricity. This is because pure water is devoid of any salts. Pure water can conduct electricity when a pinch of salt is added to it, as salt solution is conducting in nature.
Concept insight: In pure form, water does not contain any salts in it which can provide it ions to conduct electricity.
In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this.
Water usually contains salts and conducts electricity. If the electrical supply is not cut off and firemen come in contact with wet electric switches, electric wires and other electrical appliances, they may get electrocuted. So, the electrical supply is cut off to prevent electrocution of firemen.
Concept insight: Ordinary water is a good conductor of electricity.
A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason?
Water we use for drinking purposes has very less amount of salts dissolved in it while sea water has very high concentration of salts in it. So, compass needle shows more deflection in case of sea water.
Concept insight: Sea water has high concentration of salts in it, so it conducts large amount of electricity.
Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
No. It is not safe to repair electrical appliances outdoors during heavy downpour. This is because rain water contains small amounts of acids in it which makes it a conductor of electricity. So, the electrician may get electrical shocks while working outdoors during rain.
Concept insight: Rain water contains salts and acids dissolved in it, so it is a good conductor of electricity.
Paheli had heard that rain water is as good as distilled water. So, she collected some rain water in a clean glass tumbler and tested it using a tester. To her surprise, she found that the compass needle showed deflection. What could be the reasons?
Rain water is said to be pure water. But many impurities and acidic gases get dissolved in the rain water while it falls to the earth through the atmosphere. Due to the presence of these impurities and small amounts of acids in it, the rainwater conducts electricity. This explains Paheli’s observation that rain water can allow electricity to pass through it while distilled water cannot.
Prepare a list of objects around you that are electroplated.
Examples of electroplated objects are as follows:
- Chromium plating is done on different parts of cars, buses and motor cycles to give them shiny appearance.
- A fine layer of gold is deposited on the silver ornaments and they are called gold-plated ornaments.
- Iron used in constructing a building is coated with a layer of zinc (galvanization). This protects iron from corrosion and rusting.
Concept insight: Electroplating prevents corrosion of metal objects.
The process that you saw in Activity 14.7 is used for purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transferred to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why?
Copper ions are positively charged. They are attracted towards the plate which is connected to the negative terminal of the battery. As copper ions are transferred to the thin copper plate, this thin pure copper plate must be connected to the negative terminal of the battery. Consequently, impure copper rod is connected to the positive terminal of the battery.
oncept insight: When electric current is passed in this arrangement, copper ions from the solution get deposited on pure copper rod (attached to negative terminal) and copper ions from impure copper rod (attached to positive terminal) are liberated into the solution.