1. Chemical Effects Of Electric Current-
ü Passage of current through chemical solutions causes chemical reactions in the solution.
ü Chemical effects cased are the following-
n Formation of gas bubbles at electrodes,
n Deposition of metals at electrodes.
2. Conductor And Insulator-
ü Any material that allows heat and electric current to pass through it is known as a conductor. For example, metals like copper, iron etc.
ü Where as materials that do not allow the flow of heat and electric current through it are known as insulators. For example, Rubber, plastic etc.
3. Electric Circuit-
Figure 01- Closed and open circuit
ü A closed-loop path which a current take is known as an electric circuit.
ü When the path of the circuit is closed, the current flows through it, but when there is a break then, the circuit is open and is not conducting.
Figure 02- Testing of current by magnetic deflection of compass needle
Note: Tester- It is a piece of electrical equipment used to check the presence of electric current. It is usually a conductor with a led/bulb to indicate that the current is present in the circuit.
4. Conducting Liquid-
Figure 03- Conduction through lemon juice
ü Liquids also conduct electricity when there are salts dissolved in them.
ü Most liquids that conduct electricity are solutions of acids, bases or salts.
ü Acids and bases are chemical substances that dissociate to form ions when dissolved in a solution.
ü They are a good conductor of electricity because of the presence of the ions.
ü Salts when dissolved in water also conduct as they release positive/negatively charged ions.
ü Distilled water is a bad conductor of electricity because of the absence of dissolved salts and minerals.
ü Water starts conducting when acids, bases or salts are dissolved that releases ions.
ü Conductance in liquids is due to the presence of ions and thus called as ionic conductance.
5. Electrodes And Electrolyte-
Figure 04- Electrochemical/Electrolytic cell
ü A conductor, when dipped in a solution with its end connected to the terminals of a battery, thereby completing a circuit, is called as an electrode.
ü There are usually two electrodes, one cathode(negative) and one anode(positive).
ü An electrolyte is a solution in which the electrodes are submerged.
ü They dissociate on the passage of electric current.
ü The electrodes, electrolyte and the battery together form the electro-chemical and electrolytic cell.
ü The electro-chemical cell converts chemical energy to electrical energy.
ü The electrolytic cell converts electrical energy into chemical energy.
6. Electroplating-
Figure 05- Elector-platting
ü The process of depositing a layer of desired metal on another material by means of electricity is known as electroplating.
ü Copper Sulphate solution as electrolyte and copper electrodes.
ü Copper is electroplated on the negative electrode.
ü The copper in the solution is replenished due to the addition of copper ions from the positive electrode.
ü Applications of electroplating are-
n Coating zinc on the iron to prevent corrosion and rust.
n Coating silver and gold for jewelry.
n Coating tin onto iron for cans as tin is less reactive than iron.
n Chromium coating for car parts, bath fittings as it has a shiny appearance.
NCERT PRACTICE QUESTION
1. 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 _______
Ans.- (a) acids, bases, salts (b) chemical (c) negative (d) electroplating
2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?
Ans.- The deflection in magnetic needle shows that the circuit is complete and the solution conducts electricity, i.e., it is a good conductor.
3. Name three liquids, which when tested in the manner shown in Fig. 14.5. may cause the magnetic needle to deflect.
Ans.- Vinegar, lemon juice and tap water.
4. The bulb does not glow in the setup shown in Fig. 14.6. List the possible reasons. Explain your answer.
Ans.- The bulb may not glow because of the following reasons-
The wires in the circuit may be loosely connected.
The bulb may be fused.
The cells may be used up.
The liquid may be an insulator, i.e., a poor conductor of electricity.
5. 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
(i) liquid A is a better conductor than liquid B.
(ii) liquid B is a better conductor than liquid A.
(iii) both liquids are equally conducting.
(iv) conducting properties of liquid cannot be compared in this manner.
Ans.- (i) liquid A is a better conductor than liquid B.
6. Does pure water conduct electricity? If not, what can we do to make it conducting?
Ans.- No, pure water doesn’t conduct electricity. But when salt is dissolved in pure water, it conducts electricity.
7. 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.
Ans.- The water used in the water hoses is not pure water and is a good conductor of electricity. So, the fire¬men shut off the electric supply before spraying water to save themselves and other people from electrocution.
8. A child staying in a coastal region test 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?
Ans.- The seawater contains a huge amount of salts in comparison to drinking water, hence the seawater is a better conductor of electricity and it produces a stronger magnetic field in the wire and hence deflects the compass needle more.
9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain.
Ans.- No, it is highly dangerous to carry out the electrical repairs outdoors during the heavy downpour. It can cause electrocution, as water is a good conductor of electricity.
10. Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater 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?
Ans.- Rainwater is pure water which is an insulator but it gets mixed with air pollutants like sulphur dioxide and nitrogen oxides and form acidic solution, which is a good conductor of electricity. So, the compass needle showed deflection.
11. Prepare a list of objects around you that are electroplated.
Ans.- Objects that are electroplated are door handles, taps, rims of cycles, showers, the handlebar of cycles and bikes, gas burner, tin cans, metallic almirahs, buckles of belts, etc.
12. 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?
Ans.- The thick rod of impure copper plate is to be attached to the positive terminal of the battery because when electric current is passed through the copper sulphate solution, it gets dissociated into copper and sulphate. The free copper, being positively charged, gets drawn to the negative terminal of the battery and gets deposited on it. On the other hand the loss of copper from the solution is regained from the impure copper rod which is attached to the positive terminal of the battery.
Junior Science Topic - Chemical Effects Of Electric Current. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 8, CBSE syllabus, chapter 14)
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