Activity 11.1 Question:
“However, if the bulb does not glow, it means that the tester is not working. Can you think of the possible reasons?”
Answer:
The bulb may not glow if the connections are loose, if the bulb is fused, or if the cells in the tester have no power left. Tightening the connections, using a new bulb, or using fresh cells will fix the problem.
Activity 11.2 Questions:
- “Does the bulb of the tester glow? Does lemon juice or vinegar conduct electricity?”
- “How would you classify lemon juice or vinegar— a good conductor or a poor conductor?”
Answer:
- Yes, when we dip the tester’s ends into lemon juice or vinegar, the bulb usually glows. This shows that lemon juice or vinegar can conduct electricity.
- Lemon juice and vinegar contain acids, so they are good conductors of electricity.
Question after Activity 11.2 explanation:
“In some situations even though the liquid is conducting, the bulb may not glow. It may have happened in Activity 11.2. What can be the reason?”
Answer:
Even if a liquid allows electricity to pass, it might only let a very weak current flow. A weak current is not enough to heat the bulb’s filament and make it glow. Therefore, even though the liquid conducts electricity, the bulb does not glow because the current is too small.
General Question (related to making a new tester):
“What happens to a compass needle kept nearby when current flows in a wire?”
Answer:
When current passes through a wire, it creates a small magnetic field around it. This makes the nearby compass needle move or turn slightly, showing a deflection.
Activity 11.3 Question:
“Do you find a deflection in the compass needle the moment you dip the free ends of the tester in lemon juice?”
Answer:
Yes, when the tester’s ends are dipped into lemon juice, the compass needle deflects. This happens because the electric current flows through the lemon juice and creates a magnetic field.
Activity 11.4 Questions:
“What do you find? Does distilled water conduct electricity? Now dissolve a pinch of common salt in distilled water. Again test. What do you conclude this time?”
Answer:
Distilled water alone does not conduct electricity because it has no salts. After adding a pinch of salt, it starts conducting electricity. This shows that pure water does not conduct electricity, but water with dissolved salts does.
Activity 11.5 Question:
“Test which solutions conduct electricity and which do not. What results do you obtain?”
Answer:
When we add a few drops of acid or base to distilled water, it conducts electricity. But if we add sugar to distilled water, it does not conduct electricity. The result is that solutions with acids or bases conduct electricity, while sugar solution does not.
Activity 11.6 Questions:
“Do you notice any gas bubbles near the electrodes? Can we call the change taking place in the solution a chemical change?”
Answer:
Yes, we can see tiny gas bubbles near the electrodes when current passes through certain solutions. This shows that a chemical reaction is taking place. Therefore, the change is a chemical change.
From the discussion on passing current through water:
“What happens when current is passed through water?”
Answer:
When electric current passes through water (especially if some salts or acid are added), it breaks water into hydrogen and oxygen gases. You can see small bubbles forming at the electrodes.
From the potato testing example:
Boojho found a greenish-blue spot around one wire when testing a potato. What does this mean?
Answer:
The greenish-blue spot appears around the wire connected to the positive terminal. It happens because a chemical reaction takes place in the potato due to the electric current. This also shows the direction of current and helps identify the positive end.
Activity 11.7 Questions:
“Do you find any difference in any one of them (electrodes)? Do you find a coating over it? What colour is the coating? Note down the terminal of the battery with which this electrode is connected.”
Answer:
Yes, we find that the electrode connected to the negative terminal of the battery gets a coating of copper. This coating is usually reddish-brown, the color of copper.
Additional Question after Activity 11.7:
“After doing the electroplating activity, Paheli interchanged the electrodes and repeated the activity. What do you think she would observe this time?”
Answer:
If Paheli reverses the electrodes, the other electrode (now connected to the negative terminal) will get coated with copper. Electroplating depends on which side is connected to the negative terminal.
Another related question:
“Boojho replaced a copper plate with a carbon rod and succeeded in obtaining a coating of copper on it. What does this mean?”
Answer:
It means that electroplating can put a layer of one metal (in this case, copper) on many different materials, even a carbon rod, as long as it is connected to the negative terminal.
General Questions on Electroplating:
“What is electroplating?”
Answer:
Electroplating is the process of using electricity to deposit a thin layer of one metal onto another object.
“Why is electroplating done?”
Answer:
Electroplating is done to give metals a shiny look, make them resistant to rust, prevent them from getting damaged, or to make cheaper metals look like more expensive metals.
Exercise Questions:
- 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 __________.
Answers:
(a) Acids, bases, and salts
(b) Chemical
(c) Negative
(d) Electroplating
- Question: “When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason?”
Answer:
The magnetic needle deflects because the solution conducts electricity. The electric current creates a magnetic field that makes the compass needle move.
- Question: “Name three liquids, which when tested in the manner shown, may cause the magnetic needle to deflect.”
Answer:
Lemon juice, salt water, and vinegar can make the magnetic needle deflect because they conduct electricity.
- Question: “The bulb does not glow in the setup shown. List the possible reasons.”
Answer:
The bulb may not glow if:
• The bulb is fused.
• The connections are loose or broken.
• The solution does not conduct electricity well.
• The battery is weak or used up.
- Question: “A tester is used to check the conduction of electricity through two liquids, A and B. The bulb glows brightly for A and glows dimly for B. What would you conclude?”
Answer:
(i) Liquid A is a better conductor than liquid B.
(So the correct choice is (i).)
- Question: “Does pure water conduct electricity? If not, what can we do to make it conducting?”
Answer:
Pure water does not conduct electricity because it has no dissolved salts. If we add a little salt or acid to it, then it starts conducting electricity.
- Question: “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.”
Answer:
Water usually has dissolved salts and can conduct electricity. If the electrical supply is not turned off, water poured to extinguish the fire could carry electricity and give someone an electric shock. Turning off the electricity makes it safe.
- Question: “A child 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?”
Answer:
Seawater has more salts than drinking water. More salts mean better conductivity, which leads to a stronger current and a bigger deflection in the compass needle.
- Question: “Is it safe for the electrician to carry out electrical repairs outdoors during a heavy downpour? Explain.”
Answer:
No, it is not safe. Rainwater contains salts and can conduct electricity. There is a high risk of electric shock if electrical work is done in wet conditions.
- Question: “Paheli had heard that rainwater is as good as distilled water. She collected some rainwater in a clean glass and tested it. To her surprise, the compass needle showed deflection. What could be the reason?”
Answer:
Rainwater is not pure like distilled water. It dissolves impurities and salts from the air, making it conductive. This is why the compass needle showed deflection.
- Question: “Prepare a list of objects around you that are electroplated.”
Answer:
Some objects around us that are electroplated are:
• Bicycle handlebars (chrome-plated)
• Water taps (chrome-plated)
• Car parts (chrome-plated)
• Imitation jewelry (gold or silver-plated)
• Tin cans (tin-plated iron)
- Question: “In the process of purification of copper, a thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from the 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?”
Answer:
The impure copper rod should be connected to the positive terminal. When current passes, copper from the positive electrode dissolves into the solution and then deposits onto the negative electrode, which is the thin pure copper plate.