Solved examples for class 8 science : Chapter 14

    Chemical Effects of Electric Current

    I. Multiple Choice Questions (MCQs).

    Q.1. Which gases are liberated during electrolysis of water?
    a) Hydrogen and oxygen
    b) Oxygen and nitrogen
    c) Nitrogen and hydrogen
    d) Hydrogen and carbon dioxide

    Answer: (a) Hydrogen and oxygen
    Q.2. Electroplating is done on iron objects to …………………….. ,
    a) decorate it
    b) make it expensive
    c) make it smooth
    d) protect it from corrosion
    Answer. (d) protect it from corrosion

    Q.3. A liquid or solution that breaks up during electrolysis is called …………………..
    a) electrode
    b) electrolyte
    c) electroplating
    d) electrolysis

    Answer. (b) electrolyte
    Q.4. Never handle electric appliances
    a) with wet hands
    b) while standing on a wet floor
    c) both (a) and (b)
    d) none of these
    Answer. (c) both (a) and (b)

    Q.5. The rod or plate through which electric current either enters or leaves an electrolytic solution is called a/ an
    a) electrolyte
    b) electrode
    c) electrolytic plate
    d) all of these

    Answer. (b) electrode

    II. Fill in the blanks by selecting appropriate words from the given list: List: poor, shock, electroplating, momentarily, weak.

    Q.1. The electric circuit or tester is used for detecting ……………………….current.
    Q.2. Air is a………………………………… conductor of electricity.
    Q.3. To check whether the tester is working or not, connect both free ends of two wires……………….
    Q.4. Sometimes we get an electric ………………….when we touch an electric appliance with dry hand.
    Q.5. Iron objects can be protected from corrosion by ……………………it with nickel.

    Answer.
    1- weak,
    2- poor,
    3- momentarily,
    4- shock,
    5- electroplating

    III. Match the items of Column A with those of Column B.


    Answer. 1-e, 2-a, 3-d, 4-b, 5-c

    IV. Write ‘T’ for the true and ‘F’ for the false statements.

    1. The longer lead of an LED is the positive lead.

    2. LED glows only when a strong electric current flows through it.
    3. The phenomenon of causing chemical changes by passing electric current is called chemical effect of electric current.
    4. Electroplating is widely used in industry for coating metal objects with a thin layer of different metals.

    5. Most of the substances that exist as liquids at room temperature, such as alcohol, oil, etc., are good conductors of electricity.

    Answer. 1-T, 2-F, 3-T, 4-T, 5- F

    V. Very Short Answer Questions.

    Q.1. Write the full form of LED.
    Answer: Light emitting diode

    Q.2. Which of these can conduct electricity- distilled water or tap water?
    Answer. Tap water

    Q.3. Name the metal which is deposited on car parts, bath taps and bicycle handle bars.
    Answer. Chromium

    Q.4. During the electroplating of copper on stainless steel, which electrode should be connected to the positive terminal of the battery?
    Answer. Copper electrode should be connected to the positive terminal of the battery.

    Q.5. In which state are common salt, potassium iodide, etc., good conductors of electricity?
    Answer: The molten states of common salt and potassium iodide are good conductors of electricity.

    VI. Short Answer Questions.

    Q.1. In case of some conducting liquid while all components of electric circuit are working, the bulb may not glow. Why?

    Answer. In case of some conducting liquid while all components of electric circuit are working, the bulb may not glow. The reason is that if the current through a circuit is too weak, the filament of the bulb does not heated to a high temperature to start glowing.

    Q.2. Define conductor and insulator with examples.

    Answer: A conductor is a material that easily conducts electrical current. Most metals are good conductors. The best conductors are single element materials, such as copper, silver and gold etc. A material that does not conduct electric current or heat is called insulators. Examples of insulators are glass, wood, plastic etc.

    Q.3. Define electrolyte and electrode.
    Answer. The process of chemical decomposition of a liquid or solution on passing an electric current is called electrolysis and the liquid or solution that breaks up during electrolysis is called electrolyte.
    The rod (or plate) through which electric current either enters or leaves an electrolytic solution is called an electrode.

    Q.4. How can you find out the conductors and insulators amongst the solid materials?
    Answer. To find out the conductors and insulators amongst the solid materials, we need solid metals such as copper and aluminium, rubber, plastic and wood, an electric circuit.

    Procedure : Take an electric circuit as shown. To test the electric circuit whether it is working or not, join the free ends of the circuit for a moment. This completes the circuit and the bulb should glow. Remove the joint. However, if the bulb does not glow, it means that the electric circuit is not working. In this condition, check all the connections, bulb, cell, etc.


    Now to test to test the various solids, place a aluminium wire in the gap touching the point A and B. The bulb starts glowing. This shows that the aluminium is a conductor of electricity. Now repeat the above activity with other materials. You observe that aluminium and copper conduct electricity but rubber, plastic and wood do not conduct electricity.

    Conclusion : Copper and aluminium are conductors but rubber, plastic and wood are insulators or bad conductors.

    Q.5. How can you show the flow of electric current through liquids?

    Answer: Aim : To show the flow of electric current through liquids, we need, about 20 ml of liquids such as distilled water, lemon juice, vinegar, tap water, vegetable oil, milk and honey in separate beakers; electric circuit.

    Procedure : Take an electric circuit or tester

    Now dip the ends A and B of the connecting wires of the circuit into the distilled water.
    Check whether bulb glows or not.


    Take out the points A and B from distilled water and dip them into lemon juice.
    Check whether bulb glows or not.
    Repeat the activity with other liquids. In each case observe whether the bulb glows or not.
    Record your observations in table. If the bulb glows, it shows that liquid conducts electricity i.e., good conductor. If it does not, the liquid does not conduct electricity i.e. poor conductor.

    Q.6. Why is an LED or magnetic compass used as a tester?

    Answer: In some cases, even though the liquid is conducting and all components of electric circuit are working, the bulb may not glow. The reason is that if the current through a circuit is too weak, the filament of the bulb does not heated to a high temperature to start glowing. To short out this problem we need a device which can detect weak current. LED and magnetic compass can detect weak current so these are used as testers.

    VII. Long Answer Questions.

    Q.1. What do you mean by chemical effects of electric current? How can you study them? Discuss with an activity.

    Answer: When electric current is passed through a conducting solution, some chemical reaction takes place. The resulting effects are called chemical effects of electric current.
    Examples:
    (i) When electric current is passed through water, water dissociates into hydrogen and oxygen. (ii) When electric current is passed through the solution of a metal salt, such as solution of copper sulphate, metal gets deposited at the negative electrode, because metal is positively charged.

    Aim : To study the chemical effect of electric current on water.
    Requirements: A beaker, two carbon rods or two iron nails, connecting wires, two graduated tubes, battery, switch, dilute sulphuric acid.
    Procedure : Take a beaker and half fill it with water. Add a few drops of dilute sulphuric acid to it. (To make water a conducting solution a few drops of dil. $H_{2}SO_{2}$ are added. Because pure water is a poor conductor of electricity.)
    Take two carbon rods or iron rods. Now make connection with battery, switch and rods with the help of connecting wires as shown in the figure. Fill two test tubes A and B with this water. Place them over the rods by putting thumb on the open ends of tubes and by inverting them into water containing beaker. Switch on the circuit. After a few moments, bubbles of gases evolving at the ends of rods in the beaker. The gases liberated are collected in the two test-tubes by the downward displacement of water.


    After some time, volume of the two gases collected in the test tubes. The gas collected in test tube 8 is double in volume than that collected in test tube A i.e., the volumes of the two gases collected are in the ratio of 2 : 1. On testing, with a burning matchstick, the gas in test tube ‘A’ helps in burning violently. But the gas in test tube ‘B’ burns with an explosive sound. This shows that gas collected in test tube A is oxygen and gas collected in test tube ‘B’ is hydrogen.
    The process of chemical decomposition of a liquid or solution on passing an electric current is called electrolysis and the liquid or solution that breaks up during electrolysis is called electrolyte.

    Q.2. Distilled water is bad conductor of electricity. But when salt is dissolved in water, it becomes conductor of electricity. Explain all of these with a proper activity.
    Answer: To show that the Distilled water is bad conductor of electricity , we need, we need, about 20 ml of distilled water, salt and electric circuit.

    Procedure : Take an electric circuit or tester
    Now dip the ends A and B of the connecting wires of the circuit into the distilled water.
    Check whether bulb glows or not. The bulb doesn’t glowing.
    Take out the points A and B from distilled water.
    Add salt in the distilled water and dip the point A and B into salt solution.
    Check whether bulb glows or not. In this time bulb is glowing. This proves that salt solution conduct electricity i.e. conductor of electricity.


    Q.3. (a) Discuss applications of the chemical effects of electric current.
    (b) What do you understand by electroplating? How can you carry out electroplating of copper on stainless steel?

    Answer: a) The process of chemical effect has many important applications :
    (i) For extracting metals from their ores.
    (ii) For purifying metals such as copper, silver or gold.
    (iii) For electroplating things. For example, deposition of chromium on car parts, bath taps, bicycle handle bars, deposition of silver on brass or copper, etc., is done by electroplating.

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

    Aim: For electroplating of copper on stainless steel we need copper sulphate crystals, distilled water in a dry and clean beaker, 10 cm × 4 cm size copper plate, battery, stainless steel spoon, dilute sulphuric acid, sand paper, connecting wires.

    Procedure: Prepare copper sulphate solution by dissolving two tea spoonful of copper sulphate in distilled water. Add a few drops of dilute sulphuric acid to copper sulphate solution to make it more conducting. Clean copper plate and spoon with sand paper. Rinse them with water and dry them.
    Connect a copper strip to the positive terminal of the battery using a connecting wire.
    Similarly, connect the spoon to the negative terminal (acts as the cathode) of the battery using a connecting wire. Dip them in copper sulphate solution. Keep this arrangement undisturbed for 2530 minutes. Now remove both of them carefully and observe them carefully.
    You observe that a reddish coating of copper metal on the spoon is observed.

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