Class–VIII Science (Chemical Effects of Electric Current
Q.1Fill in the blanks
a. Electrolysis is used for ______ one metal over another metal.
b. A combination of cells is known as ______ .
c. In liquid the moving charges are called ______ .
d. The driving force that carries charges around a circuit is ______ force.
e. Electric current is the flow of negatively charged particles called ______
Ans.electroplating, battery, ions, electromotive, electrons.
Q2.Fill in the blanks :-
a. An electric current can bring about a ______ change.
b. An ______ when dissolved in water, breaks up into ions.
c. ______ are materials that allow electricity to flow through them.
d. ______ are also called as insulators.
e. A source of electricity is called a ______ .
Ans.chemical, electrolyte, conductors, non- conductors, cell.
Q2. What is the actual direction of electric current?
Ans.Electrons flow from an electron rich body/object to an electron deficit one. In other
words from a body that is negatively charged to a body that is positively charged.
However, scientists from earlier days thought that electric current is the flow of
positive charge from a positively charged body to a negatively charged body.
Therefore, they took this as the direction of flow of current.
Today, we consider the flow of conventional electric current to be from positive to
negative electrode
3. Why is it dangerous to handle electrical appliances with wet hands or while standing
on a wet floor ?
Ans.Small amounts of mineral salts are present naturally in water. They are beneficial for
human health. However, these salts make water conducting as such if we touch an
electrical appliance with wet hands or while standing on a wet floor, current will flow
through our body and result will be an electric shock.
4.Define electrolysis. Through diagram represent movement of ions during electrolysis.
Ans.If any substance conducts electricity when dissolved in water and breaks up into its
constituents during the process of dissolving, is known as an electrolyte. The process
of breaking down of an electrolyte on passing electric current through it is called
electrolysis.
5. Write one important application of electrolysis in our daily life.
Ans.Electrolysis has many applications in our daily life but the most common of all the
applications is electroplating. In the process of electroplating, a thin layer of a metal
like gold, silver, chromium, tin, nickel etc is coated over another cheaper metal either
to protect the cheaper metal or to make that metal look attractive
6. Pick the odd word out of the following:
salt solution, sugar solution, lime water, tap water, river water.
Ans.Except sugar all solution will conduct electricity
7. Is electric shock lethal? What should be done in case of electric shock?
Ans.The electric shocks vary in their effect, as they can be mild to severe, sometimes they
even lead to death i.e. lethal.
Electric shocks can cause severe burns and tissue damage. The function of the heart
can also be disrupted by a strong electric shock. Irreparable injury to vital organs
result into lethality. If shock occurs indoors, immediate disconnection of power supply
is required. If a person is in contact with source of electric current, stand on a dry
rubber mat, wooden slab etc and try to move the person using a completely dry
wooden pole.
8. What is deplating? How is it useful?
Ans.The process just opposite to electroplating is deplating. In this process, the object to
be deplated is used as the anode ( positively charged electrode) and the recovered
metal is deposited on a cathode (negatively charged electrode) usually of the same
metal.
To recover gold and other valuable metals from the circuits of computers, mobiles,
other electronic goods, the process of deplating is very useful.
9.Describe the specific features of an LED?
Ans.An LED or light emitting diode is an electronic device. It lights up (starts emitting
light) even when a very weak current flows through it. It contains two leads (as shown
in worksheet 68) one of which is longer than the other. The longer lead is connected to
the cell/battery. Hence when current flowing through the circuit is too weak then bulb
may not glow at all so for such cases we can utilise LED which is a more sensitive
detector of current.
10. Chrome plating is very popular in the industry. What are its pros and cons?
Ans.Chrome plated objects have a good lustrous shine and appear attractive. They are
scratch and corrosion resistant. Therefore, chromium plating is done on large number
of objects, like fancy lights, bathroom accessories, kitchen appliances, automobile
parts etc.
However, chrome plating is done in a bath (solution) of chromic acid, which is
considered carcinogenic and has been known to be poisonous/toxic and cause sores
and ulcers etc.
11.Are conductors and electrolytes same? If not explain differences between them with
examples.
Ans.Materials that allow electricity to flow through them are called conductors.
Conductors like, silver, gold, copper, mercury, aluminium, iron, graphite etc are
elements, and remain unchanged when they conduct electricity.
Substances which conduct electricity when dissolved in water or when melted are
called electrolytes. In solution or in molten state, electrolyte decomposes into ions that
are charged particles and conduct electricity. Example: common salt, vinegar, caustic
soda etc.
12. Purification of metals is possible through electrolysis. Describe this application.
Ans.Purification of metals can be done by electrorefining, where metals are refined by
electrolytic method. For example: copper, silver, nickel, gold, aluminium etc purified
through electrorefipning.
The purified metal after electrolysis deposits at the negatively charged electrode i.e.
the cathode and is collected from time to time.
Q13.State whether the following statements are true or false :-
a. Natural water that runs down the hills is 100% pure water.
b. Formation of a new chemical compound by electricity is electrolysis.
c. Kerosene is a non electrolyte.
d. Lemon juice is an electrolyte.
e. All liquids conduct electricity.
Ans.false, false, true, true, false.
Q14.State whether the following statements are true or false :-
a. Passing electric currents through a conducting liquid causes chemical changes.
b. Electrolysis is an application of electroplating.
c. Vinegar is a conductor of electricity.
d. A solution that contains oppositely charged ions conducts electricity.
e. Glucose solution is an electrolyte and hence conducts electricity.
Ans.true, false, true, true, false.
Q15.State whether the following statements are true or false :-
a. Every ion has both positive as well as negative charges.
b. Electricity is a form of energy.
c. Asbestos is a good conductor of electricity.
d. Current flows in a closed circuit as well as an open circuit.
e. Different LEDs may give out light of different colours.
Ans.false, true, false, false, true.
