Revision questions for practice class 8 cbse

1. Maths questions for chapter Comparing Quantities:-
1. Express as a trinomial.
2.
3. Obtain the identity for the product of (x + a) and (x + b).
4. If x + y = 4 and xy = 2, find x2
+ y2
.
5. Find the value of expression(x + y)2
- (x - y)2
, if .
6. Define monomial, binomial and trinomial and give two examples of
each.
7. Find the product: .
8. Simplify:
9.
10.
11.
12. If then find
13.
14.
Science questions-
1. Which of the following is not a strong electrolyte?
 Acetic acid solution
2. Define electroplating. List any three uses of it.
 The method of coating one metal with another using an electric current is
called electroplating.
Uses:
i) Metals that corrode easily can be protected by coating them with a metal
that does not corrode easily.
 ii) An attractive and shiny appearance is given to the objects made of iron or
steel.
 iii) Ornaments made of a cheap metal are coated with a more expensive
metal to make them look more attractive.
3. What is an electrolyte? Give one example.
 A liquid that conducts electricity because of the presence of the ions is called
an electrolyte.

2. Example: a solution of copper sulphate salt in water.
4. A metal is released in the electrolysis of a salt. It gets deposited on the
cathode.
5. How will you identify the positive terminal of a battery if it is hidden in a box
and you can only see two of its leads.
6. Solution
 Both the leads of the battery can be dipped into a single piece of potato for
long time. After that, it can be observed that there is green coloration near
one of the terminals. This is the positive terminal and the other one is the
negative terminal.
6. Chemical effects in the solution are produced when an electric current flows through a
solution.
7. LED glows even with weak currents but bulbs do not. So, an LED is sometimes used in
the circuit to detect weak currents passing through weak electrolytes.
8. A child staying in a coastal region tests the drinking water and also the sea water with his
tester. He finds that the compass needle deflects more in the case of sea water. Can you
explain the reason?
9. Solution
 Sea water contains more dissolved salts than drinking water, which makes it
more conducting. Because of this reason, the compass needle deflects more
in sea water than in drinking water.
10. Electroplating is one of the chemical effects of electric current.
11. Applications of electrolysis:
(i) Used in the production of many metals and non metals, such as aluminum,
magnesium, chlorine.
(ii) Used for coating one metal with another.
12. The bulb does not glow in the setup shown in Figure. List the possible reasons. Explain
your answer.
13. Solution

3.  The bulb may not glow because of the following reasons:
(i)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.
(ii) 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.
(iii) The battery may be exhausted and does not have sufficient energy to generate
electricity.
(iv) The bulb may be fuse or the circuit connections may be loose.
14. When an electric current flows through the conducting solution, it causes a chemical
reaction (or chemical change).
These chemical reactions may produce one or more of the following effects:
i) Bubbles of gas (or gases) may be formed on the electrodes.
ii) Deposits of metals may form on electrodes.
iii) Change in colour of solution may occur.
15. Using the technique of electroplating, iron is coated with zinc which does not corrode.
16. Is it safe for the electrician to carry out electrical repairs outdoors during heavy
downpour? Explain
17. Solution
18. No, it is not at all safe for an electrician to carry out electrical repairs during heavy
downpour. Rather, it is highly dangerous because during heavy downpour there is a high
risk of electrocution as water is a good conductor of electricity.
19. Electrolyte: A liquid that conducts electricity due to the presence of ions is called an
electrolyte.
(ii) Cathode: The electrode connected to the negative terminal of the battery is called
cathode.
(iii) Anode: The electrode connected to the positive terminal of the battery is called
anode.
20.When the liquid between two ends of a tester is a good conductor, the bulb glows
because-
A conducting liquid allows electric current to pass through it and hence completes the circuit.
This makes the bulb connected in the circuit glow.