List of top Science Questions asked in CBSE Class X

An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focused image can be obtained? Find the size and the nature of the image.
Find the focal length of a convex mirror whose radius of curvature is 32 cm.
Show how you would connect three resistors, each of resistance 6 Ω, so that the combination has a resistance of 
  1. 9 Ω
  2. 4 Ω
A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
Explain the following.
  1. Why is the tungsten used almost exclusively for filament of electric lamps?
  2. Why are the conductors of electric heating devices, such as bread-toasters and electric irons, made of an alloy rather than a pure metal?
  3. Why is the series arrangement not used for domestic circuits?
  4. How does the resistance of a wire vary with its area of cross-section?
  5. Why are copper and aluminium wires usually employed for electricity transmission?
Why do we apply paint on iron articles?
The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
Draw the structure of a neuron and explain its function.
Explain the following terms with one example each. 
(a) Corrosion 
(b) Rancidity
A battery of 9 V is connected in series with resistors of 0.2 Ω, 0.3 Ω, 0.4 Ω , 0.5 Ω and 12 Ω, respectively. How much current would flow through the 12 Ω resistor?
Balance the following chemical equations.
(a) HNO3 +Ca(OH)2 \(→\) Ca(NO3)2 + H2O 
(b) NaOH + H2SO4 \(→\) Na2SO4 + H2O 
(c) NaCl + AgNO3 \(→\) AgCl + NaNO3 
(d) BaCl + H2 SO4 \(→\) BaSO4 + HCl
Oil and fat containing food items are flushed with nitrogen. Why?

A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?

What is the difference between the manner in which movement takes place in a sensitive plant and the movement in our legs?

How are fats digested in our bodies? Where does this process take place?

State two ways to prevent the rusting of iron.

What are the differences between aerobic and anaerobic respiration? Name some organisms that use the anaerobic mode of respiration.

Why does the sky appear dark instead of blue to an astronaut?
Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 × 108 m s−1.
Which of the statements about the reaction below are incorrect?
\(\text{ 2PbO(s) + C(s) → 2Pb(s) + C}O_2\text{(g)}\)
(a) Lead is getting reduced. 
(b) Carbon dioxide is getting oxidized. 
(c) Carbon is getting oxidized. 
(d) Lead oxide is getting reduced.
How does phototropism occur in plants?

Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an Activity to prove it.

One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.
You must have seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why these sour substances are effective in cleaning the vessels.
Why is a normal eye not able to see clearly the objects placed closer than 25 cm?