List of top Science Questions asked in CBSE Class X

Why are testes in human males located outside the abdominal cavity in scrotum? 

Define Puberty. List any two changes seen in boys at the time of puberty. 

What are soaps? Write the structure of a soap molecule. Explain the cleansing action of a soap. Why are soaps not considered suitable for washing clothes in a region where water is hard? How is this problem overcome? 

Name an alcohol and a carboxylic acid having two carbon atoms in their structures. Draw their structures and state how this alcohol and carboxylic acid can be converted into a carboxylic acid. What happens when these two compounds react in the presence of an acid? Write chemical equations for the reactions involved in the two cases mentioned above. 

What do the symbols given below represent in an electric circuit? Write one function of each. 

Explain the statement "Potential difference between two points is 1 volt".

Determine the focal length of the lenses used in the spectacles.
List two causes of this defect.
Name the defect of vision the person is suffering from.
Draw a ray diagram to show image formation for any correct set of observations.
Which one of the sets of observations is not correct and why?

Write one difference between dominant and recessive trait.

Give the (i) percentage of white flowered plants and (ii) ratio of the gene combinations WW, Ww, and ww in F2 generation.

Name the blood vessel that brings (i) oxygenated blood (ii) deoxygenated blood, to the human heart. Also name that chamber of the heart which receives deoxygenated blood and state how deoxygenated blood from this chamber is sent to the lungs for oxygenation. 

Carbonate.
Base.
Metal.
In soda-acid fire extinguishers.
As a constituent in making baking powder.
As an anti-acid.
Glucose reacts with oxygen in the cells of our body and provides energy.
Natural gas is burnt in air.
Steam is passed over red hot iron.

We do not clean natural ponds or lakes, whereas an aquarium or a swimming pool needs to be cleaned regularly. Why? 

Give reasons:  
(i) The sky appears dark to passengers flying at very high altitudes. 
At very high altitudes, passengers are above the atmosphere where there is less scattering of sunlight. As a result, they do not see the scattered blue light and the sky appears dark, similar to the condition experienced by astronauts in space. 
(ii) 'Danger' signal lights are red in color.