Science (English Medium) Class 12 - CBSE Important Questions

If light of wavelength 412.5 nm is incident on each of the metals given below, which ones will show photoelectric emission and why?

A hydrogen atom initially in the ground level absorbs a photon, which excites it to the n = 4 level. Determine the wavelength and frequency of the photon.

State Bohr's postulate to define stable orbits in the hydrogen atom. How does de Broglie's hypothesis explain the stability of these orbits?

Explain the processes of nuclear fission and nuclear fusion by using the plot of binding energy per nucleon (BE/A) versus the mass number A

A student wants to use two p-n junction diodes to convert alternating current into direct current. Draw the labelled circuit diagram she would use and explain how it works.

Give three reasons why modulation of a message signal is necessary for long-distance transmission

Show graphically an audio signal, a carrier wave and an amplitude modulated wave

A carrier wave of peak voltage 15 V is used to transmit a message signal. Find the peak voltage of the modulating signal in order to have a modulation index of 60%

How do you convert the following:

Aniline to phenol

How do you convert the Ethanal to Propanone

When concentrated sulphuric acid was added to an unknown salt present in a test tube a brown gas (A) was evolved. This gas intensified when copper turnings were added to this test tube. On cooling, the gas (A) changed into a colourless solid (B).

1) Identify (A) and (B).

2) Write the structures of (A) and (B).

3) Why does gas (A) change to solid on cooling?

Arrange the following in the decreasing order of their reducing character :

HF, HCl, HBr, HI

Account for the following:

pK_a value of 4-nitrobenzoic acid is lower than that of benzoic acid.

Give a simple chemical test to distinguish between Aniline and N, N-dimethylaniline

In a potentiometer arrangement for determining the emf of a cell, the balance point of the cell in open circuit is 350 cm. When a resistance of 9 Ω is used in the external circuit of the cell, the balance point shifts to 300 cm. Determine the internal resistance of the cell.

Write the underlying principle of a moving coil galvanometer.

Define mutual inductance.

What do you understand by the statement, "Electromagnetic waves transport momentum"?

Draw a diagram of the microscopic structure of human sperm. Label the following parts in it and write their functions:

1. Acrosome
2. Nucleus
3. Middle piece

Explain the effect on the characteristics of a river when urban sewage is discharged into it.