List of top Questions asked in CBSE CLASS XII- 2025

Assertion (A): The boiling points of alkyl halides decrease in the order: RI>RBr>RCl>RF. 
Reason (R): The boiling points of alkyl chlorides, bromides and iodides are considerably higher than that of the hydrocarbon of comparable molecular mass.

Arrange the following compounds in increasing order of their boiling points:
 increasing order of their boiling points

Acidified \(KMnO_4\) oxidizes sulphite to: 

In case of association, abnormal molar mass of solute will: 

Your school is organizing an Inter-House Science Model-Making Competition. As President of the Science Club, draft a notice to inform all House members from IX – XII about the competition and specify the number of registrations invited per house. Include other necessary details. You are Mitali/Mukesh. Put your notice in a box.
A right-angled prism \( ABC \) (refractive index \( \sqrt{2} \)) is kept on a plane mirror as shown in the figure. A ray of light is incident normally on the face \( AC \). Find the angle of deviation produced by the prism.

(i) State Lenz's law and explain how this law is a consequence of conservation of energy principle.

(ii) A square shaped loop of side \(\dfrac{\ell}{2}\) is initially lying outside a region of uniform magnetic field \(\vec{B}\) as shown in the figure. The loop is moved towards right with a constant velocity \(\vec{v}\) till it goes out of the region of magnetic field.

(I) What will be the directions of induced current when the loop enters the field and when it leaves the field ?

(II) Draw the plots showing the variation of magnetic flux \(\phi\) linked with the loop with time \(t\) and variation of induced emf \(E\) with time \(t\). Mark the relevant values of \(E\), \(\phi\) and \(t\) on the graphs.


A soft iron rod X is allowed to fall on the two poles of a U shaped permanent magnet as shown in figure. A coil is wrapped over one arm of the U shaped magnet. During fall of the rod, the current in the coil will be
The number of turns between different pairs of output terminals are shown for a step-up transformer. Input voltage of 20 V is applied between A and B. Between which two terminals will the output be 120 V ?
Which pair of readings of ideal voltmeter and ideal ammeter in the given circuit is possible when a suitable power source of 3 \( \Omega \) internal resistance is connected between P and Q ?
An ac source of voltage \( V = V_0 \sin \omega t \) is connected to a circuit element \( X \). It is observed that the current flowing through \( X \) varies as \( I = I_0 \sin\left(\omega t - \dfrac{\pi}{2}\right) \).
(c) Draw plots showing the variation of voltage and current with time over one cycle of applied ac.

The minimum energy required to free the electron from the ground state of the hydrogen atom is :

An ac source of voltage \( V = V_0 \sin \omega t \) is connected to a circuit element \( X \). It is observed that the current flowing through \( X \) varies as \( I = I_0 \sin\left(\omega t - \dfrac{\pi}{2}\right) \).
(b) Plot a graph to show the variation of reactance of the element with the frequency of the applied voltage.
Draw a graph of potential energy between a pair of nucleons as a function of their separation.

Find the \(Q\) value of the following nuclear reaction:

\({}_6^{12}\text{C} + {}_6^{12}\text{C} \longrightarrow {}_{10}^{20}\text{Ne} + {}_2^{4}\text{He}\)

Is this reaction exothermic or endothermic ?

Given: \(m({}_6^{12}\text{C}) = 12.000000\ \text{u}\), \(m({}_{10}^{20}\text{Ne}) = 19.992439\ \text{u}\), \(m({}_2^{4}\text{He}) = 4.002603\ \text{u}\), \(1\ \text{u} = 931\ \text{MeV}/c^2\)

The magnetic moment (\(5\;\text{J/T}\)) of a bar magnet points along a uniform magnetic field \(0.4\;\text{T}\).

(a) Calculate (i) the potential energy of the bar magnet, and (ii) the work done in turning the magnet by \(180^{\circ}\).

(b) In which case is the potential energy of the magnet minimum ?

A capacitor of plate area \(A\) and plate separation \(d\) is charged by a battery to voltage \(V\). The battery is disconnected and plates are slowly pulled apart till the separation becomes \(2d\). Find the value of :

(a) potential difference between the plates,

(b) electric field between the plates,

(c) work done in pulling the plates apart.

Using the Huygens' principle, briefly describe reflection of a plane wavefront from a reflecting surface. Hence, prove the laws of reflection.

(a) Write the conditions under which two light waves originating from two coherent sources can interfere each other (i) constructively, and (ii) destructively, in terms of wavelength. Can these be applied for two lights originating from two sodium lamps ? Give reason.

(b) Monochromatic light of green colour is used in Young's double slit experiment and an interference pattern is observed on a screen. If the green light is replaced by red monochromatic light of the same intensity, how will the fringe width of interference pattern be affected ? Justify your answer.

A particle of mass \(m\) and charge \(q\) moves along \(y\)-axis in a region in which a uniform magnetic field \(\vec{B}\) is pointing along \(x\)-axis. The Lorentz force acting on the charge will point along :

In Bohr's model of hydrogen atom, find the percentage change in the radius of its orbit when an electron makes a transition from \(n = 3\) state to \(n = 2\) state.

The two surfaces of a biconvex lens are of radius of curvature '\(R\)' each. Obtain the condition under which its focal length '\(f\)' be equal to '\(R\)'. If one of the two surfaces of this lens is made plane, what will be the new focal length of the lens ?

In Young's double slit experiment, the screen is moved 30 cm towards the slits. As a consequence, the fringe width of the pattern changes by 0.09 mm. If the slits separation used is 2 mm, calculate the wavelength of light used in the experiment.
A charged particle \(q\) moving with a velocity \(\vec{v}\) is subjected to a uniform magnetic field \(\vec{B}\) acting perpendicular to \(\vec{v}\). If a uniform electric field \(\vec{E}\) is also set up in the region along the direction of \(\vec{B}\), describe the path followed by the particle and draw its shape.
In a double slit experiment, the two slits are 1.5 mm apart. The slits are illuminated by a mixture of lights of wavelengths of 600 nm and 400 nm and the interference pattern is observed on a screen 1.5 m away from the slits. Find the minimum distance of the point from the central maximum at which bright fringes of the interference patterns of the two wavelengths coincide.