During a process work equivalent to $ 400J $ is done on a system, which gives out $ 125J $ of energy. The change in internal energy is:
(A) $ 525J $
(B) $ 375J $
(C) $ 275J $
(D) $ 200J $

Hint: Atoms are not so much stable they have high energies so to acquire stability they form molecules in which same atoms or different atoms combine to each other to attain the stability. Because the higher the energy of the atom or molecule the less stable will be that atom or molecule.

Complete step by step solution:
Atoms are not so stable they have high energies so to acquire stability they form molecules in which the same atoms or different atoms combine to each other to attain the stability. Because the higher the energy of the atom or molecule the less stable will be that atom or molecule.
Exothermic reaction: The reactions in which energy in the form of heat and light is released after the reaction, is known as exothermic reactions. In these reactions $ Q $ (symbol of energy) is written on the product side. Although, a small amount of energy is required to start a reaction but in these reactions the given energy is greater than the released energy.
Endothermic reaction: The reactions in which the energy is required to start the reaction, are known as endothermic reactions. In these reactions $ Q $ (symbol of energy) is written on the reactant side.
Work done in a reversible system is defined as the product of constant pressure (which is the same for the system and surrounding) and change in volume.
Internal energy: It is defined as the energy with the random or distorted motion of the molecule.
Here we are given a process work equivalent to $ 400J $ is done on a system, which gives out $ 125J $ of energy. Now we know that change in internal energy is equal to the sum of work done on a system and the energy given out by the system.
So here work done on the system is $ 400J $ and the energy given out by the system is $ 125J $ so change in internal energy is $ 400 + 125 = 525J $ . Hence option a is correct.

Note:
The negative energy in the reaction is defined as the exothermic reaction i.e. the reaction in which heat is evolved during the reaction. The positive energy in the reaction is defined as the endothermic reaction i.e. the reaction in which heat is required for the reaction.