NIOS Class 12 Chemistry Important Questions 2026 | Most Important questions for board exam preparation
“Paper 1”
Important Questions and Answers
Q: What is the SI unit of mass for a substance? Explain it.
Answer: The SI unit of mass for a substance is the kilogram (kg). It is represented by the symbol ‘kg’.
Q: Define the ‘Mole’.
Answer: A mole is that quantity of a substance which contains the same number of fundamental particles (atoms, molecules, etc.) as there are atoms in exactly 0.012 kilograms (12 grams) of the carbon-12 isotope.
Q:How many picometers are there in 1 meter?
Answer: 10¹² (10 raised to the power of 12).
Q: On what grounds was Rutherford’s model rejected?
Answer: Rutherford’s model failed to explain the stability of the atom.
Q: Write down the objective and the limitations (defects) of Bohr’s model.
Answer: Bohr’s model explains the stability of the atom and the hydrogen spectrum.
Limitations: It failed to explain the line spectra of other elements.
Q:What do you understand by a ‘Wave Function’?
Answer: It is a mathematical function that describes the motion of an electron within an atom. It contains all the information regarding the system.
Q: Explain Heisenberg’s Uncertainty Principle.
**Answer:** It is impossible to simultaneously determine and measure with absolute precision both the exact position and the exact momentum of a particle.
Q:Explain the ‘Law of Octaves’.
Answer: When elements are arranged in the increasing order of their atomic masses, the properties of every eighth element are similar to those of the first element, much like the notes in a musical scale.
Q:Explain Mendeleev’s Periodic Law.
Answer: The physical and chemical properties of elements are a periodic function of their atomic masses.
Q:What is the effect on atomic size (radius) when moving from top to bottom within a group, and from left to right within a period in the periodic table?
Answer: In a given period, the atomic size (radius) decreases as one moves from left to right; and… It increases from top to bottom.
Q: Explain periodicity.
Answer: The recurrence of the properties of elements at regular intervals is called periodicity.
Q: Explain hybridization.
Answer: Under hybridization, orbitals of slightly different energies belonging to the same atom combine their energies to form new orbitals of equal energy before forming chemical bonds. These newly formed orbitals are called hybrid orbitals.
Q: Explain what a real gas is.
Answer: A gas that obeys the gas laws only at low pressures and very high temperatures is called a real gas.
Q: Explain Standard Temperature and Pressure (STP).
Answer: Standard temperature is 0°C (or 273 K), and standard pressure is 1 bar (or 760 mm of mercury).
Q: What volume does 1 mole of gas occupy at STP?
Answer: 22.4 litres.
Q: State Avogadro’s Law.
Answer: At the same temperature and pressure, equal volumes of different gases contain an equal number of molecules.
Q: What is critical temperature?
Answer: It is the temperature above which a gas cannot be liquefied, no matter how high the applied pressure may be.
Q: Explain Dalton’s Law of Partial Pressures.
Answer: According to this law, for a mixture of gases that do not react with one another, the total pressure of the mixture is equal to the sum of the partial pressures of the individual gases present in the mixture.
Q: Why is it difficult to cook vegetables in hilly regions?
Answer: In hilly regions, the boiling point of water decreases because the atmospheric pressure is lower.
Q: Explain Graham’s Law of Diffusion.
Answer: At constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its density. If the density of one gas is *d₁* and its molar mass is *M₁*, and the density of a second gas is *d₂* and its molar mass is *M₂*… 201 Mi 2012 M2
