1. Diffusion is not used in which of the following?
a) Doping of semiconductors
b) Manufacturing of alloys
c) Heat treatment of metals
d) Catalysis
Explanation: Alloy manufacturing is a bulk process, achieved by mixing metal melts. Whereas diffusion is a surface phenomenon.
2. Carburisation is a heat treatment used for case hardening steels. Carbon is trapped on steel surface by:
a) Osmosis
b) Interstitial diffusion
c) Vacancy diffusion
d) None of the mentioned
Explanation: Carbon forms interstitial carbides with iron due to the large difference in atomic radii.
3. You want to demonstrate the phenomenon of diffusion to a group of school kids using two coloured gases. For this, you need to slow down the process. Which of the following tricks will help you achieve this feat?
a) Cooling the gases
b) Using gases of large molecular weights
c) Decreasing the size of orifice
d) All of the mentioned
Explanation: Rate of diffusion is directly proportional to temperature and it is indirectly proportional to molecular weights.
4. In steady state diffusion which of the following remains constant?
a) Concentration gradient
b) Kinetic energy of particles
c) Potential energy of particles
d) Change of concentration with respect to temperature
Explanation: In steady state diffusion, the concentration of the diffusing with respect to space at any point remains constant. Kinetic energy and potential energy need not be constant.
5. Which of the following gases can be purified by allowing it to diffuse through palladium cap?
a) Oxygen
b) Hydrogen
c) Chlorine
d) Nitrogen
Explanation: When a gas at constant pressure pervades through a thin walled metal foil, steady state diffusion occurs. Hydrogen is purified by making it pass through a palladium cap after going through steady state diffusion.
6. Which of the following law is used for steady state diffusion?
a) Fick’s law
b) Newton’s law of diffusion
c) Bragg’s law
d) Charles’s law
Explanation: Fick’s law is used under steady state conditions to obtain the diffusion coefficient. Bragg’s law is used in diffraction of light while Charles’s law is used in thermodynamics and is also known as the law of volumes.
7. Which of the following is not a part of Fick’s first law?
a) Diffusion flux
b) Diffusion coefficient
c) Change in concentration with respect to the position in space
d) Change in concentration with respect to time
Explanation: Fick’s first law is given by: J = – D (dc / dx), where J is diffusion flux or diffusivity, D is diffusion coefficient and dc/dx stands for change in concentration with respect to the position in space. Hence change in concentration with respect to time is not a part of Flick’s first law
8. Velocity of diffusing particles does not depend on _______
a) Temperature
b) Viscosity of the fluid
c) Size of the particles
d) Pressure
Explanation: Diffusion coefficient is proportional to square of velocity which depends on the temperature of the fluid, viscosity of the fluid and the size of particles. The velocity of particle does not depend on pressure.
9. Flux direction is opposite to the direction of the concentration gradient.
a) True
b) False
Explanation: Flux direction is opposite to the direction of the concentration gradient. Because of which there is a negative sign in Fick’s first law of diffusion on the right side of the equation.
10. The diffusivity of copper in copper increases by how many times when the temperature is increased from 500 ° C to 1000 °C?
a) 200000
b) 100000
c) 150000
d) 170000
Explanation: The diffusivity of copper in copper at 500 °C is 1e-18 while diffusivity at 1000 °C is 2e-13. By dividing the values, we find that the diffusivity increases by 200000 times fold.