If a gold bar and a silver bar are tied together firmly and left for years, some of the gold particles will be found in the silver bar due to

Step 1: Analyze the question Two solid metal bars are held in close contact for a long time. Gold particles end up inside the silver bar. We need the process that explains particles of one substance moving into another.

Step 2: Explain the concept

• Diffusion: the spontaneous movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion. It happens in gases and liquids, and very slowly in solids.
• Brownian movement: the random zig-zag motion of visible particles (like pollen or smoke) suspended in a fluid, caused by collisions with fluid molecules. It needs a fluid, not two solid bars.
• Displacement: a more reactive element replacing a less reactive one from its compound (e.g. Zn + CuSO₄ → ZnSO₄ + Cu). No compounds are involved here.
• Osmosis: the movement of solvent (usually water) through a semi-permeable membrane. There is no membrane or solvent here.

Step 3: Apply to the question The gold and silver atoms vibrate and slowly migrate across the contact surface. Gold atoms move into the silver, and silver atoms move into the gold, because each metal has a high concentration of its own atoms at its end of the system. This is solid-state diffusion. It is extremely slow, which is why the question says "left for years."

Step 4: Answer The correct option is B. diffusion.

Tip: Link each term to its setting:

• Diffusion → particles spreading out (gas, liquid, even solid)
• Brownian movement → tiny particles jiggling in a fluid
• Osmosis → water through a membrane
• Displacement → reactivity series reactions

If the question mentions particles of one substance mixing into another with no membrane, think diffusion.