A few grains of table salt were put in a cup of cold water, kept at constant temperature and left…

Step 1: Analyze the question Salt grains are placed in cold water that is left undisturbed at constant temperature (no stirring, no heating). Over time, the salty taste spreads through all the water. We need the process responsible.

Step 2: Explain the concepts

• Convection: Heat transfer or movement in fluids caused by temperature differences (hot fluid rises, cold fluid sinks). Here the temperature is constant, so this is not the cause.
• Osmosis: Movement of solvent (water) through a semi-permeable membrane from a dilute to a concentrated solution. No membrane is involved here.
• Capillarity: Rise or movement of liquid in narrow tubes or porous materials due to adhesion and surface tension. It does not apply to salt spreading in a cup.
• Diffusion: The spreading of particles from a region of high concentration to a region of low concentration due to random molecular motion, until they are evenly distributed. No stirring or heating is needed.

Step 3: Apply to the problem The salt dissolves where the grains sit, so the concentration is highest there. The salt particles (ions) move randomly and gradually spread into the rest of the water, where the concentration is lower. Eventually the salt is evenly distributed, so all the water tastes salty. This is exactly what diffusion describes.

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

Exam tip: Remember "Diffusion = Distribution." If particles spread out by themselves with no stirring, no heating and no membrane, the answer is diffusion. Key words like "undisturbed" and "constant temperature" rule out convection, and "membrane" would point to osmosis.