When a bottle of perfume is opened in one corner of a room and the smell soon spreads throughout the room, which physical process is mainly responsible for this spreading?

Introduction / Context:
Everyday observations often illustrate important physical processes. The spreading of perfume smell across a room is a classic example used in school physics to explain how particles move from regions of high concentration to regions of low concentration. This question tests your ability to connect such a familiar phenomenon with the correct scientific term for the underlying process.

Given Data / Assumptions:

• A bottle of perfume is opened in one corner of a closed or semi-closed room.
• Initially, perfume molecules are concentrated near the bottle.
• After some time, people in other parts of the room can smell the perfume.
• We neglect air currents produced by fans or wind for the basic conceptual explanation.

Concept / Approach:
Diffusion is the process by which particles of a substance move from a region of higher concentration to a region of lower concentration due to their random thermal motion. Gas molecules are in constant random motion, and their collisions cause them to spread out over time. Surface tension deals with liquid surfaces, capillarity refers to rise or fall of liquids in narrow tubes, and viscosity relates to internal friction in fluids. None of these explain the gradual spreading of perfume molecules through air. Therefore, diffusion is the correct process that describes how the scent reaches all parts of the room.

Step-by-Step Solution:
Step 1: Recognise that perfume consists of volatile molecules that readily evaporate into the air. Step 2: Near the open bottle, the concentration of perfume molecules in air is high. Step 3: Over time, these molecules undergo random motion and collide with air molecules, gradually spreading out. Step 4: This random spreading from high to low concentration regions is defined as diffusion. Step 5: Capillarity, surface tension, and viscosity have no direct role in transporting perfume scent across the entire room. Step 6: Therefore, the process responsible for the spreading of smell is diffusion.

Verification / Alternative check:
The explanation is consistent with experiments such as placing a bottle of ammonia or other strong-smelling gas in one place and observing how the smell reaches different parts of a room even without visible bulk airflow. Diffusion is a fundamental concept in kinetic theory of gases and in chemistry, where it also explains mixing of gases and liquids at the molecular level. Even if convection currents or airflow are present, diffusion still plays a role at the microscopic scale and is the core process highlighted in basic science examples like this one.

Why Other Options Are Wrong:
Surface tension: Acts at the interface between liquid and air, influencing droplet shape but not responsible for the spread of gas molecules through the room.
Capillarity: Describes liquid rise or fall in thin tubes due to surface tension, not gas spreading in open air.
Viscosity: Refers to internal friction in liquids and gases that resists flow; it can slow down motion but does not by itself cause spreading from high to low concentration.

Common Pitfalls:
Some learners confuse diffusion with convection (bulk movement of fluid) or think that smell travels only due to wind. While air currents can speed up the distribution of perfume molecules, the fundamental microscopic process is diffusion driven by random molecular motion. It is also easy to be distracted by other fluid properties listed in options without recalling their exact definitions. To avoid confusion, always link diffusion with concentration gradients and random molecular motion in gases or liquids.

Final Answer:
The smell of perfume spreading throughout the room is mainly due to the process of Diffusion.