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Expansibility is an intrinsic property of gases that enables them to spread out and occupy all the available volume in a container. In the case of a bicycle tire, when air is pumped into the tire, the gas molecules naturally move to fill the entire space within the tire. This is why a flat tire, once filled with air, will appear inflated. The tire's volume increases due to the gas molecules' innate tendency to move apart from each other, reducing their concentration and leading to an even distribution throughout the tire.

The mixture of gases that constitute air, primarily nitrogen and oxygen, has no fixed shape or volume, unlike solids or liquids. This ability to expand is what helps the tire regain its shape and form when air is introduced. In practical terms, a bike rider exploits the property of expansibility to transform a compact stream of compressed air into a firm tire that's capable of holding weight and absorbing shock.

Compressibility refers to the ability of a gas to be compacted or squeezed into a smaller volume under pressure. Gases are highly compressible compared to liquids and solids due to the significant amount of space between the gas particles. This space allows the particles to be pushed closer together when pressure is applied.

Imagine a bicycle pump; when the handle is pressed down, the air inside the pump is subjected to pressure and is consequently forced into the tire. The tire can hold a considerable amount of air because the gas compresses under the pressure exerted by the pump's plunger. This compressibility is crucial in numerous applications, not just bicycle tires. It also plays a role in the functioning of gas storage tanks, engines, and even our lungs, where air must be compressed to facilitate various processes.

Gas pressure is the force exerted by the motion of gas particles as they collide with the walls of their container. It's an essential concept to grasp when explaining why air, once pumped into a bicycle tire, causes the tire to expand. These collisions are continual and occur in all directions, and the combined effect of these impacts results in the gas pressure we can measure.

When more air is introduced into the tire, more gas molecules are present to bump against the tire's inner surfaces, increasing the pressure. If air continues to be pumped in, pressure builds up until it balances with the tire's ability to contain it, ensuring the tire stays firm and round. This property is not only relevant for bike tires but also influences weather patterns, flight, and even deep sea diving, where understanding and managing gas pressure is critical.

Diffusion in gases is a process where gas molecules spread out and mix with other gases due to their kinetic energy. This spontaneous mixing occurs until a uniform mixture is formed. In our everyday experience, this is evident when a fragrance spreads from one part of a room to another.

Considering the bicycle tire scenario, while diffusion is less directly observed when pumping air into a tire, it is still a fundamental aspect of gases that is continuously at play. When different air is pumped into the tire, the existing and the new air molecules will gradually diffuse into one another, leading to a uniform distribution of the mixture within the tire. On a microscopic level, it is this property of gases that ensures that the air inside the tire doesn't have varying concentrations of oxygen and nitrogen but rather a smooth and even mix, contributing to consistent tire performance.