As an expert in the field of thermodynamics and fluid dynamics, I can provide a detailed explanation for why air becomes less dense as it is warmed. The behavior of air, or any gas for that matter, is governed by the principles of kinetic theory and the ideal gas law.

**Step 1: Understanding the Kinetic Theory of Gases**

The kinetic theory of gases is a fundamental concept that explains the macroscopic properties of gases in terms of the motion of their constituent particles. According to this theory, a gas is composed of a large number of small particles (molecules) that are in constant, random motion. These molecules are very far apart compared to their size, and the forces between them are negligible except during collisions. The pressure exerted by a gas is due to the frequent collisions of these molecules with the walls of the container.

When heat is added to a gas, the kinetic energy of the molecules increases. This increase in kinetic energy results in the molecules moving more quickly and more forcefully. As the molecules move faster, they collide with the walls of the container with greater frequency and force, which can lead to an increase in pressure if the volume is held constant.

Step 2: The Ideal Gas Law

The ideal gas law is given by the equation PV = nRT, where:
- P is the pressure of the gas,
- V is the volume of the gas,
- n is the number of moles of the gas,
- R is the ideal gas constant, and
- T is the temperature of the gas in Kelvin.

This law describes how the pressure, volume, and temperature of a gas are related when the gas behaves ideally. For an ideal gas, the volume occupied by the gas molecules themselves is negligible compared to the volume of the container, and the average kinetic energy of the molecules is directly proportional to the absolute temperature of the gas.

When the temperature (T) of a gas increases, and if the pressure (P) is held constant, the volume (V) must increase to maintain the equality in the ideal gas law equation. An increase in volume with a constant amount of gas means that the gas has become less dense. Density (ρ) is defined as mass (m) per unit volume (V), so mathematically, ρ = m/V. If V increases while m remains constant, ρ decreases.

Step 3: Expansion and Density

As mentioned earlier, adding heat energy to a gas causes the molecules to move more quickly and take up a greater amount of space. This is because the added energy is distributed among the molecules, increasing their velocity. The increased velocity means that the molecules will cover more ground in the same amount of time, effectively expanding the volume they occupy.

The concept of expansion is crucial to understanding why air becomes less dense when warmed. As the gas expands, the distance between molecules increases. Since the mass of the gas remains constant, the density, which is the mass per unit volume, decreases. This is why warm air is less dense than cool air; the molecules of warm air are moving faster and are more spread out, occupying a larger volume for the same mass.

Step 4: Practical Implications

This principle has numerous practical applications. For instance, it is the reason why hot air balloons can rise. The heated air inside the balloon is less dense than the cooler air outside, so the balloon experiences a net upward buoyant force. It is also the reason why the atmosphere has layers with different temperatures and densities, such as the troposphere, stratosphere, and so on.

In summary, the warming of air leads to an increase in the kinetic energy of its molecules, causing them to move faster and occupy a larger volume for the same mass, thus reducing the density of the air.

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This represents molecules in a liquid state. They are moving more quickly and taking up a greater amount of space. ... Adding heat energy causes molecules to move more quickly. Adding heat energy to most liquids and gases causes them to expand (become less dense).read more >>