According to the kinetic theory of gases, air expands when it ______.

Introduction / Context:

The behaviour of gases under changes in temperature and pressure is described by gas laws such as Charles law and the ideal gas equation. Everyday observations, such as hot air balloons rising or air inside a tyre expanding in the heat, are direct consequences of these laws. This question asks you to recall under which condition air expands according to basic kinetic theory and gas law ideas.

Given Data / Assumptions:

• Air is treated as an ideal gas for simplicity.
• We consider typical conditions where the gas laws hold reasonably well.
• Expansion means the gas occupies a larger volume.
• We assume pressure is held constant when considering the effect of temperature on volume, as in Charles law.

Concept / Approach:

Charles law states that for a given mass of gas at constant pressure, the volume is directly proportional to its absolute temperature. In symbols, V proportional to T when pressure is constant. This means that as the temperature of a gas increases, its volume increases; in other words, the gas expands when heated. Cooling the air would have the opposite effect, causing it to contract. Increasing pressure at constant temperature would compress the air, reducing its volume. Humidity affects the composition of air but is not the primary factor in simple expansion described by the gas laws.

Step-by-Step Solution:

Verification / Alternative check:

An everyday demonstration is a balloon partially filled with air. If you warm the balloon, the air inside heats up, the particles move more rapidly and push outward more strongly on the balloon wall, causing the balloon to expand. If you cool the same balloon, the air inside contracts and the balloon shrinks. Similarly, hot air balloons rise because heated air inside the balloon expands, becomes less dense and rises in the cooler surrounding air. These observations match the prediction that air expands when heated.

Why Other Options Are Wrong:

Option A (is cooled): Cooling reduces the average kinetic energy of air molecules and leads to contraction, not expansion.

Option C (is placed under very high pressure at constant temperature): High pressure at constant temperature compresses the air, reducing its volume rather than expanding it.

Option D (becomes more humid): Adding water vapour changes the composition and density of air but is not the simple temperature driven expansion described by basic gas laws.

Option E (is compressed into a smaller volume at constant temperature): This statement describes the opposite of expansion, that is, compression.

Common Pitfalls:

Students sometimes confuse the roles of temperature and pressure in gas behaviour. It is useful to remember that at constant pressure, heating a gas makes it expand, whereas at constant volume, heating increases the pressure. Similarly, at constant temperature, increasing pressure compresses the gas. Keeping track of which variable is held constant in the gas laws helps avoid mistakes in conceptual questions about expansion and compression.

Final Answer:

Air expands when it is heated.