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A hot-air balloon rises in the atmosphere because the hot air in the balloon

Rationale

is less dense than the cooler air in the atmosphere.

Hot air inside the balloon is less dense than the surrounding cooler air, which creates a buoyant force that causes the balloon to rise. This principle is based on Archimedes' principle, where an object will rise when the weight of the air displaced is greater than the weight of the object itself.

A (has a lower pressure than the atmospheric pressure) YOUR ANSWER

While the pressure of the air in the balloon may be lower at certain points, it is not the primary reason for the balloon's ascent. The buoyancy effect relies on density differences rather than pressure differences, making this choice misleading regarding the mechanics of rising.

B (is less dense than the cooler air in the atmosphere) CORRECT

This is the correct answer because the hot air in the balloon has a lower density compared to the cooler surrounding air. According to the principles of buoyancy, an object (or air) that is less dense than its surroundings will rise, which is exactly what occurs with the hot-air balloon.

C (has more inertia than the cooler air in the atmosphere) YOUR ANSWER

Inertia refers to the resistance of an object to change its state of motion. The buoyancy and rising of the balloon are not determined by inertia, but rather by the differences in density. Therefore, this choice does not accurately describe the reason for the balloon's ascent.

D (contains one gas only, whereas the atmosphere is a mixture of gases) YOUR ANSWER

The composition of gases inside the balloon versus the atmosphere is not relevant to the balloon's ability to rise. The crucial factor is the density of the air inside the balloon compared to the surrounding air, not the number of gases present.

Conclusion

The ability of a hot-air balloon to rise is fundamentally linked to the principle of buoyancy, which is influenced by the density of the air inside the balloon. Because the hot air is less dense than the cooler air outside, it creates an upward force that allows the balloon to ascend. Other factors, such as pressure, inertia, and gas composition, do not play a significant role in this phenomenon.

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