In basic science about gases, what generally happens to air when it is heated in a normal situation at constant pressure?

Introduction / Context:
Understanding what happens to air when it is heated is a fundamental concept in physics, meteorology and everyday life. This principle explains why hot air rises, how weather patterns form and why devices like hot air balloons work. Many school level science questions test this concept because it is simple in theory but very important in practice.

Given Data / Assumptions:

• The question deals with air, which can be treated as a typical gas mixture.
• We consider a normal situation where air is heated at or near constant pressure.
• The focus is on what happens to volume and density when temperature increases.
• Phase changes such as condensation into liquid are not considered in this basic scenario.

Concept / Approach:
The behaviour of gases can be described by simple gas laws such as Charles law and the ideal gas law. In simple terms, when a gas is heated at constant pressure, its volume increases and its density decreases, because the same mass occupies a larger space. As the gas becomes less dense than the surrounding cooler air, it tends to rise. Since air is a gas, it follows the same basic rules. Therefore, we expect heated air to expand and become lighter, not to contract or remain unchanged.

Step-by-Step Solution:
Step 1: Recall that for gases at constant pressure, an increase in temperature leads to an increase in volume, according to Charles law. Step 2: Realise that density is defined as mass divided by volume. When volume increases but mass stays the same, density decreases. Step 3: Apply this to air: when air is heated, its molecules move faster and spread out, making the air expand. Step 4: As the heated air expands, its density becomes lower than that of the surrounding cooler air. Step 5: Because less dense air tends to rise above more dense air, hot air will move upward, which is why smoke and hot air balloons rise. Step 6: This confirms that heated air expands and becomes lighter, matching option A.

Verification / Alternative check:
A simple everyday experiment can verify this concept. When you place your hand carefully above a candle or a cup of hot tea, you can feel warm air moving upward. Weather diagrams explaining convection currents in the atmosphere also show warm air rising and cool air sinking, which is consistent with expansion and reduced density on heating. These observations match the predictions of the gas laws and support the answer.

Why Other Options Are Wrong:
The option stating that air contracts and becomes heavier is opposite to well established gas laws for heating at constant pressure. The idea that air remains unchanged ignores the strong temperature effect on gas volume and motion of molecules. The suggestion that heated air immediately turns into liquid water is incorrect, because air is a gas mixture and does not transform into liquid simply by heating under normal atmospheric conditions.

Therefore, none of the other options properly describe the typical behaviour of air when it is heated.

Common Pitfalls:
Some learners confuse the behaviour of solids, liquids and gases and may imagine that heating always leads to contraction or that heating must always change the state. Others may forget that density depends on both mass and volume. To avoid these errors, it is helpful to review basic gas laws and to connect them with practical examples such as hot air balloons, chimneys and rising warm air in rooms.

Final Answer:
When air is heated at normal conditions, it expands and becomes lighter, reducing its density.