When small bubbles of air rise upward through a liquid at rest, which main physical effect is responsible for their upward motion?

Introduction / Context:
The motion of bubbles in liquids is a familiar sight in boiling water, carbonated drinks, and underwater scenes. From a physics point of view, bubbles provide a clear example of how forces act on bodies immersed in fluids. When a small pocket of air or gas is trapped in a liquid, it tends to rise upward. This question asks you to identify the main physical effect responsible for this upward motion. Understanding the role of buoyant force and density differences is fundamental for topics like flotation, fluid statics, and engineering applications involving gases in liquids.

Given Data / Assumptions:
• The system consists of small bubbles of air inside a liquid. • The liquid is initially at rest and is not flowing violently. • The bubbles are fully surrounded by the liquid and move upward through it. • We apply Archimedes principle and basic fluid statics concepts.

Concept / Approach:
According to Archimedes principle, any body or volume of fluid immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced. Air bubbles inside a liquid displace liquid whose density is much greater than that of the air inside the bubble. Consequently, the buoyant force on the bubble is significantly larger than the weight of the air it contains. The net result is an upward force (upthrust) that accelerates the bubble upward. Surface tension, viscous forces, and external air currents can influence the shape and speed of the bubble, but they do not provide the main driving force for its rise; buoyancy due to density difference is the primary cause.

Step-by-Step Solution:
Step 1: Recognise that the bubble of air occupies a certain volume within the liquid and displaces an equal volume of liquid. Step 2: Apply Archimedes principle, which states that the buoyant force equals the weight of the displaced liquid. Step 3: Compare the weight of the displaced liquid with the weight of the air inside the bubble; the liquid is much denser, so its weight is greater. Step 4: Calculate that the upward buoyant force therefore exceeds the downward weight of the bubble, resulting in a net upward force. Step 5: Conclude that this net upward force causes the bubble to accelerate upwards until other factors such as viscous drag balance the motion at some terminal speed. Step 6: Identify this net upward effect as buoyant force or upthrust due to density differences, which is the main cause of the bubbles rising.

Verification / Alternative check:
You can verify this explanation by analogy with floating objects. Objects less dense than water, such as wood, float on the surface due to buoyant force exceeding their weight. Similarly, hot air balloons rise in the atmosphere because the hot air inside the balloon is less dense than the surrounding cooler air. In all these cases, density differences lead to buoyant forces that cause upward motion. Surface tension mainly affects the shape of the bubble interface, and viscosity provides resistance to motion rather than an upward push. Air currents above the liquid surface can disturb the surface but cannot directly push bubbles deep inside the liquid upwards. This comparison confirms that buoyancy is the dominant effect.

Why Other Options Are Wrong:
Option B, surface tension, acts along the interface between air and liquid and tends to minimise surface area, but it does not provide a net upward force that can consistently overcome gravity for the bubble volume. Option C, viscous drag, is actually a resistive force that opposes the motion of the bubble through the liquid rather than causing it to rise. Option D, air currents over the surface, may disturb the surface but do not explain the initial upward motion of bubbles within the liquid. None of these alternatives match the fundamental buoyant effect described by Archimedes principle.

Common Pitfalls:
Students sometimes focus on more complicated effects like turbulence, surface tension shapes, or bubble bursting and overlook the simple density argument. Another mistake is to think of the bubble as being somehow naturally attracted to the surface without considering forces. Remember that in fluid statics, density differences create pressure differences that translate into buoyant forces. Whenever you see a less dense region inside a more dense fluid, such as gas bubbles in water, you should immediately think of upthrust as the primary reason for upward motion.

Final Answer:
The correct choice is Buoyant force (upthrust) due to the air having lower density than the liquid, because this density difference produces a net upward force on the bubble that makes it rise through the liquid.