For safety and buoyancy, which of the following gases is most commonly used to fill balloons, especially weather balloons and decorative floating balloons?

Introduction / Context:
Gases are used for many practical purposes, and filling balloons is a familiar everyday example that also appears in basic science questions. A gas used to fill balloons needs to be lighter than air so that the balloon can float, and for safety reasons it should ideally be non flammable. This question asks which gas best satisfies these requirements and is therefore commonly used in balloons.

Given Data / Assumptions:

• The context is filling balloons that float, including weather balloons and decorative balloons.

• The gas should be lighter than air to provide lift.

• Safety is an important consideration in common practice.

• Options include oxygen, nitrogen, argon, helium and hydrogen.

Concept / Approach:
Air is composed mainly of nitrogen and oxygen and has a certain average density. To make a balloon rise, the gas inside must have a lower density than air. Hydrogen is lighter than air and could be used, but it is highly flammable and therefore dangerous, especially near sparks or flames. Helium is an inert noble gas that is lighter than air and non flammable, making it ideal and widely used for balloons. In contrast, nitrogen, oxygen and argon have densities too close to that of air to provide significant lift.

Step-by-Step Solution:
Step 1: Recall that a balloon rises when its average density (balloon plus gas) is less than the density of air. Step 2: Identify gases that are lighter than air. Hydrogen and helium are lighter; nitrogen and oxygen are not significantly lighter, and argon is heavier than air. Step 3: Consider safety. Hydrogen is very flammable and can cause explosions, as shown historically by accidents. Step 4: Recognise that helium is a noble gas, chemically inert and non flammable, yet still less dense than air. Step 5: Conclude that helium is the most suitable and commonly used gas for filling floating balloons.

Verification / Alternative check:
General science references and practical guides on weather balloons state that helium is the standard gas for safe balloon launches. Commercial party balloons that float for hours are typically labelled as helium filled. Safety guidelines discourage the use of hydrogen in public environments. These real world practices align with textbook statements that helium is preferred due to its low density and lack of reactivity.

Why Other Options Are Wrong:
Oxygen supports combustion and has a density similar to air, so it does not provide adequate lift and poses a fire risk. Nitrogen makes up most of the air and therefore has almost the same density, so a nitrogen filled balloon will not float significantly. Argon is heavier than air and will make a balloon sink. Hydrogen would make a balloon rise strongly but is dangerous due to its flammability, so it is not the common safe choice, especially in general knowledge context.

Common Pitfalls:
A frequent error is to choose hydrogen because it is lighter than air and students remember diagrams of hydrogen filled balloons. However, many examination questions expect the safer practical answer, helium. Another pitfall is ignoring density and thinking that any gas will work, including oxygen or nitrogen. Remembering that safe floating balloons almost always use helium helps avoid these misunderstandings.

Final Answer:
Balloons used for safe floating applications are most commonly filled with helium.