In arc welding, why is argon gas commonly used around the metal being welded?

Introduction / Context:
This question connects chemistry with engineering practice by asking about the role of argon gas in arc welding. Arc welding involves very high temperatures and molten metal, and shielding gases like argon play an important role in controlling the welding environment. Understanding why argon is chosen relies on knowledge of inert gases and their chemical properties.

Given Data / Assumptions:

• Process: arc welding of metals.
• Shielding gas used: argon.
• We must identify the main reason for using argon.
• Options mention reactivity, melting point, flammability, calorific value and oxidising nature.

Concept / Approach:
Argon is a noble gas and is chemically inert under most conditions. In arc welding, a shielding gas is required to prevent the hot metal and molten weld pool from reacting with air, which contains oxygen and nitrogen. If these reactive gases reach the weld, they can cause oxidation, porosity and weak joints. An inert gas like argon provides a protective atmosphere that does not react with the metal. Therefore, low reactivity with the metal is the key reason for using argon.

Step-by-Step Solution:
Step 1: Recall that argon is a group 18 element, so it has a full valence shell and is chemically inert. Step 2: In arc welding, an electric arc melts the metal at the joint, creating a molten pool that is very hot and reactive. Step 3: If this hot metal is exposed to air, oxygen and nitrogen can react with it, forming oxides or nitrides that weaken the weld. Step 4: To prevent this, a shielding gas is used to surround the arc and the molten pool, driving away air. Step 5: Argon is chosen because it does not react with the molten metal and does not introduce any unwanted chemical species. Step 6: Therefore the best description is that argon has low reactivity with the metal.

Verification / Alternative check:
In welding technology, arc processes are often named after their shielding gas, such as gas tungsten arc welding, where argon is commonly used, or mixtures of argon and other gases. Technical manuals emphasise that the shielding gas must be inert or nearly inert to protect the weld from atmospheric contamination. This confirms that the main requirement for argon is its inert character, not its energy content or flammability.

Why Other Options Are Wrong:
Argon does not lower the melting point of the metal; the melting point is determined mainly by the metal itself and any alloying elements. Argon is not flammable and does not have a high calorific value because it is not a fuel. It is also not a strong oxidising agent, which would be undesirable, since oxidation would damage the weld. These properties make the other options incorrect for the question asked.

Common Pitfalls:
Some students might assume that any gas used in welding must be combustible, but in reality the purpose of the shielding gas is to protect, not to fuel the arc. Another confusion is to think that argon somehow cools or alters melting points; in fact, its role is mainly chemical protection. Remember that noble gases like argon and helium are used in situations where an inert protective atmosphere is needed.

Final Answer:
Argon is used in arc welding mainly because of its low reactivity with metal, providing an inert shielding atmosphere around the weld.