Which of the following gaseous hydrocarbons is commonly used as an illuminating gas in lamps and welding torches?

Introduction / Context:
This question relates to the practical uses of hydrocarbons, specifically which gas is used as an illuminating gas. Illuminating gases were historically used in lamps and continue to be used for welding and cutting metals. Knowing which hydrocarbon is employed in such applications deepens your understanding of fuel properties, such as flame temperature and brightness.

Given Data / Assumptions:

• The options are methane, propane, ethyne, ethane and butane.
• The question asks which gaseous hydrocarbon is used as an illuminating gas.
• We assume standard school level knowledge of hydrocarbon uses.

Concept / Approach:
Ethyne, also called acetylene, is a hydrocarbon with formula C2H2 that burns in oxygen with a very hot and luminous flame. This property makes it useful as an illuminating gas and in oxy acetylene welding. Methane, propane, ethane and butane are also fuels but are more commonly used for domestic cooking, heating and as LPG components, not specifically referred to as illuminating gas in basic chemistry textbooks.

Step-by-Step Solution:
Step 1: Recall that ethyne (acetylene) has a triple bond and burns with a very bright flame in oxygen. Step 2: Historically, ethyne was used in lamps because its flame is luminous and can provide bright light, which is why it is called an illuminating gas. Step 3: In modern applications, ethyne is mainly used in oxy acetylene welding, where its high flame temperature is important. Step 4: Methane is the main component of natural gas and is used for heating and cooking, but it is not commonly referred to as illuminating gas in school chemistry. Step 5: Propane and butane are components of LPG used in household gas cylinders, again not described as illuminating gas in this context. Step 6: Ethane is a simple alkane used in some industrial processes, but again not the typical illuminating gas mentioned in textbooks. Step 7: Therefore, ethyne is the correct answer.

Verification / Alternative check:
A quick memory check is that many textbooks specifically state that acetylene is used as an illuminating gas and in welding. The brightness of its flame and its ability to burn at a high temperature in oxygen make it suitable for these tasks. If you have seen diagrams of oxy acetylene torches in your chemistry book, that is a strong hint that ethyne is the special case among the listed hydrocarbons.

Why Other Options Are Wrong:
Methane, propane, ethane and butane are fuels, but they are not usually labelled as illuminating gas in school level terminology. Methane is associated with natural gas, propane and butane with LPG cylinders, and ethane with petrochemical feedstock. The question expects the specific textbook phrase illuminating gas, which is tied to ethyne. Therefore the other options, while combustible, are not the intended answer here.

Common Pitfalls:
Students may think of methane because it is the simplest hydrocarbon gas, or propane and butane because they are common in domestic gas cylinders. However, the key phrase illuminating gas points to older historical uses where acetylene lamps were common. Another pitfall is confusing ethane and ethyne due to similar names. Remember that the triple bond in ethyne gives it distinctive burning characteristics compared to ethane.

Final Answer:
The hydrocarbon commonly used as an illuminating gas is ethyne (acetylene).