Final answer:
The early name for 'flammable air' refers to C. hydrogen gas, which is highly flammable and has been used historically in airships, such as the Hindenburg, with disastrous results. Helium is now used due to its nonflammable nature.
Step-by-step explanation:
“Flammable air” was an early name for hydrogen gas. Hydrogen is known to be highly flammable, producing a significant amount of heat when it reacts with oxygen, as demonstrated by the chemical reaction
2H₂(g) + O₂(g) → 2H₂O(l) +572kJ.
This characteristic of hydrogen has had both beneficial and tragic outcomes in history. For example, the Hindenburg airship was filled with hydrogen gas due to its lifting properties but exploded due to hydrogen's flammable nature. In contrast, helium, which is nonflammable, is now the preferred choice for lighter-than-air craft. Despite hydrogen's flammability, it remains a potent energy source and is considered for use in fuel-cell vehicles (FCV), which could revolutionize transportation if we can produce hydrogen economically and in an environmentally sound way.