For a gas, if the specific gravity is less than 1, what does this indicate about the gas?

For a gas, if the specific gravity is less than 1, what does this indicate about the gas?

• A. The gas is heavier than air.
• B. The gas has SG greater than 1.
• C. The gas is lighter than air.
• D. The SG cannot be less than 1.

Answer: (C)

Specific gravity compares a gas’s density to that of air at the same temperature and pressure. For gases, density is proportional to molar mass, so SG is essentially the gas’s molar mass divided by the molar mass of air (about 29 g/mol). If SG is less than 1, the gas has a smaller molar mass than air, making it less dense than air. That means it is lighter than air and will tend to rise, as with helium or hydrogen. Conversely, SG greater than 1 would indicate a heavier-than-air gas.

Specific gravity compares a gas’s density to that of air at the same temperature and pressure. For gases, density is proportional to molar mass, so SG is essentially the gas’s molar mass divided by the molar mass of air (about 29 g/mol). If SG is less than 1, the gas has a smaller molar mass than air, making it less dense than air. That means it is lighter than air and will tend to rise, as with helium or hydrogen. Conversely, SG greater than 1 would indicate a heavier-than-air gas.