ANS:A - < 1 and ≈ 1

The Schmidt number (𝑆𝑐Sc) for pure air at atmospheric conditions is approximately greater than 1, indicating that momentum diffusivity is greater than mass diffusivity in air. This is because air is a mixture of gases, and the molecular weight of the gases present in air differs from that of individual gases. The Lewis number (𝐿𝑒Le) for pure air is approximately equal to 1. The Lewis number represents the ratio of thermal diffusivity to mass diffusivity. For most gases at moderate temperatures and pressures, the thermal diffusivity and mass diffusivity are of the same order of magnitude, resulting in a Lewis number close to 1. Therefore, the correct option is:

1 and ≈ 1