If van der Waals constant "a" for NH₃, H₂ and CO₂ are respectively 4.17, 0.244, 1.36 and 3.59, then which one of the following gases is most easily liquefied?

$1.$ ${\mathrm{NH}}_3$
$2.$ ${\mathrm{H}}_2$
$3.$ ${\mathrm{O}}_2$
$4.$ ${\mathrm{CO}}_2$

The surface tension of which of the following liquid is maximum:

1. C₆H₆

2. H₂O

3. C₂H₅OH

4. CH₃OH