Fill in the blank :
$1.$ In collision theory the molecules are hard .........
$2.$ In a reality the molecules are hard ..........
$3.$ Collision take place with .......... species.

(N/A) $1.$ In collision theory,the molecules are considered as hard $\text{spheres}$.
$2.$ In reality,the molecules are hard $\text{ellipsoids}$ or have complex shapes.
$3.$ Collision takes place with $\text{reacting}$ species.