The nurse instills 5 ml of normal saline before suctioning a client's tracheostomy tube. the instillation is effective when:

The answer to this question would be: the secretion are thinned

Secretion in the tracheostomy might be thick and that can obstruct the suction tube. Thick secretion mostly will adhere stronger, thus harder to be cleaned off.
The nurse giving normal saline to make the secretion become thinned and easier to be sucked up by the suction. 

The answer would be dogs.