The heat that enters the system is used to do work such as melting ice to form water.
(The heat; enters; the system) (The heat that enters the system; is used; to do work such as melting ice) (The heat that enters the system; to do; work such as melting ice)
The heat The heat that enters the system The heat that enters the system
enters is used to do
the system
to do work such as melting ice
work such as melting ice