State what the variable represents in the equation.

This is a gas.

A heat filter is used so that it can absorb the heat energy and hence controlling temperature as a variable

The difference between a controlled variable and a variable is in their state. A controlled variable is something which is rigid and constant while a variable is liable to change and inconsistent.

Any variable, which holds a value that represent the state of something, like:

int door_is_open; /* 0/1 = closed/opened */