´identify the independent, dependent, and controlled variables in your experiment which is how does adding salt affect the freezing point of water?

An independent variable is a variable that is not affected in the experiment. It is what experimenter controls. Here, the concentration of salt is the independent variable.
On the other side, a dependent variable is called dependent because it depends on the independent variables. It is what is affected and observed during the experiment. Here, the freezing point of water is affected and, thus, it is the dependent variable.
A controlled variable is a variable that must remain unchanged (must be constant) during the experiment so that the effect on the dependent variable depends only on the independent variable. Here, the volume of water must be constant, so it is the controlled variable.

They carry out the exchange of gases as we breathe ( oxygen and carbon dioxide).

Sorry i dont understand the question can you rephase it?