Question

state the chemical reagent that is used to demonstrate that expired air contains carbon dioxide

Answer 1

It is proven by the colour of the limewater that CO2 is present in the expired/exhaled air after air is blown in the beaker

Answer 2

Yes ,I agree with Pathneythe lime water will turn milky with the presence of the expired air
Carbondioxide

Answer 3

And the chemical reaction for the expirement showing how expired air contains carbon dioxide is

Ca(0H)2+Co2-CaCo3+H20