Draw a vertical line up to the line of best ... Add 10 cm 3 of dilute hydrochloric acid to the conical flask and swirl it. Start a stopwatch. Record the time taken for the cross to disappear.

A cotton swab is dipped into concentrated hydrochloric acid (producing hydrogen chloride gas) while a second on is dipped into concentrated aqueous ammonia (producing ammonia gas). Both cotton swabs ...