Fig. 1: A graph of the total volume of gas in the syringe, in millilitres, on the vertical axis, from 0 to 160 mL, against the volume of gas Y2 added, in millilitres, on the horizontal axis, from 0 to 180 mL. Seven measured points are marked, at 0, 30, 60, 90, 120, 150 and 180 mL of Y2, with their total volumes written beside them: 40, 50, 60, 70, 80, 110 and 140 mL. The points lie on two straight lines: a gentle rise from 40 mL to 80 mL, then a steeper rise. The point where the slope changes is marked with dashed lines to both axes, at 120 mL of Y2 and 80 mL in total.