What is the heating curve on the horizontal line

The plateaus or horizontal lines on the graph represent the transition between states of the sample. The first plateau represents the melting (or transition from solid to liquid) and the second plateau represents boiling (or transition from liquid to gas).

Explanation:

the answer is learned

a gas, having a molecular weight of 13 kg/kmol and a kinematic viscosity of 0.25 cm2/s, flows through a pipe 0.25 m internal diameter and 5 km long at the rate of 0.4 m3/s and is delivered at atmospheric pressure.