The number of statement's, which are correct with respect to the compression of carbon dioxide from point (a) in the Andrews isotherm from the following is _____.

A. Carbon dioxide remains as a gas upto point (b)
B. Liquid carbon dioxide appears at point (c)
C. Liquid and gaseous carbon dioxide coexist between points (b) and (c)
D. As the volume decreases from (b) to (c), the amount of liquid decreases

The Andrews isotherm for a temperature lower than the critical temperature has three characteristic points when a sample of $$CO_2$$ is compressed isothermally.

1. Point (a)  →  Region of pure gaseous $$CO_2$$ (unsaturated vapour).
2. Point (b)  →  Saturated vapour just about to condense; gas is still the only phase present.
3. Point (c)  →  Saturated liquid just after the first condensation has been completed; a tiny amount of liquid has appeared and both phases now coexist.

Between points (b) and (c) the horizontal portion of the isotherm is obtained. Pressure remains constant while the volume keeps on decreasing; every decrease in volume converts a part of the vapour into the liquid phase until, at point (c), the gas is in equilibrium with a small amount of liquid.

Now examine the four statements one by one.

Statement A “Carbon dioxide remains as a gas up to point (b)”
Until the isotherm reaches the saturated-vapour point (b) the sample is entirely in the gaseous state. Hence, statement A is correct.

Statement B “Liquid carbon dioxide appears at point (c)”
The very first trace of liquid is detected only when the volume is decreased just beyond point (b) and reaches point (c). Therefore statement B is correct.

Statement C “Liquid and gaseous carbon dioxide coexist between points (b) and (c)”
Along the horizontal segment from (b) to (c) the system contains both liquid and vapour at the same pressure. Hence, statement C is correct.

Statement D “As the volume decreases from (b) to (c), the amount of liquid decreases”
When the volume decreases along the horizontal segment, more vapour condenses and the quantity of liquid increases, not decreases. So statement D is incorrect.

Thus, out of the four statements, A, B and C are correct.

Number of correct statements = 3.