The phase diagram of CO₂ and the heat needed to warm it

Problem statement

The figure shows the phase diagram of $\mathrm{CO}_2$. Answer the following questions:

[Figure: phase diagram with pressure in atmospheres on the vertical axis and temperature in degrees Celsius on the horizontal axis, with no numerical values. Three regions: solid, liquid and gas, separated by three curves that meet at the triple point. A horizontal dashed line, above the triple point, crosses the solid-liquid curve at point A and the liquid-gas curve at point B.]

a) If the temperature is kept constant, which variable must be changed, and how, to turn gaseous $\mathrm{CO}_2$ into a liquid? (0.2 marks)

b) For a given value of pressure, what are the phenomena that occur at points A and B called if we increase the temperature? And if we lower it? (0.4 marks)

c) What is meant by the specific heat capacity of a substance? (0.7 marks)

d) Calculate the heat needed to take $0.5\ \text{L}$ of $\mathrm{CO}_2$ from $25\ ^\circ\text{C}$ to $75\ ^\circ\text{C}$ (1.2 marks). Data: $c_e(\mathrm{CO}_2)=849\ \text{J}\cdot\text{kg}^{-1}\cdot\text{K}^{-1}$; density $(\mathrm{CO}_2)=1.9\cdot10^{-3}\ \text{kg/L}$.

Source: Madrid university entrance exam (PAU, Spain) · 2025-2026 · Ordinary session · General Science · Question 4B. Official exam set by the public universities of the Community of Madrid, to whom it belongs; translated from Spanish and reproduced with its source in order to explain it for teaching purposes.