Answer» - The curve labelled l – v represents those points where the liquid and vapour phases are in equilibrium.
- It is a graph of boiling point versus pressure.
- The l – v curve of water correctly shows that at a pressure of 1 atmosphere, the boiling point of water is 100 °C and the boiling point gets lowered for a decreased pressure.
- The l – v curve for CO2 yields that CO2 cannot exist as a liquid under normal atmospheric pressure conditions.
- The curve l – s represents the points where the solid and liquid phases coexist in equilibrium.
- It is a graph of the freezing point versus pressure.
- At one standard atmosphere pressure, the freezing point of water is 0 °C which can be depicted using l – s curve of water.
- At a pressure of one standard atmosphere water is in the liquid phase if the temperature is between 0 °C and 100 °C but is in the solid or vapour phase if the temperature is below 0 °C or above 100 °C.
- Also, l – s curve for water slopes upward to the left i.e., fusion curve of water has a slightly negative slope.
- This is true only of substances that expand upon freezing.
- However, for most materials like CO2, the l – s curve slopes upwards to the right i.e., fusion curve has a positive slope. The melting point of CO2 is -56 °C at higher pressure of 5.11 atm.
- The curve labelled s – v is the sublimation point versus pressure curve.
- Water sublimates at pressure less than 0.0060 atmosphere, while carbon dioxide, which in the solid state is called dry ice, sublimates even at atmospheric pressure at temperature as low as -78 °C.
- The temperature and pressure at which the fusion curve, the vaporisation curve and the sublimation curve meet and all the three phases of a substance coexist is called the triple point of the substance.
- The triple point of water is that point where water in solid, liquid and gaseous states coexist in equilibrium and this occurs only at a unique temperature and pressure.
- The triple point of water is 273.16 K and 6.11 × 10-3 Pa and that of CO2 is -56.6 °C and 5.1 × 10-5 Pa.
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