Freezing And Boiling Point Graph Answer Key

Freezing and Boiling Point Graph: A Comprehensive Guide with Answer Key

Understanding phase transitions, particularly freezing and boiling points, is fundamental to chemistry and physics. This comprehensive guide will delve into the intricacies of representing these transitions graphically, providing a clear explanation of how to interpret phase diagrams, and offering an answer key to common practice problems. We'll explore various scenarios, including the impact of pressure and impurities on these crucial points.

Understanding Phase Diagrams

A phase diagram is a graphical representation of the physical states of a substance under varying conditions of temperature and pressure. It shows the boundaries between different phases – solid, liquid, and gas – and how these boundaries change with alterations in temperature and pressure. The most important points on a phase diagram are the triple point (where all three phases coexist in equilibrium), the critical point (beyond which the distinction between liquid and gas disappears), and the freezing and boiling points (the temperatures at which phase transitions occur at a given pressure).

Key Features of a Phase Diagram

• Temperature Axis (x-axis): Usually displayed horizontally, representing the temperature of the substance.
• Pressure Axis (y-axis): Usually displayed vertically, representing the pressure exerted on the substance.
• Phase Boundaries: Lines separating the different phases (solid, liquid, gas). These lines represent the conditions under which two phases can coexist in equilibrium.
• Triple Point: The point where all three phases (solid, liquid, gas) coexist in equilibrium.
• Critical Point: The point beyond which the distinction between liquid and gas disappears, forming a supercritical fluid.
• Freezing Point Line (Solid-Liquid Boundary): Shows the temperature at which a substance freezes (liquid to solid) or melts (solid to liquid) at a given pressure. Typically, this line slopes slightly to the right for most substances, indicating that increased pressure favors the denser phase (usually solid). However, water is a notable exception, with its solid-liquid boundary sloping to the left.
• Boiling Point Line (Liquid-Gas Boundary): Shows the temperature at which a substance boils (liquid to gas) or condenses (gas to liquid) at a given pressure. This line shows a strong positive correlation with pressure: higher pressure requires a higher temperature to boil.

Interpreting Freezing and Boiling Point Graphs

Interpreting a phase diagram requires understanding the relationship between temperature, pressure, and the phase of the substance. Let's consider a few examples:

Example 1: Determining the freezing point at a specific pressure.

Find the pressure value on the y-axis. Trace a horizontal line across to the solid-liquid boundary. The temperature where this line intersects the boundary represents the freezing point at that pressure.

Example 2: Determining the boiling point at a specific pressure.

Similar to the freezing point, locate the pressure value on the y-axis. Trace a horizontal line across to the liquid-gas boundary. The temperature at the intersection is the boiling point at that pressure.

Example 3: Determining the phase at a specific temperature and pressure.

Locate the temperature and pressure values on their respective axes. The region of the phase diagram where this point falls (solid, liquid, or gas) indicates the phase of the substance under those conditions.

Example 4: Understanding the effect of pressure on boiling point.

Observe the liquid-gas boundary. Note that as pressure increases, the boiling point also increases. This is because higher pressure requires more energy to overcome the intermolecular forces holding the liquid together, thus requiring a higher temperature.

Practice Problems and Answer Key

Here are some practice problems to solidify your understanding. The answers are provided below.

Problem 1: A substance has a triple point at 0.01 atm and -56.6°C. Its critical point is at 72.9 atm and 304.2°C. At 1 atm pressure, the substance freezes at -114.5°C and boils at -85°C. Sketch a phase diagram for this substance, labeling all key features.

Problem 2: Using the phase diagram you sketched in Problem 1, what phase is the substance in at:

a) 1 atm and -100°C?
b) 1 atm and -70°C?
c) 50 atm and 200°C?

Problem 3: Explain why water's solid-liquid boundary has a negative slope, while most other substances have a positive slope.

Problem 4: Describe how the presence of impurities might affect the freezing and boiling points of a substance.

Answer Key

Problem 1: The sketch should show a typical phase diagram with a solid-liquid boundary sloping slightly to the right (most substances), a liquid-gas boundary that curves upwards to the right (reflecting pressure dependence), and clearly labeled triple and critical points. The freezing and boiling points at 1 atm should also be indicated. The precise shape is not critical, but the general trend and labeled points are crucial.

Problem 2:

a) Solid
b) Liquid
c) Supercritical fluid (or a high-pressure gas-like state beyond the critical point)

Problem 3: Water is unique because ice is less dense than liquid water. Increased pressure favors the denser phase. Therefore, for water, increased pressure favors the liquid phase, leading to a decrease in the freezing point with increased pressure – hence the negative slope. Most substances have a positive slope because their solid phases are denser than their liquid phases.

Problem 4: Impurities generally lower the freezing point and raise the boiling point of a substance. This is because impurities disrupt the regular arrangement of molecules in both the solid and liquid states, making it easier for the substance to transition to the liquid phase at a lower temperature (freezing point depression) and requiring a higher temperature to transition to the gas phase (boiling point elevation). The magnitude of these effects depends on the concentration of impurities.

Conclusion

Understanding phase diagrams and interpreting freezing and boiling point data is critical in various scientific and engineering applications. By mastering the concepts and techniques discussed in this guide, you can confidently analyze phase behavior and predict the state of a substance under varying conditions of temperature and pressure. Remember that practice is key. Continue working through problems and analyzing different phase diagrams to solidify your understanding. The more you work with these concepts, the clearer they will become, and the better you'll be at problem-solving in this area. Remember to always consider the effects of pressure and impurities when analysing these phase transitions.