Question

Use a phase diagram to show the difference in freezing points and boiling points between an aqueous urea solution and pure water.

Short answer

An aqueous urea solution has a lower freezing point and a higher boiling point than pure water.

Step-by-step solution

Understand the Properties of Solutions

Before plotting a phase diagram, it's crucial to know that adding a solute like urea to water changes its physical properties. This results in freezing point depression and boiling point elevation compared to pure water. In essence, the solution has a lower freezing point and a higher boiling point.

Identify the Basics of a Phase Diagram

A phase diagram typically plots temperature against pressure and illustrates the state (solid, liquid, gas) of a substance. Key features include lines indicating the boundaries between different states, such as the freezing point and boiling point at atmospheric pressure.

Plot the Phase Diagram for Pure Water

Draw a simple phase diagram for pure water with temperature on the x-axis and pressure on the y-axis. Indicate the freezing point at 0°C and boiling point at 100°C under 1 atm pressure. The lines between solid and liquid (melting line) and liquid and gas (vaporization line) are plotted.

Adjust the Diagram for the Aqueous Urea Solution

On the same phase diagram, adjust the melting and vaporization lines for the aqueous urea solution. The melting line will shift to the left, indicating a decrease in freezing point, while the vaporization line will shift to the right, indicating an increase in boiling point.

Compare the Freezing and Boiling Points

Clearly label the new freezing and boiling points for the aqueous urea solution on the phase diagram. The freezing point will be below 0°C, and the boiling point will be above 100°C, reflecting the impact of urea on the solution's properties.

Key concepts

Freezing Point Depression

When a solute like urea is added to a solvent such as water, the freezing point of the resulting solution is lower than that of the pure solvent. This phenomenon is known as "freezing point depression". It occurs because the solute molecules interfere with the formation of the solid lattice structure, which is necessary for the substance to freeze.
A direct consequence is that more energy (in the form of a lower temperature) is needed to overcome these disturbances and allow the substance to solidify. The extent of freezing point depression can be calculated using the formula: \[ \Delta T_f = i \cdot K_f \cdot m \]where:

• \(\Delta T_f\) is the freezing point depression.
• \(i\) is the van 't Hoff factor (1 for urea, since it does not dissociate).
• \(K_f\) is the cryoscopic constant of the solvent water.
• \(m\) is the molality of the solution.

Understanding this calculation helps in predicting how much the freezing point of water will lower when urea is dissolved.

Boiling Point Elevation

In contrast to freezing point depression, when a solute like urea is dissolved in water, the boiling point of the solution is higher than that of the pure solvent. This change is referred to as "boiling point elevation".
The reason behind this is that the presence of solute particles in the liquid phase hinders the escape of solvent molecules, therefore requiring more heat to reach the point where vapor pressure equals atmospheric pressure. This phenomenon is quantitatively expressed with the following equation: \[ \Delta T_b = i \cdot K_b \cdot m \]where:

• \(\Delta T_b\) is the boiling point elevation.
• \(i\) is the van 't Hoff factor.
• \(K_b\) is the ebullioscopic constant of the solvent water.
• \(m\) is the molality of the solution.

These concepts are crucial for understanding how solutions behave under temperature variations.

Aqueous Solution

An aqueous solution is a type of solution where the solvent is water. Water is often referred to as the "universal solvent" due to its ability to dissolve many different substances, making it an ideal medium for chemical reactions and processes.
In the context of urea solute, the urea is the solute, and water is the solvent in the aqueous solution. The behavior of this solution can be influenced by the interactions between urea molecules and water, which impact properties such as boiling point and freezing point. Understanding how urea interacts with water on a molecular level is fundamental to grasping the modifications in its physical properties. Aqueous solutions are commonplace in biological systems and are essential for life, given that many physiological processes happen in such a medium.

Urea Solute

Urea is a simple organic compound that is highly soluble in water, making it an excellent choice for solute studies in solution chemistry. In an aqueous solution, urea does not dissociate into ions but remains as individual molecules surrounded by water molecules.
This non-dissociative behavior impacts the colligative properties of the solution, such as freezing point depression and boiling point elevation. As discussed earlier, in both phenomena, the non-ionic nature of urea means it affects these properties differently when compared to ionic solutes.Because urea molecules do not break into ions, the van 't Hoff factor, \(i\), remains equal to 1. Understanding these interactions and properties improves comprehension of broader concepts in chemistry, such as solubility and molecular interactions.