Question

A substance, X, has the following properties: Sketch a heating curve for substance \(X\) starting at \(-50 .^{\circ} \mathrm{C}\)

Short answer

To sketch a heating curve for substance X starting at -50°C: 1. Create a graph with x-axis (time) and y-axis (temperature) and plot the starting point at (-50°C, 0 min). 2. Draw a line with a positive slope representing the initial solid phase until it reaches the melting point. 3. Draw a horizontal line at the melting point, representing the phase transition from solid to liquid. 4. Draw a line with a positive slope representing the liquid phase until it reaches the boiling point. 5. Draw a horizontal line at the boiling point, representing the phase transition from liquid to gas. The final heating curve will show the temperature changes across solid, melting, liquid, and boiling phases.

Step-by-step solution

1. Initial Solid Phase

Substance X begins at a temperature of -50°C. Start by drawing the x-axis (time) and y-axis (temperature) on the graph. Plot the point (-50°C, 0 min) as the starting point of the curve. As the substance is heated, the temperature increases until it reaches its melting point.

2. Melting at the Phase Transition from Solid to Liquid

At the melting point, the substance will start to transition from a solid to a liquid. During this phase transition, the temperature will remain constant until all the solid has melted. Draw a horizontal line from the end of the solid phase to represent the melting.

3. Liquid Phase

Once the entire substance has melted, it will be in the liquid phase, and the temperature will again start to increase as it absorbs more heat. Draw a line with a positive slope to represent the liquid phase, indicating the temperature increase over time.

4. Boiling at the Phase Transition from Liquid to Gas

At the boiling point, the substance will start to transition from a liquid to a gas. Again, during this phase transition, the temperature will remain constant until all the liquid has turned to gas. Draw a horizontal line from the end of the liquid phase to represent the boiling.

5. Final Heating Curve

Finally, the heating curve for substance X should show the initial solid phase with an increasing temperature, a horizontal line at the melting point, the liquid phase with an increasing temperature, and another horizontal line at the boiling point. This graph allows us to visualize how the temperature of substance X changes as it is heated from -50°C.

Key concepts

Phase Transitions

Phase transitions are essential concepts in understanding how substances change states when energy is added or removed. These transitions occur at specific temperatures and involve a substance moving between solid, liquid, and gas phases.

• Melting: This is the transition from solid to liquid. During melting, energy breaks down the rigid structure of a solid, allowing it to flow as a liquid. The temperature remains constant until the complete transition has taken place.
• Boiling: This is the transition from liquid to gas. Here, sufficient energy allows molecules to overcome attractive forces in the liquid, escaping into the gas phase. Like melting, the temperature stays constant until the transition is complete.
• Freezing and Condensation: These are the opposite transitions of melting and boiling, respectively.

Each phase transition involves energy changes without temperature change, focusing on overcoming forces keeping molecules together.

Melting Point

The melting point of a substance is a distinct temperature where it changes from a solid to a liquid. When reaching the melting point, a substance absorbs heat without an increase in temperature, signifying the phase transition process.

• Characteristics: Different substances have different melting points, largely determined by the strength of forces between particles.
• Influence of Pressure: Although generally stable, pressure changes can affect melting points in some substances.

Knowing the melting point helps in identifying substances and is crucial for applications like material design and understanding natural processes.

Boiling Point

The boiling point refers to the temperature at which a liquid changes to a gas throughout the substance, not just at the surface. This critical point marks the start of the boiling phase transition.

• Vaporization Process: During boiling, added heat turns the liquid into vapor. The entire liquid must reach the boiling point for boiling to occur.
• Pressure Dependence: Unlike the melting point, the boiling point is highly sensitive to pressure changes. For example, water boils at lower temperatures at higher altitudes due to lower atmospheric pressure.

Understanding the boiling point is crucial in cooking, industrial applications and scientific research where precise temperature control is necessary.

Temperature

Temperature is a crucial factor determining the state of a substance. It measures the average kinetic energy of particles, influencing phase transitions.

• Effect on Phases: As temperature rises, substances move from solid to liquid to gas, given enough heat is provided.
• Constant During Transitions: During phase transitions like melting or boiling, despite constant heat addition, the temperature remains unchanged until the transition is complete.

Temperature acts as an indicator of molecular energy levels and plays a significant role in predicting and understanding phase behavior in substances.

Graphing

Graphing a heating curve visually represents how the temperature of a substance changes over time as it is heated. This graph helps in understanding thermal properties and phase transitions effectively.

• Axes Orientation: Typically, the x-axis represents time, while the y-axis shows temperature. This framework aids in understanding the relationship between heat addition and temperature changes.
• Key Features: The curve usually includes increasing temperature sections and flat sections at melting and boiling points, indicating phase transitions.
• Interpreting Curves: The slope of the curve indicates the rate of temperature change. Flat portions highlight where energy goes into changing state rather than raising temperature.

Graphs are powerful tools for students and scientists alike, offering clear insights into complex processes by visualizing data and relationships.