Question

Sketch a heating/cooling curve for water, starting out at -20 \({ }^{\circ} \mathrm{C}\) and going up to 120 \({ }^{\circ} \mathrm{C}\), applying heat to the sample at a constant rate. Mark on your sketch the portions of the curve that represent the melting of the solid and the boiling of the liquid.

Short answer

To sketch a heating/cooling curve for water from -20°C to 120°C, start with labeled axes for temperature and time/heat. The curve begins with a diagonal line from -20°C to 0°C (heating of the ice). At 0°C, draw a horizontal line (melting phase). From here, draw another diagonal line up to 100°C (heating of the liquid water). At 100°C, draw another horizontal line (boiling phase). Finally, draw a diagonal line from 100°C to 120°C (heating of the water vapor). Label each portion of the curve to represent solid, liquid, and gas phases, as well as the melting and boiling points.

Step-by-step solution

To start, we need to draw our graph axes. Create a vertical axis (y-axis) representing the temperature (in °C), ranging from -20°C to 120°C, and a horizontal axis (x-axis) representing time or amount of heat added to the system. #Step 2: Plot the starting point#

Mark an initial point on the curve to represent the beginning temperature of -20°C on the temperature axis. #Step 3: Draw the heating curve for the solid phase#

From the initial point at -20°C, draw a diagonal line moving upwards to the right until it reaches the freezing/melting temperature of water (0°C). This portion of the curve represents the heating of the water in its solid form (ice). #Step 4: Draw the melting phase#

At the melting temperature (0°C), draw a horizontal line, indicating the phase change from solid to liquid. During this phase, the temperature remains constant and heat is absorbed to break the bonds between the solid particles. #Step 5: Draw the heating curve for the liquid phase#

Once the melting process is complete, draw another diagonal line moving upwards to the right, starting from the end of the horizontal line at 0°C. This line should end at the boiling temperature of water (100°C). This portion of the curve represents the heating of the liquid water. #Step 6:Draw the boiling phase#

At the boiling temperature (100°C), draw another horizontal line, indicating the phase change from liquid to gas. During this phase, the temperature remains constant, and heat is absorbed to break the bonds between the liquid particles and convert them into a gaseous state. #Step 7: Draw the heating curve for the gas phase#

Finally, from the end of the horizontal line at 100°C, draw a diagonal line moving upwards to the right, ending at the final temperature (120°C). This portion of the curve represents the heating of the water vapor. #Step 8: Label the graph#

Label the different parts of the curve to represent the heating and phase transitions: - Solid heating segment (below 0°C) - Melting phase (horizontal line at 0°C) - Liquid heating segment (curve between 0°C and 100°C) - Boiling phase (horizontal line at 100°C) - Gas heating segment (curve above 100°C) Now that all of the steps are completed, we have successfully sketched the heating/cooling curve for water, identified the melting and boiling portions, and marked the graph accordingly.

Key concepts

Phase Transitions

Phase transitions are the transformations of a substance from one state of matter to another, such as from solid to liquid or from liquid to gas, and vice versa. These transitions occur when energy, often in the form of heat, is added to or removed from a substance, causing changes in its physical properties. For example, when ice (solid water) is heated, it reaches a point where it transitions into liquid water; this is called the melting point. Similarly, when liquid water is heated further, it eventually reaches the boiling point where it transitions into water vapor (gas).

During these transitions, the temperature of the substance does not change despite the heat being added or removed. This is because the energy is being used to break or form the intermolecular bonds that hold the molecules in their respective states. Understanding phase transitions is crucial for comprehending various natural phenomena and technological applications, such as climate patterns and cooling systems.

Melting Point

The melting point of a substance is the temperature at which it changes from a solid to a liquid. For water, this occurs at 0°C or 32°F under standard atmospheric pressure. The melting point is an important physical property, not just for water, but for all materials, as it helps us identify and classify substances. At the melting point, the solid and liquid phases coexist in equilibrium, and the addition of heat goes directly into breaking the bonds that hold the molecules in a rigid structure, rather than increasing the temperature of the substance. During this phase change, even though energy is constantly being added, the temperature remains stable until all the solid has become liquid. This creates a flat, horizontal segment on a heating curve, which visually represents the consistent temperature during the melting process.

Boiling Point

Similar to the melting point, the boiling point is the temperature at which a liquid changes to a gas. This occurs for water at 100°C or 212°F at sea level. At the boiling point, the energy being added is used to overcome the liquid's atmospheric pressure and allow the molecules to spread out and form a gas, rather than increase its temperature. This again results in a phase change where temperature does not rise during the transition, creating another flat, horizontal line on the heating curve. This is where water molecules move vigorously enough to break free into the air as steam or vapor. The boiling point is a crucial concept in many areas, including cooking, industrial processes, and the function of engines. Heat absorbed during boiling converts water into vapor without changing the temperature, which reflects on the curve only as a continuation of the previous phase until the transition is complete.

Temperature Graphing

Temperature graphing involves plotting how the temperature of a substance changes over time or as heat is added. A heating/cooling curve is a type of temperature graph that illustrates the phase transitions of a substance such as water. It shows the constant-rate heating process: starting with the substance in its solid state, transitioning through melting, then heating in its liquid state, boiling, and finally, heating as a gas.

On the graph, the upward-sloping lines represent increases in temperature as heat is added, while the flat, horizontal sections indicate phase changes occurring at constant temperature. A well-constructed temperature graph can provide immediate, visual insight into the behavior of a substance during heating or cooling. Labeling each section of the graph corresponding to different states and transitions is a helpful strategy for distinguishing between phases and understanding the properties of the substance across various temperatures.