Heating and cooling curves are important visuals for chemistry students to understand when they are studying phase changes and thermal energy. While most textbooks and other traditional resources present heating and cooling curves and explain what the graphs show, it is impactful when students discover the shape of the curves through their own data collection. The process of collecting and plotting their own data can also help dispel the common misconceptions that substances continue to rise in temperature when undergoing a phase change. The process of collecting and graphing their own data will help students understand how and why substances stay at a certain temperature while going through a phase change.
Do you know the expression “a watched pot never boils”? Well, this lab activity guides students through the process of collecting temperature data as they watch water boil. Don’t worry, the water will still boil even when they are making careful measurements and observations! When they graph their data, they will see the shape of a heating curve. The advantages of the lab include its relatively safe nature, the low teacher prep required, the simplicity of equipment and materials, and the fact that it can even be done at home.
The most impactful part of this activity is the possible “aha!” moment when students connect their hypotheses to their resulting graphs. It is recommended to do this lab before showing or assigning reading about heating / cooling curves so that students can truly discover it themselves. This is also a great opportunity to have students create a hypothesis by sketching a graph of what they think their data will look like in the end. While written hypotheses, especially in the “if…then…” format, are vital for high school science courses, thinking ahead about graphs and relationships between variables is a high-order skill crucial for scientists. If students are struggling with where to start when making a hypothesis-graph, the teacher can provide the x- and y-axes and can show several options of graph ideas to get them started.
If working in the lab, students work in groups of 2-3 students. It is helpful to have at least 2 people working together, as one can keep an eye on the stopwatch and record data while the other looks at the thermometer and the water for observations.
The procedure and data collection would take a 45-minute class period. Post-lab graphing and the conclusion can be completed as homework if the students don't finish it in class.
- Hot plate
- Boiling flask or small cooking pot
- Thermometer
- Heat proof glove / tongs
- Tap water
- Stopwatch
An example student procedure and teacher’s guide are provided in the Supporting Information below, but the main steps of the lab are fairly simple. Many different combinations of equipment can work. The most important parts are a heat source, such as a hot plate or stove; a heat-proof container for the boiling water, such as a heat-proof beaker or small kitchen pot; a thermometer; and a stopwatch.
Students begin by responding to the following question with a prediction.
How will the temperature of water change over time as you apply heat to bring it to a boil?
Students are then directed to sketch a graph of that hypothesis. Developing a hypothesis in the form of a graph is most likely new for students, so they might need extra guidance to do this. One idea is to show example graphs that they can choose from or use as a starting point (examples are offered in the Supporting Information).
The students turn on the heat source and begin to take temperature readings at regular time intervals. Qualitative observations should also be recorded. When the water reaches a rolling boil, students take temperature readings for two additional minutes. Then, they can graph and analyze their data.
Figure 1 is an example of a student graph made using spreadsheet software.
Figure 1: Temperature vs Time graph in spreadsheet software
Students then analyze their graphs, compare them to heating curves from their textbook and answer conclusion questions.
As far as safety goes, boiling water and steam can both cause burns. Students should use heat-safe gloves or potholders when handling equipment, should tie long hair and clothing back, and should not put their hands or faces over boiling water or steam.
This lab has successfully been done at home with a kitchen stove, pot, and meat thermometer. Distance learning or homeschool courses could take advantage of this simple set up to deliver a rigorous lab environment at home.
Overall, this lab activity packs a lot of punch for a relatively simple set up. As chemistry teachers, we want to maximize the time in class where students are doing hands-on and high-order thinking tasks. However, we teachers are usually short on prep time and resources. This kind of lab activity can strike a good balance between these desires and constraints.
SUPPORTING INFORMATION:
There is a full-length student lab handout with procedure and post-lab questions, as well as a teacher guide with NGSS standards, helpful tips, and detailed procedure notes. The Supporting Information can be found below when you are logged into your ChemEd X account. Not a member? Register for FREE!
Prepare materials for groups within the classroom or advise students how to set up the heating curve materials at home. It is helpful to test the equipment to be used and determine a rough idea of how many minutes the water will take to boil with your specific equipment. The heat source and volume of container will determine this and it is useful to give students a rough idea of how long they will need to record data.
Thank you to my chemistry students who tested this lab for me and helped to refine it!
Safety
General Safety
General Safety
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
NGSS
Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data.
Analyzing data in 9–12 builds on K–8 and progresses to introducing more detailed statistical analysis, the comparison of data sets for consistency, and the use of models to generate and analyze data. Analyze data using tools, technologies, and/or models (e.g., computational, mathematical) in order to make valid and reliable scientific claims or determine an optimal design solution.
Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.
Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.
questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of a design.
Scientific questions arise in a variety of ways. They can be driven by curiosity about the world (e.g., Why is the sky blue?). They can be inspired by a model’s or theory’s predictions or by attempts to extend or refine a model or theory (e.g., How does the particle model of matter explain the incompressibility of liquids?). Or they can result from the need to provide better solutions to a problem. For example, the question of why it is impossible to siphon water above a height of 32 feet led Evangelista Torricelli (17th-century inventor of the barometer) to his discoveries about the atmosphere and the identification of a vacuum.
Questions are also important in engineering. Engineers must be able to ask probing questions in order to define an engineering problem. For example, they may ask: What is the need or desire that underlies the problem? What are the criteria (specifications) for a successful solution? What are the constraints? Other questions arise when generating possible solutions: Will this solution meet the design criteria? Can two or more ideas be combined to produce a better solution?
Modeling in 9–12 builds on K–8 and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds.
Modeling in 9–12 builds on K–8 and progresses to using, synthesizing, and developing models to predict and show relationships among variables between systems and their components in the natural and designed worlds. Use a model to predict the relationships between systems or between components of a system.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science.
Engaging in argument from evidence in 9–12 builds on K–8 experiences and progresses to using appropriate and sufficient evidence and scientific reasoning to defend and critique claims and explanations about natural and designed worlds. Arguments may also come from current scientific or historical episodes in science.
Evaluate the claims, evidence, and reasoning behind currently accepted explanations or solutions to determine the merits of arguments.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.
Students who demonstrate understanding can develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.
Students who demonstrate understanding can develop a model to illustrate that the release or absorption of energy from a chemical reaction system depends upon the changes in total bond energy.
Assessment does not include calculating the total bond energy changes during a chemical reaction from the bond energies of reactants and products.
Emphasis is on the idea that a chemical reaction is a system that affects the energy change. Examples of models could include molecular-level drawings and diagrams of reactions, graphs showing the relative energies of reactants and products, and representations showing energy is conserved.
Students who demonstrate understanding can create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.
*More information about all DCI for HS-PS3 can be found at https://www.nextgenscience.org/topic-arrangement/hsenergy.
Students who demonstrate understanding can create a computational model to calculate the change in the energy of one component in a system when the change in energy of the other component(s) and energy flows in and out of the system are known.
Assessment is limited to basic algebraic expressions or computations; to systems of two or three components; and to thermal energy, kinetic energy, and/or the energies in gravitational, magnetic, or electric fields.
Emphasis is on explaining the meaning of mathematical expressions used in the model.
Students who demonstrate understanding can develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative position of particles (objects).
*More information about all DCI for HS-PS3 can be found at https://www.nextgenscience.org/topic-arrangement/hsenergy.
Students who demonstrate understanding can develop and use models to illustrate that energy at the macroscopic scale can be accounted for as a combination of energy associated with the motions of particles (objects) and energy associated with the relative position of particles (objects).
Examples of phenomena at the macroscopic scale could include the conversion of kinetic energy to thermal energy, the energy stored due to position of an object above the earth, and the energy stored between two electrically-charged plates. Examples of models could include diagrams, drawings, descriptions, and computer simulations.