Boiling water demo: Much more than meets the eye

High school chemistry students often have a decent understanding of kinetic and potential energies from their previous work with Newton’s laws, but kinetic energy (KE) and potential energy (PE), as they apply in the world of chemistry, are often brand new concepts. I’ve also found that my sophomores have trouble connecting the abstract microscopic perspective of chemistry to the much more tangible macroscopic one. On top of all this, high school students (and probably the general public as well!) have a broad misunderstanding of heat and temperature and the relationship between the two. All these factors, and probably more, make it challenging for students to thoroughly grasp what happens as a pot of water is warmed and boils. To help connect the dots, I’ve developed a simple demo that I accompany with timely, thought-provoking questions to help my students understand what happens at the microscopic level when water warms and boils.

When the students walk in to class on the demo day, I have a 600 mL beaker containing about 450 mL of water sitting on a hot plate (Fig. 1). A Vernier temperature probe is clamped to a ring stand, immersed in the water and connected to my computer through a LabPro. Logger Pro has been set up to collect 12 temperature readings per minute for 45 minutes — the length of our class period. The range for the y-axis of the graph has been set to stretch from 10 to 110 ºC, preparing us to see room temperature water warmed to its boiling point.

As I get class started, I enthusiastically tell the students that we’re going to do something really fun today — we’re going to boil water!! Before I begin warming the water I start collecting temperature data — but I don’t yet display it to the class — and ask the students a number of questions:

  • The first is a very simple question, yet tricky for some: Sketch a graph of temperature vs. time, assuming the hot plate is not turned on. Once they are done, I show them a live view of the graph so they can check their drawings. Then I hide the live graph and secretly turn on the hot plate.
  • Next, I ask what the particles are doing. (They’re moving around and bouncing off one another.) Then I ask what macroscopic evidence they have of this since we can’t actually see the particles. (The temperature of the water is not zero Kelvin; therefore, the particles must have KE so they must be moving.) This helps the students make connections between the macro and microscopic worlds of chemistry.

Computer collecting data from a beaker.

Fig.1. Setup to collect temperature vs time of water boiling.

  • Then, to get students thinking about heat and how it’s different from KE, PE and temperature, I ask them if the particles have heat. (No.) Do they have a temperature?  (No. Temperature is a macroscopic property that is proportional to, but not the same thing as, the average KE of the particles at the microscopic level.)
  • Finally, to lead them on to the next phase of this demo, I ask how we could increase the energy of the particles. (Turn on the hot plate to warm up the water thereby increasing the KE of the particles.)
  • I then ask the students to extend the line in their graphs to show what will happen when we turn on the hot plate and warm the water for a few minutes (assuming it doesn’t get warm enough to boil). Once again, the students check their sketches against the live view of the graph that I display when everyone is finished.
  • Next, I ask the students how the behavior of the particles has changed since we turned on the hot plate. Has the KE changed? What about the PE? What macroscopic evidence of these changes is there?  (The particles are moving around at higher speeds, their KE has increased as evidenced by the increase in temperature of the water, and the PE has not changed since there is no phase change yet.) For simplicity, we ignore evaporation.
  • Oftentimes I need to adjust the hot plate dial to get the water to boil at the right time but I don’t mention this to the students. Later when a change in the slope of the graph becomes apparent (Fig. 2), I ask the students if anyone can explain why the slope of the line has changed.

Finally, we’re approaching the climax of the demo — boiling of the water!  If the water is close to boiling, I hide the graph so as not to reveal the outcome; otherwise, I continue to display the live view so the kids can watch the drama unfold in real time after they answer the next set of questions.

  • First, I ask the students a few basic questions about boiling: What is going to happen to the water if we continue to warm it? (It will boil.) At what temperature will this happen? (100 ºC) Will the temperature continue to rise as the water boils and it’s still sitting on the hot plate absorbing heat? (No.) There is rarely a consensus on this question and I let the students argue their opinions for a while before allowing them to see for themselves with the live graph.
  • Now, to dig in deeper, I ask what the particles are doing when the water boils. (They’re overcoming their intermolecular forces and turning into gaseous particles.) What is changing: PE or KE or both? What evidence do you have? (Their energy must be increasing since they are absorbing heat. KE can’t be changing since the temperature is constant, so it must be the PE that changes. PE is changing because the water is turning gaseous, meaning the particles are overcoming their intermolecular forces, and thereby increasing their PE.)

Graph of temperature of boiling water over time.

  • What happens to the heat now that the water is boiling? Is the heat “heating up” the water? (The heat is being converted into PE as evidenced by the bubbles of gaseous water rising in the liquid; the heat is no longer “heating up” the water.)
  • Why isn’t the temperature exactly 100 ºC? (We’re slightly above sea level.)
  • What will happen to the line if we turn the hot plate up? (As long as the water continues to boil, the temperature will remain constant but the rate of boiling will change.)

My class sizes are pretty small (about 18 – 20) so I usually ask these questions orally to the whole class and manage to hear from all the kids, but students could easily work in small groups answering the questions on whiteboards. Besides being a highly effective demo, cleanup is a snap: you can safely pour the water down the sink!