Last week I went to Robert Goddard Space Flight Center. It was really cool! There was lots of information about space missions, and outer space. I recommend anyone who likes space to go and see all of the cool information, and the awesome science on a sphere, which is looking at pictures and video on a sphere screen.

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I like it when a little information from chemistry class can add a layer of depth to something that seems ordinary and unworthy of notice.  A melting ice cube falls into this category rather nicely.

My favorite behind-the-scenes fact about melting (and freezing, boiling, etc.) is that the temperature stays the same while the change is happening. That is a very cool little fact, because to explain it you have to go one more step and consider molecules, atoms and their motion. How does heat affect a molecule?

With some household items and a thermometer, you can gather some data in your kitchen that will demonstrate for yourself and the kids that while the ice cube is melting the temperature stays the same.

The experimental procedure is at the end of this post, but the basic idea is to freeze the bulb of a thermometer into a pan of water.

Preparing ice cube

(Focus on the set up here, and not what’s in my freezer!)

The ice cube will be the size of the pan, and the thermometer will record the temperature at the middle of the ice cube. Melt the ice cube at low heat. Record the temperature of the ice every 30 seconds until the ice is completely melted.

Yes, we actually did this:

Ice cube - Kelly taking data

I had help, but I think Plum refused to be photographed so there would be no evidence that she was involved.

Here is what you will notice while you are taking data:

Near 0 degrees Celcius (or 32 degrees Fahrenheit) you will record the same temperature for several minutes.

At this point, recording the data might get even more boring. Back when I taught high school, I could tell when students got to the melting point because they would start yelling out, “The thermometer is stuck!” or “Is it broken?!”

Here is what the data looks like on a graph:

Graph - ice cube

Starting at the bottom left of this graph, the temperature of the ice cube starts to rise from the heat of the stove. Then the temperature tapers off and stays constant while the ice melts. After that, the liquid water rises in temperature.

Why does the temperature stay the same while the ice cube melts if the pan is still on the heat?

Think of the solid ice cube for a moment. All of the water molecules are packed together in an orderly way so that the positive areas of one molecule can be near the negative areas of another. Positive and negative attract. This organization makes it a solid. Any single molecule would need extra energy to separate itself from the postitive and negative charges of the nearby molecules. You can feel this when you pull apart two magnets.

Temperature measures motion.

Although each molecule is locked into position, it is still able to vibrate or rotate in place. The temperature gives you an idea of how fast molecules are vibrating, rotating, or moving. As heat from the stove warms the ice cube, the frozen water molecules vibrate more and the temperature rises. Eventually there is enough energy for the molecules to overcome the force of attraction that holds them in place as a solid. That is melting. During melting, the extra energy that is available to the molecules is used to overcome their attractions to each other and convert from solid to liquid. The heat energy is not used to increase their motion, so the temperature on the thermometer stays the same.

Once the water molecules are all in liquid form, the heat of the stove causes them to move faster, so the temperature rises.

The key moment in this activity comes when people get annoyed that the temperature is not changing. The fact that the thermometer comes to a halt always seems to elicit a response. It’s an opening for discussion.

The frustrating part about coming to conclusions is that you cannot see the molecules in action, so you have to take what you know about ice and water, look at the data, think about what might be going on at the molecular level, and do some research. If nothing else, this actvity is a good lesson in data recording; we spent about an hour recording temperatures.

Main ideas

  • Molecules are always in motion.
  • Temperature measures their motion.
  • Applying heat to molecules can make them move faster.
  • If there is enough heat energy, the molecules overcome their attractions to each other and reorganize.
  • The reorganization is called phase change, in this case, melting.

Experimental Procedure

It is much easier to take the data for this experiment with two people.

Materials

  • ThermometerThermometer (Preferably a nice tall one that has a range from roughly -20 to 100 degrees Celcius)
  • Small sauce pan
  • Stopwatch (or method of measuring in seconds and minutes)
  • String
  • Duct tape
  • Space in the freezer
  • Tap water
  • Data table with two columns: Time (s) and Temperature (C). Write in the times before you start with intervals of 30 seconds (0:00, 0:30, 1:00, 1:30, etc). Our times went up to 65:00.
  • Lots of graph paper or a spreadsheet program

Ice cube data table

Procedure

Day 1

  1. Taped thermometerFill the saucepan almost to the top with water and put it on a shelf in the freezer. There should be enough space above the pan to suspend the thermometer. I used the rack above my saucepan to tape the thermometer into place.
  2. Use the duct tape to attach the thermometer to the rack above the saucepan so that the height of the thermometer bulb is in the middle of the water. Don’t let it rest on the bottom of the pan.
  3. Attach string to the top of the thermometer. It will be used during the melting to keep the thermometer in place while the ice cube melts and moves around.

Day 2

  1. Have the data table and stop watch ready.
  2. Turn the stove on low. If electric, allow it to heat up so that the temperature is constant. Leave the stove at the same setting throughout the process.
  3. Carefully get the pan out of the freezer and put it on the stove.
  4. Take the first temperature measurement and start the stopwatch immediately.
  5. Record the temperature every 30 seconds. A good method is to have one person keep their eyes on the thermometer and stopwatch, ready to call out the temperature measurements. The other should write them down on the data table.
  6. Use duct tape to attach the thermometer string to a cabinet directly above the saucepan. Ideally the string will keep the thermometer at the same height and in the same general spot in the pan as the ice starts to melt.
  7. Get comfortable and record the temperature every thirty seconds until all of the ice is melted or until any remaining ice floats across the pan away from the thermometer.
  8. Enter the data into a spreadsheet program and plot a graph (chart) with it.