Water (~4 hrs.)
On a sunny Spring day at the New England beach pictured here, the air temperature increases substantially. The air temperature can also drop rapidly. It was 18 °C at noon yesterday, but by this morning, the temperature had dropped below 0°C, and there was frost on the ground.
On the other hand, the temperature of the ocean changes only very slightly--the surface temperature changes just a couple of degrees a month. It is slow to heat up, and once warmed, it is slow to cool down.
The difference between the change of temperature in the air and in the water has to do with their specific heat capacity, that is the heat required to change the temperature of one gram of a substance by 1 degree. It is a property of a material; it stays the same, no matter what the size of the sample. It is a measure of how well a substance can store energy.
The amount of heat needed to change the temperature varies from one substance to the next. It takes a lot of energy to change the temperature of water. How do other materials compare to water?
Before you compare water to another material, explore the specific heat capacity of water. You can look up the value of the specific heat capacity of water and many other materials: 1 gram of water requires 1 calorie of energy to raise its temperature 1°C. The challenge here is to explore specific heat capacity qualitatively in terms of heat and temperature.
To get started, think about the distinction between heat and temperature in Hewitt's definition of specific heat capacity:
Specific Heat Capacity
The specific heat capacity of any substance is defined as the quantity of heat required to change the temperature of a unit mass of the substance by 1 degree. --from Conceptual Physics
To help you think about this, consider what happens when you double the mass of water. Remember the burner/beaker/water/air system you investigated in Session 4? When the same amount of heat is transferred to 100 grams of water and to 200 grams of water, how will the heat and temperature of the two systems compare?
To answer this question, use the following two resources to gather evidence from a macroscopic perspective and particle perspective:
- Compare change in temperature over time curves for 100 grams of water and 200 grams of water that were heated simultaneously on a burner.
- Compare what happens to the Molecular Workbench representations of two masses of water when you add energy to them by clicking the sun icon.
- Compare the average energy of motion of the particles when you add the same amount of energy (number of clicks) to the two systems.
- Compare the amount of energy (number of clicks) necessary to raise the temperature the same amount in the two systems.
Use the evidence from the graph and the molecular representations to explain your ideas about the specific heat capacity of water.
Turn your attention to a much larger mass of water: Earth's oceans
Heat and temperature in ocean waters is one of the subjects of active investigation by climate scientists who monitor possible changes in the global energy balance. What do you think that a 1 degree change in ocean temperature would indicate about energy transfer in the Earth system?
To find out more about ongoing research on ocean temperatures, read the NASA article, Earth's Big Heat Bucket.
B. Plan an investigation
How do other materials compare to water when it comes to cooling?
Brainstorm comparisons about heat storage and temperature change in different materials, for example, 100 grams of sand or clay vs. 100 grams of water, 100 grams of mashed potato or soup vs. 100 grams of water.
1. Here's a question about the world outside - The sun comes up on a summer day at the beach, and all around are sand and water. The air is cool, but the forecaster has predicted that temperatures will rise to 35°C and then drop to 20°C after sunset. What will happen to the temperature of the sand and the water? Do they change at the same rate?
2. Here's a more domestic question: Water stays warm for a long time in a hot water bottle. What if you filled your hot water bottle with oil? Do oil and water cool down at the same rate?
3. Does beef stew cool at the same rate as coffee?
Take 20 minutes to explore some ideas about the rate at which different materials might cool. Then, select 1 material that interests you, and design a fair test to compare it to water.
We've put a few constraints on this investigation so that in your discussion, you can compare similar temperature vs. time curves:
- Use 100 grams of water and 100 grams of the other material.. Use 400 ml glass beakers.
- Heat the 2 materials to 50°C. (E.g., use a hot water bath (~80°C) or microwave oven.)
- Collect data while the materials cool for 20 minutes.
A Fair Test
- Consider how you'll create identical conditions for each material so that you can compare the temperature change in the 2 different materials when the same amount of heat is transferred from each of them.
- It's tricky to find a "fair" way to transfer energy into, out of, and throughout the mass of 2 different materials. ( Is there air circulating between the sand particles? Are there convection currents in the water? Could the containers be absorbing or radiating energy differently?) We recommend using the probes to stir.
In your notebook, a.) write your investigation question, b.) describe the systems (e.g., the boundaries and components) that you will investigate, and c.) describe your experimental design.
Before you make a prediction, set out 100 grams of each material and take stock of the differences between them.
Then, sketch the temperature vs. time graph that you predict you will collect for each material. Provide a rationale for your prediction. Don't forget to take both a macroscopic and particle perspective.
Use your probes to collect data and create temperature vs. time graphs.
E. Interpret, make sense, and explain Change
Where and how is heat transferred?
Where and how is heat transferred into, out of, and within the 2 materials? How could there be differences in the kind or amount of heat transfer in the 2 systems?
Analyze the data on your graph
Compare the change in temperature of the 2 materials. Annotate each curve, identifying common patterns of temperature change, describing how you think energy may be transferred to and from the materials as time passes, and explaining why you think that the curve looks the way it does. How might specific heat capacity affect the cooling curves?
Take A Molecular Perspective
What might be happening on a molecular level? Read Specific Heat Capacity at the Molecular Level by Roger Tobin.
Keeping Tobin's description of specific heart capacity at the molecular level in mind, return to the graph of the 2 materials you are investigating. Use a molecular perspective to explain any differences in temperature change.
For more information about specific heat capacity refer to Conceptual Physics (pp. 294-296 in the tenth edition).
F. Report on your investigation
In your report, include data from your investigation as evidence to support your conclusions.
What was your investigation question and the system you investigated, and how did you set up a fair comparison?
What does the data tell you about specific heat capacity of the two different materials? Use evidence from your investigation on a macroscopic scale using temperature probes, and describe your ideas about heat transfer on a molecular scale.
Go back to your prediction graph and compare it to your data. Any surprises? Can you find a reasonable explanation for any discrepancies between your prediction and your results?
Heat Transfer in Your Life
This week, look at the world around you focusing on specific heat capacity in your surroundings. Use the Heat Transfer in Your Life section of your journal to record information about at least three examples. In Little House on the Prairie, people used baked potatoes to keep their hands warm on cold nights - did that have something to do with specific heat capacity? What examples can you find of specific heat capacity in books you've read, and in your everyday life? What questions do you have about it?