||Heating and cooling of the Earth's surface
Author: Jennifer Skene
Overview: Students conduct an experiment to determine the rate at which two materials, sand and water, heat up and cool down. Then, based on their observations, they make hypotheses about why materials heat up and cool down at different rates. Students conduct experiments to test their hypotheses and share their results with the class.
- Science is both a process and a body of knowledge.
- Science aims to build increasingly broad and coherent explanations of the natural world.
- The process of science involves testing ideas about the natural world with data from the natural world.
- Hypotheses are potential explanations for natural phenomena.
- Hypotheses are based on observations, prior knowledge, and logic.
- Graphs help scientists interpret and share their data.
- Different materials heat up and cool down at different rates; this has important implications for life on Earth.
Grade span: 6-8
- 500 ml beakers, two per group of four students (beakers can be any size, but all beakers should be the same size)
- Thermometers, two per group of four students
- Lights with incandescent bulbs, per group of four students
- Stopwatches, one per group of four students
- A variety of other materials. Suggestions:
- Sand and water mixed together
- Salt water
- Tea (room temperature)
- Dirt (or potting soil)
- Mud (try several types of mud of different thicknesses, with different amounts
- Cut grass
- Data sheets (pdf)
- Graph paper (pdf)
Advance preparation: Gather materials.
Time: Two class periods
Grouping: The entire class, then groups of four students, then the entire class
Science content: The core physical principle at work in this lab is the specific heat capacity of different materials. If this is an introductory activity, then it is really not important that students understand the details of specific heat capacity at this point! It is just important that students understand that different materials heat up and cool down at different rates; this has important implications for life on Earth.
A material's specific heat capacity is the amount of heat (or energy) required to increase the temperature of one kilogram of the material by 1°C. You can think about heat capacity as "thermal inertia." Similarly, for objects with a lot of physical inertia, you have to put in a lot of energy to get them to move. For objects with a lot of thermal inertia, you have to put in a lot of energy to get them to heat up. Every material has its own specific heat capacity.
A material's specific heat capacity has to do with many factors, but one of the most important is the mass of its individual atoms or molecules. It turns out that materials with lighter atoms tend to have higher specific heat capacities (take more energy to heat up) than materials with heavier atoms. This might seem counterintuitive at first, but think about it this way. If we have a material with very light atoms, like hydrogen, how many atoms will it take to make 1 kg? Lots! If we have a material with mostly heavy atoms, like lead, how many atoms does it take to make up 1 kg? Fewer. Heat has to do with the vibrations of all those molecules in the material. So to heat up 1 kg of hydrogen, you have to put in lots of energy to get all those molecules moving. Heating up 1kg of lead means getting fewer atoms vibrating and so takes less energy. Though it's not an absolute rule, denser materials tend to have lower atomic masses and lower specific heats, while the reverse tends to be true for less dense materials.
Nature/process of science: What is a hypothesis? The word "hypothesis" is
often used incorrectly in textbooks. A hypothesis should involve an explanation,
based on observations, prior knowledge and logic. A hypothesis is not just a
guess. Watch out for the misuse of the word "hypothesis" in textbooks and tell
your students to be on the lookout too!
Graphs: Graphs and figures are an important part of the scientific
process. Many textbook labs require students to make graphs of their data, but
then provide no guidance about what to do with those graphs. Graphs are not an
end in themselves. In fact, they serve a dual purpose: graphs (along with statistical
analyses) help scientists interpret their data, and graphs help scientists communicate
their findings with other people. In this, lab students use their graphs to compare
the heating and cooling rates of different materials. They also use their graphs
to share their results with the rest of the class.
Vocabulary: Hypothesis, evidence, experiment, data
- Ask the students to think about their own experiences involving the temperature of sand and water; they can later base their explanations and predictions on this knowledge. Prompt them to think about walking on the beach or on pavement on a very hot day with bare feet. How did it feel (cold, warm, hot)? Now imagine walking into some water near the beach or pavement. How did that feel? (warmer or cooler than the sand or pavement)?
- Give each group of students two beakers, one filled with sand and one filled with an equal volume of water. Ask them to make a prediction or guess which they think will heat up faster, sand or water, and why they think that. At this point, their explanations are unlikely to be real hypotheses about why some materials heat up faster than others but they should be able to describe a personal experience that informs their guess.
- Write the students' ideas on the board. Suggest that they can gather some data to find out if their ideas are correct.
- Pass out the data sheets and graph paper.
- Tell the students to place each beaker under the light. Put one thermometer in each beaker, holding the thermometer so it is in the middle of the sand or water, not resting on the bottom of the beaker. Tell the students to record the temperature of the sand and the water on the data sheet. Tell the students to simultaneously turn on the light and start their timers. They will record the temperature of the sand and water every minute for 15 minutes. After 15 minutes, they will turn off the light, and again record the temperature of the sand and water every minute for 15 minutes.
- Have the students graph their data on the graph paper, with time on the x-axis and temperature on the y-axis. They will draw two separate lines, one for sand and one for water.
- Have the students examine their graphs. Point out that graphs can help us see patterns in our data. Which heated up faster, sand or water? Which cooled down faster? Ask students why they think this happened. Their responses might be something like this:
They may need prodding to come up with some of these ideas.
- Sand heated up faster than water because sand has a color and water is clear.
- Sand heated up faster than water because sand is darker than water; dark colored materials will absorb more light.
- Sand heated up faster than water because sand is a solid and water is a liquid.
- Students can now use these ideas to make and test hypotheses about heating. Based upon what they have already observed, ask students to make a hypothesis about the rate at which different materials will heat up and cool down. Remind students that a hypothesis should involve an explanation. To help students to write a good hypothesis, you might ask them to fill in this sentence: "Sand heats up more quickly than water because __________." This can give them ideas about what factors might cause different rates of heating and cooling. Students will likely need guidance on this. If you would prefer, the whole class could brainstorm together to come up with factors that might explain what they have observed. They might consider:
Of course, students probably won't come up with the explanation for specific heat capacity; the important point is that they come up with some explanation that could help answer the question of *why* some materials heat/cool faster than others.
- Color: students might think that darker colored materials will heat up more quickly than lighter colored materials. Darker colors absorb more light (and heat), and lighter colors reflect more light (and heat).
- Weight or Density: students might think that heavier or denser materials heat up more slowly than lighter or less-dense materials.
- Liquid versus solid: students might think that liquids will heat up more quickly than solids, or vice versa.
- Each student group should come up with (or choose) a hypothesis to test. Ask students if their hypothesis is testable and ask them to think about what they could do to test it.
- Point out that they will do an experiment exactly the same way that scientists do experiments. Show the students the other materials. Ask each group to choose one of the materials that would be appropriate for testing their hypothesis. They will be comparing the heating and cooling of this material to sand and water. Ask the students to write down their hypothesis (explanation) and, based on this hypothesis, what they expect to happen (their prediction): will their material heat up and cool down faster or slower than sand or water?
- Tell students to take the temperature of the new material with the light on for 15 minutes, and with the light off for 15 minutes. Meanwhile make a summary chart on the chalkboard to show the relative heating and cooling rates of all of the materials see the sample summary chart (page 2 of the pdf).
- Have each group graph their results and discuss what happened within their groups. Did their results support their hypothesis, contradict their hypothesis, or could they tell?
- In order to share class results, have one student from each group come to the front of the classroom and present their findings to the class. They should bring their graph, tape it to the board, write the results of their experiments in the summary chart, and briefly explain to the class their hypothesis, prediction, results, and what they think these results mean for their hypothesis.
- After each group has presented their results, ask the
class if they see any patterns in the results. Ask students if they have changed
their minds about which hypothesis is the best one. Can they come up with any
new explanations after doing this experiment? How might they test those explanations?
There may not be any real pattern in the heating and cooling rates of different
materials the main point is that different materials heat up and
cool down at different rates. Don't be troubled if there is no clear pattern;
you can use this to point out to students that this is often how science works.
We did not find a definitive answer to the question "why do some materials
heat up and cool down at different rates?" but we know a whole lot more than
we did when we started! And we have lots of ideas about other tests we could
do to learn more. This is how the process of science works often, scientists
can't answer a question with a single experiment. It can take many experiments,
over many years, to slowly learn more about it.
- To emphasize the process of science more explicitly, offer students the
opportunity to reflect on how they approached this investigation, or to revisit
key concepts about the nature and process of science, have students chart their
pathways using the Science Flowchart. This can be done through a class
discussion or in their small groups. Provide a flowchart to each student and
have them reflect on what they did during this investigation. Students place
a #1 on the flowchart indicating the first thing they did, a #2 for the next,
etc. At the end, they simply connect the numbers to provide a portrayal of
the pathway they followed. Students can then compare their pathways and discuss
differences and commonalities. This helps to emphasize that science is a non-linear,
dynamic process involving observation, exploration, discovery, testing, communication,
"Things we wonder why" bulletin board: Students can pin science questions on the board. Questions can stem from ambiguous labs like this one, or from class discussions. Questions can also address things individual students are curious about. If anyone gains new information, has a flash of insight, or makes a relevant observation during the semester, it could be added to the relevant question on the bulletin board.
Connect heating and cooling to Earth's biomes: Ask students to think about places
on Earth where there is a lot of water, and places where there is a lot of
sand. Coastal areas are an example of places near a lot of water. Deserts are
an example of places with a lot of sand. The presence of water or sand affects
the climate in each of these areas. Ask students what temperature patterns
they would expect to see in these areas? How could they test their ideas?
Background information: It takes a lot of energy to heat up water. Large bodies of water, like the ocean, don't change temperature very much throughout the year. That is partly why, in coastal areas, the temperature doesn't change much throughout the year, or between day and night. The ocean keeps the temperature on land relatively stable.
It takes less energy to heat up sand. That is part of the reason why deserts
like the Sahara and the Mojave are so hot the sand heats up very quickly.
And deserts are usually pretty cold at night because once the sun sets,
the sand cools off very quickly. (Note that deserts don't necessarily have
sand, and deserts are not necessarily hot! By definition, deserts are places
that get very little precipitation the Arctic and the Antarctic are
deserts, and they are neither sandy nor hot!)
The materials that make up the Earth's surface (water, sand, rock) are not the
only things that determine an area's climate. Characteristics like latitude,
altitude, wind and current patterns, and precipitation are also really important
in determining an area's climate.