Thermal Expansion of Water
Lesson 10 of 10
Objective: SWBAT determine the relationship between heat and the expansion of water, represent graphically how water expands as it is heated, and connect this information to what is happening on a global scale.
Sea level is the level of the sea after averaging out the short-term variations due to wind waves. Regional, short-term sea level is constantly fluctuating due to tidal and wind-driven effects on the ocean. Global sea level has fluctuated throughout Earth's history. These long-term, widespread changes in the mean sea level are known as eustatic sea level changes.
Over the twentieth century, global sea level has risen on the average of 2.0 millimeters (0.08 inches) per year for a total of 10 to 25 centimeters (3.9 to 9.8 inches). According to a recent Environmental Protection Agency study, it is expected to rise at least another 20 centimeters (7.8 inches) by 2100. Most experts agree that human-induced global warming is the force behind the current global sea-level rise.
Causes of Eustatic Variations
Eustatic variations in sea level are the effects of external forces. There are three eustatic factors that contribute to sea level rise:
- Regional subsidence and land movements including, tectonic displacements;
- Thermal expansion of the water in the ocean; and
- Exchange of water stored on land by glaciers and ice sheets.
As the global climate warms, the average level of the ocean is gradually increasing, because warmer water occupies a greater volume. The warmer climate also is causing the melting of mountain and non polar glaciers, which adds volume to the oceans.
Impacts of Rising Sea Level
Rising sea level has many impacts on coastal areas. Although an increase in sea level will not necessarily affect the intensity of storms, such as hurricanes, it does increase the vulnerability of coastal areas to severe storms. Coasts are especially densely populated. More than 100 million people are estimated to live within 1 meter (approximately 3 feet) of the present-day sea level.
A rise in sea level causes five primary physical effects:
- Erosion of beaches and bluffs
- Increased flooding and storm damage
- Inundation of low-lying areas
- Salt-water intrusion into aquifers and surface waters
- Higher water tables
The primary reason for inconsistencies in relative sea level is regional subsidence and emergence of land due to postglacial rebound. During the last glacial period, approximately 18,000 years ago, enormous glaciers covered continental areas. The land beneath these glaciers subsided due to the added weight. When the ice melted at the end of this period, the land began to emerge again, and continues to do so today. Other factors that can causes changes in relative sea level are either uplift or subsidence related to tectonic processes, fluid withdrawal, and sediment deposition and compaction.
During a period of glaciation, the average global temperature drops considerably and the volume of the ocean decreases greatly. The water that would otherwise be in the ocean is frozen as ice in continental glaciers, or as sea ice in the oceans. During the peak of the last glacial period, global sea level was approximately 100 meters (328 feet) lower than it is today. Only when the temperature began to warm did the glaciers melt and flow back into the ocean. This is just one example of how the global climate can have a substantial effect on sea level.
It is estimated that most of the increase in sea level will be as the result of global warming, which causes thermal expansion of the oceans. Thermal expansion occurs because seawater expands when the temperature of water increases. Since the oceans absorb heat from the atmosphere, when the atmosphere becomes warmer so will the oceans. Warm seawater has a greater volume than cold seawater. As the temperature of the ocean increases so will the total ocean volume. The increased volume will cause the level of the water in the oceans to rise.
Two factors contribute to this accelerated rise in sea level. First, although the oceans have an enormous heat storage capacity, if global atmospheric temperatures rise, the oceans will absorb heat and expand. A greater volume of ocean water due to thermal expansion will lead to a rise in sea level. Second, rising temperatures cause the ice and snowfields to melt, thereby increasing the amount of water in the oceans. It should be noted that only the melting of landbased ice and snow increases sea level. The melting of floating ice will not affect sea level. This can be demonstrated to your students by partially filling a glass container with ice and water and marking the water level on the glass. When the ice cubes melt, note that the water level has not changed.
*From the Water Encyclopedia
This lesson was adapted from a lesson by Leah Reidenbach, Joshua Reece, and Reed Noss at University of Central Florida.
- Graduated Cylinder
- Funnel 500- milliliter conical flask (Erlenmeyer Flask)
- Electronic scale
- Weight boat Tissue or paper towels
- Hot plate
- 1 liter bottle to mix salt and water
- Thermal Glove
- Two-hole stopper
- Hollow glass tube (long)
- Metric ruler
- Clear tape
- Students worksheet for recording data
Prepare salt water that has a similar salinity to that of the average of the Earth’s oceans (about 35 ppt). To do this, measure 35 grams (~ 2 tbsp.) of salt on the electronic scale and mix it with 1 liter of water (a typical water bottle is usually 1 liter). Shake the bottle to make sure the salt completely dissolves. Make enough for each group to have 1 liter of salt water.
For this activity students should work with small groups, but each student should work on their own worksheet.
Begin the class by asking students what they they think are main contributing factors to sea level rise. Record their ideas on the board. Most of my students will not have considered the thermal expansion of water so I push their thinking.
"In the albedo lab we learned that as sea and land ice melt, oceans absorb more solar radiation. What happens to the volume of water as it heats up?"
Since I have previously taught lessons around heat transfer and movement of molecules as they heat up I would refer back to those lessons here. If you have not taught these concepts you may wish to stop here and move on with the lesson as the science will emerge after completing the lab.
Previous lessons include:
Show the class the experimental set up. Walk them through both parts of the lab and be sure to stress lab safety protocols as they will be working with heat.
Students should look for the water height to increase as the temperature increase. Ask them why this is happening? What is happening to the water molecules? Since the flasks are closed, we are not adding more water, so what explains the height increase?
I ask students to share out their data on chart paper so that we can compare and discuss our results. Hopefully you have a similar routine for your science classroom.
Clarify any questions that students may have then let them get to work.
Monitor and support as needed.
After collecting data and completing the lab, ask students to complete the discussion questions and read the "What's Going On?" section. Facilitate a discussion with your students that incorporates parts of the global system that contribute to thermal expansion of water.
Discussion Questions and Answers
What caused the change in the water level over time?
Heat caused the water to expand which increase the volume over time.
2. Why do you think water expands when it is heated?
The molecules of the water absorb the heat which gives them more energy. This energy causes the molecules to move around and vibrate more and ultimately take up more space.
3. What do you think your graph would look like if you continued to heat up the water?
The water level would increase until the water turning into gas, and then the water level would eventually decrease.
4. Use the following formula to find the coefficient of volume expansion (β) using the data from your experiment.
Volume expansion = change in volume
initial volume x change in temperature
5. Now that you know the coefficient of volume expansion of the saltwater from your experiment, use it to find the change in volume of Earth’s ocean if the average temperature of the Earth raises by 2C. Currently the ocean has a volume of 332,519,000 cubic miles of water .
6. Do you think determining sea level rise this way is over-simplified? Why or why not?
There are many different variables that contribute to the change in sea level due to thermal expansion of water. For instance, the temperature of the ocean varies all over the world and at different depths. The salinity of the water also effects how much water will expand when it is heated.
7. What are other factors that could be involved in sea level rise?
Other factors that could contribute to sea level rise include melting ice caps and groundwater depletion.
What’s going on?
In general, as water gets warmer it takes up more space. Each drop of water only expands by a little bit, but when you multiply this thermal expansion of water over the entire depth of the ocean, it all adds up and causes sea level to rise. Sea level is also rising because melting glaciers on land are adding more water to the oceans.
Glaciers are large sheets of snow and ice that are found on land all year long. They are found in the western United States, Alaska, the mountains of Europe and Asia, and many other parts of the world. The giant ice sheets on Greenland and Antarctica are also considered glaciers. Warmer temperatures cause glaciers to melt faster than they can accumulate new snow.
As giant ice sheets and smaller glaciers melt, they add more water into the ocean, which causes sea level to rise. If people keep adding greenhouse gases to the atmosphere, the average sea level around the world by the end of this century (the year 2099) could be anywhere from 7 to 40 inches higher than it was in 2000. Sea level could rise even more if the big ice sheets in Greenland and Antarctica were to melt more quickly.
If temperatures keep rising, glaciers will continue melting, and some could disappear completely. Rising sea level is a threat to people who live near the ocean. Hundreds of millions of people around the world live in low-lying areas near the coast that could be flooded as sea level rises. Some low-lying areas will have more frequent flooding, and very low-lying land could be submerged completely.
Rising sea level also threatens the buildings and infrastructure of cities located along coastlines, as well as coastal ecosystems such as mangrove forests and coral reefs. Rising sea level and stronger storms caused by warmer oceans could erode beaches, damage many coastal wetlands, and even completely wipe out certain beaches and islands.
This lesson uses the practice of developing and using models. Students are using a model to generate data about phenomena in a natural system, looking at the inputs and outputs in the system, and relating this to a much larger scale model, Earth's oceans.