Hydrosphere: Why are Oceans Salty?

27 teachers like this lesson
Print Lesson


SWBAT graph and explain the distribution of fresh water on Earth.

Big Idea

In this lesson, students explore why there is salt and fresh water on Earth through class demonstrations and research. Then, students graph and describe the distribution of fresh water resources on Earth.

Lesson Overview

Inquiry Based Instructional Model

To intertwine scientific knowledge and practices and to empower students to learn through exploration, it is essential for scientific inquiry to be embedded in science education. While there are many types of inquiry-based models, one model that I've grown to appreciate and use is called the FERA Learning Cycle, developed by the National Science Resources Center (NSRC):

1. Focus

2. Explore

3. Reflect

4. Apply

A framework for implementation can be found here

I absolutely love how the Center for Inquiry Science at the Institute for Systems Biology explains that this is "not a locked-step method" but "rather a cyclical process," meaning that some lessons may start off at the focus phase while others may begin at the explore phase. 

Finally, an amazing article found at Edudemic.com, How Inquiry-Based Learning Works with STEM, very clearly outlines how inquiry based learning "paves the way for effective learning in science" and supports College and Career Readiness, particularly in the area of STEM career choices. 

Unit Explanation

In this unit, students will study each of Earth's major systems: biosphere, geosphere, hydrosphere, and atmosphere. In addition, students will investigate how these systems interact in multiple ways to affect Earth's materials and processes by conducting research, constructing graphs, creating models, carrying out scientific investigations, and analyzing real-world applications. 

Summary of Lesson

Today, I open the lesson by showing a video and completing two investigations on salt and fresh water. Students then explore why we have salt and fresh water on Earth. At the end of the lesson, students reflect and apply their new understanding of the distribution of fresh water by graphing the fresh water resources on Earth. 

Next Generation Science Standards  

This lesson will support the following NGSS Standard(s):'

5-ESS2-2. Describe and graph the amounts and percentages of water and fresh water in various reservoirs to provide evidence about the distribution of water on Earth.  

Scientific & Engineering Practices

For this lesson, students are engaged in Science & Engineering Practice:

Science & Engineering Practice 4: Analyzing and Interpreting Data 

Students represent and evaluate data on fresh water resources in a pie chart.

Crosscutting Concepts

To relate ideas across disciplinary content, during this lesson I focus on the following Crosscutting Concept

Crosscutting Concept 2: Cause and Effect 

Students examine how the water cycle and the process of erosion help create fresh and salt water reservoirs on Earth. 

Disciplinary Core Ideas

In addition, this lesson also aligns with the following Disciplinary Core Ideas

ESS2.A:  Earth Materials and Systems

Earth’s major systems are the geosphere (solid and molten rock, soil, and sediments), the hydrosphere (water and ice), the atmosphere (air), and the biosphere (living things, including humans). These systems interact in multiple ways to affect Earth’s surface materials and processes. The ocean supports a variety of ecosystems and organisms, shapes landforms, and influences climate. Winds and clouds in the atmosphere interact with the landforms to determine patterns of weather. (5-ESS2-1)

ESS2.C:  The Roles of Water in Earth’s Surface Processes

Nearly all of Earth’s available water is in the ocean. Most fresh water is in glaciers or underground; only a tiny fraction is in streams, lakes, wetlands, and the atmosphere. (5-ESS2-2)

ELA Integration

To add depth to student understanding, when I can, I'll often integrate ELA standards with science lessons. Today, students will work on meeting CCSS.ELA-LITERACY.RI.5.7: Draw on information from multiple print or digital sources, demonstrating the ability to locate an answer to a question quickly or to solve a problem efficiently. In this lesson, students will be using multiple resources to locate key information involving an environmental issue. 

Choosing Science Teams

With science, it is often difficult to find a balance between providing students with as many hands-on experiences as possible, having plenty of science materials, and offering students a collaborative setting to solve problems. Any time groups have four or more students, the opportunities for individual students to speak and take part in the exploration process decreases. With groups of two, I often struggle to find enough science materials to go around. So this year, I chose to place students in teams of two or three! Picking science teams is always easy as I already have students placed in desk groups based upon behavior, abilities, and communication skills. Each desk group has about six kids, so I simply divide this larger group in half or thirds. 

Gathering Supplies & Assigning Roles

To encourage a smooth running classroom, I ask students to decide who is a 1, 2, or 3 in their groups of three students (without talking). In no time, each student has a number in the air. I'll then ask the "threes" to get certain supplies, "ones" to grab their computers, and "twos" to hand out papers (or whatever is needed for the lesson). This management strategy has proven to be effective when cleaning up and returning supplies as well!  


20 minutes

Lesson Introduction & Goal 

I introduce today's learning goal: I can graph and explain the distribution of fresh water on Earth. I ask students to turn and talk: Why do you think we have salt water and fresh water on Earth?

One student suggests, "I think the reason the water is salty is because of the salt and minerals in the rocks."

Another student says, "When the Earth was first created, the water was exposed to salt, but it didn't turn all of the water to salty water. There's 3% that stayed fresh water."

Then a student mentions, "I think salt water has always been here. When the salt water evaporates, it leaves all the salt minerals behind. Then, it rains into little ravines and eventually into a lake. This then becomes a freshwater lake."


I want to inspire interest in today's lesson and capitalize on student curiosity, so I show the following video clip, (until 2:10). Each time I mention a video, students excitedly turn off the lights and reposition themselves closer to the screen! 

Turn & Talk Moments

During the video, I pause a couple times to make sure students are actively listening and gleaning new knowledge. At 0:27, I pause and ask: What eventually happens to all fresh water? Later on, at 1:14, I ask: Why are oceans salty? 

Salt Water Evaporation Investigation

During yesterday's lesson, a team of students suggested that we investigate whether or not salt evaporates using two cups: one with fresh water and one with salt water. I set out two cups filled half full with water, epson salt (the only salt I have on hand), and a measuring spoon. I then ask: How can we test whether or not salt evaporates? A student suggests that we place salt in one cup and no salt in the other cup: Does Salt Evaporate Investigation.

I then stir salt into one cup and then decide that we should use less water so that the evaporation process doesn't take so long: Stirring in Salt! I ask two student volunteers to pour out some water, label the cups, and place them next to the window: Fresh Water & Salt Water Cups. (It's not until now that I realize that the water levels are not the same, however, for the purpose of observing the salt left in the bottom of the salt water cup, this variable will not matter.)

Teacher Note: Several weeks later, after our winter break, students return to find the water in both cups evaporated, but not the salt: Fresh & Salt Water Cups (After). What's neat is that the epson salt created a layer of salt that trapped water underneath. For the next week, students poked their fingers into the cup, to break up the layers of salt and to test whether or not all of the water would evaporate. This was such a simple and effective investigation to demonstrate how the evaporation process in the water cycle filters the salt out of salt water in order to rain fresh water on land. 

Eroding Salt Demonstration 

I decide to also model how erosion transports salt minerals in rocks on land to the ocean. To prepare for this demosntration, I drilled a hole in the bottom of a paint tray (creating a stream table), placed soil inside the tray, and set a white tub below the tray to catch dripping water. 

Then, with students by my side, I dyed some epsom salt red with food coloring, sprinkled the salt over the land (Sprinkling Red Salt), and simulated rain by pouring water through a cup with holes (Pouring Water). I want students to see that the water picks up the red salt and carries it to the ocean: Stream Table Investigation. Here's what the demonstration looked like at the end: After Investigation

Following the investigation, I ask for students to make "scientific observations." One  student says, "I observed that when it rained, some of the salt washed away into the rivers." Another student adds, "I predicted there would be a lot more water, but some of it was absorbed by the soil." We discuss how this is similar to precipitation being stored in soil and as groundwater in the real world. 


30 minutes

At this point, I want to give students the opportunity to study salt and fresh water by reading an online resource and completing a fill-in-the-blank handout.

Google Email & Links to Resources

Each of my students has a google email account, so emailing students links to online resources is quick and easy! Prior to this lesson, I email the following link to students.

Getting Ready

I explain: Today you will be working in groups of two. Students know that this means they will work with their elbow partners (the students sitting right next to them). I then ask partners to silently show me which partner is a #1 and #2 using their fingers. Students know that I expect them to do this quickly and without talking. Setting this expectation ahead of time saves instructional time.

I then ask #1 students to grab their computers and #2 students to grab a copy of the following handout, Why do we have salty and fresh water on Earth, to share with his/her partner. 

Today's assignment is sort of like a fill in the blank puzzle! As you read pages 182-183 in this online chapter out of a textbook, look for the correct words that fill in the blanks! Students couldn't wait to start, almost as if this would be like a game! 

Monitoring Student Understanding

Once students begin working, I conference with every group. My goal is to support students by asking guiding questions (listed below). I also want to encourage students to engage in Science & Engineering Practice 7: Engaging in Argument from Evidence

  1. What patterns have you noticed? 
  2. Why do you suppose ____? 
  3. What have you found so far? 
  4. Has your thinking changed? 
  5. What evidence do you have? 
  6. How did you decide _____?
  7. What conclusion can you draw about ____?
  8. Can you explain why oceans are salty?
  9. Why do we have fresh water on land? 

Student Conferences

Here, Conferencing With Students #1, I support students by pointing to a sentence in the text. I then try to connect their findings with the stream table demonstration earlier. I wish I had asked them to explain HOW this question connects with the demonstration.

During this conference, Conferencing With Students #2, I ask the students to use what their research to explain how fresh rainwater turns into saltwater. We then discuss how the salt levels in oceans may increase and how this might impact aquatic and terrestrial biomes.  

Student Work

As students finish the fill-in-the-blank activity (Completed Student Handout), I ask them to take turns "teaching" each other what they have learned about salty and fresh water on Earth. We then go over the answers as a class. Students are eager to find out if they got all the answers correct! 

Reflect & Apply

30 minutes

Constructing Circle Graphs

Now that students have built meaning and understanding by observing, questioning, and exploring, I pass out a Freshwater Resources on Earth Chart to each group and a Circle Graph & Observations to each student. In order to meet NGSS Standard 5-ESS2-2 (Describe and graph the amounts and percentages of water and fresh water in various reservoirs to provide evidence about the distribution of water on Earth), I want to provide students with the opportunity to describe and graph the fresh water reservoirs on Earth. 

During yesterday's lesson, Hydrosphere: Water Distribution on Earth, I modeled how to use a circle graph to represent the salt and fresh water resources on Earth. Knowing this, I provide students with very little instruction, hoping that they can complete today's circle graph with their partners. I simply ask a few questions to get students started: 

What percent does the entire circle graph represent? (100%)

What percent does one pie piece represent? (10%)

If you're graphing the information in the chart, what would be an appropriate title for this graph? (Fresh Water Resources on Earth)


During this time, the majority of my conferences with students focus on the pie chart and percentages. Here, a student beautifully explains how he divided the pie chart up to represent the data and how he double checks that the percentages add up to 100%: Student Explaining Percentages.

Sharing Out

As students finish completing their circle graph, they move on to writing observations: Student Graph Example 1 and Student Graph Example 2. I then invite students to make a circle on the carpet when they are ready to share their observations with the class. 

Here, Students Sharing 1, we discuss what would happen if we melted the glaciers and ice caps in order to get more fresh water. While these students are talking, I hear another student say, "I'm sure glad we have the water cycle." I ask him to share next: Students Sharing 2. I'm happy to hear him bring up the water cycle because that is one of the key points to this lesson! Without the water cycle replenishing fresh water on land, all liquid water on Earth would eventually make it's way to the ocean as oceans are located at the lowest part of the Earth's crust.