How Does Saltwater Differ From Freshwater?

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Objective

SWBAT distinguish between saltwater and freshwater.

Big Idea

Students will investigate how saltwater's density differs from freshwater.

Lesson Overview

5e Lesson Plan Model

Many of my science lessons are based upon and taught using the 5E lesson plan model: Engage, Explore, Explain, Elaborate, and Evaluate. This lesson plan model allows me to incorporate a variety of learning opportunities and strategies for students.  With multiple learning experiences, students can gain new ideas, demonstrate thinking, draw conclusions, develop critical thinking skills, and interact with peers through discussions and hands-on activities.  With each stage in this lesson model, I select strategies that will serve students best for the concepts and content being delivered to them.  These strategies were selected for this lesson to facilitate peer discussions, participation in a group activity, reflective learning practices, and accountability for learning.

Unit Focus  

The Water on Earth Unit focuses on the interaction of the hydrosphere with other Earth systems including the geosphere, biosphere, and atmosphere. Through models, investigations, research, graphing, and multimedia, students learn that the amount of water on Earth never changes and the amount available for human consumption is small. They identify and calculate the distribution of water sources on Earth, distinguish the properties of various forms of water, and recognize the cycling of water in and out of the atmosphere. 

Lesson Synopsis

This lesson, How Does Saltwater Differ from Freshwater, begins with a review of water on Earth. Students then locate examples of freshwater and saltwater using atlas'. Next, they begin an investigation to identify some differences between them. They discover that ocean water contains large amounts of salt and is heavier than freshwater water. The terms salinity and density are introduced at this point. Then students further investigate how the salinity of water impacts its density. They perform an experiment to test this out. After a guided discussion on what they discovered, students fill out an exit ticket to share what they have learned. I collect this and use it as a formative assessment.

 

Next Generation Science Standards  

This lesson will address and support future lessons on 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

Students are engaged in the following scientific and engineering practices:

2.) Developing and Using Models- Students create models of saltwater and freshwater to describe how the salinity and density vary between them.

Crosscutting Concepts

The How Does Saltwater Differ from Freshwater lesson will correlate to other interdisciplinary areas.  These Crosscutting Concepts include:

4. Systems and System Models- Students investigate how salt ends up in the ocean and develop a model to illustrate the effects of salt in water.

 

Disciplinary Core Ideas

Disciplinary Core Ideas within this lesson include:

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

 

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Classroom Management Methods

Importance of Modeling to Develop Student 

Responsibility, Accountability, and Independence 

Depending upon the time of year, this lesson is taught, teachers should consider modeling how groups should work together; establish group norms for activities, class discussions, and partner talks.  In addition, it is important to model think aloud strategies.  This sets up students to be more expressive and develop thinking skills during an activity.  The first half of the year, I model what group work and/or talks “look like and sound like.”  I intervene the moment students are off task with reminders and redirection.  By the second and last half of the year, I am able to ask students, “Who can give of three reminders for group activities to be successful?” Who can tell us two reminders for partner talks?”  Students take responsibility for becoming successful learners.  Again before teaching this lesson, consider the time of year, it may be necessary to do a lot of front loading to get students to eventually become more independent and transition through the lessons in a timely manner.

EXPLORE TEAMS (Pre-Set)

For time management purposes, I use “lab rats ” where each student has a number on the back of his or her chair, 1,2,3,4 (students sit in groups of 4)and displayed on the board.  For each activity I use lab rats, I switch up the roles randomly so students are experiencing different task responsibilities which include:  Director, Materials Manager, Reporter, and Technician.  It makes for smooth transitions and efficiency for set up, work, and clean-up.  

Engage

15 minutes

Activating Prior Knowledge

Today I begin by handing out hand out atlas' to each groupI explain their task is to locate examples of freshwater and saltwater on Earth. They record each example in a t-chart in their interactive notebook. I want them to notice that there are many examples of freshwaters, and fewer examples of salt waters (oceans and seas), but the the amount of water in the oceans and seas are greater than the freshwaters. 

We reconvene as a whole class and discuss the examples they noted.

t chart examples

Explore / Explain

25 minutes

Saltwater vs Freshwater

After reviewing, I explain that we are going to further investigate the differences between saltwater and freshwater. I hand out a tray with two containers on it to each group. One container, labeled A has saltwater in it, and the other labeled B has fresh water in it. (I purposefully do not tell them which container is saltwater or freshwater as I want them to explore the properties and make observations about each one.) I hand out a Freshwater vs SaltWater task card recording sheet that helps students make observations about each container. In part 1 they are feeling the water and writing down their observations. In part 2, they add drops of blue dye to each container and write what they notice. My intention if for them to notice that when dye is added to saltwater, it will actually float on top of the salt water.  As students notice this, they are writing down what they see. Then they determine which container they believe is saltwater and freshwater and to state why they think that.

Dye floats on water

Once students complete their observations, we discuss the activity as a whole class. I look for students to share how the waters felt and what happened to the added water. After we discuss their observations, we identify each container, A=saltwater and B=freshwater. I explain that the amount of salt in water is called salinity (I post this on the board) and that the salinity in oceans and seas differ from one another. I continue explaining that when water evaporates from the ocean, the salt stays behind, increasing the salinity of the ocean water. I further explain that besides salinity, another difference between ocean water and freshwater is density. I state that density refers to the amount of particles in a given space or packed in a space. Salt makes ocean water denser than freshwater, which is why the freshwater was able to float on top of the saltwater. 

I have students note the terms salinity and density under their t-chart from earlier in the lesson in their interactive notebook.

How and Why is the Ocean Salty?

I ask "Have you ever tasted ocean water? How did it taste?"  I ask a student to share out loud. I am looking for them to say salty. I explain that is why we cannot drink ocean water because it has a large amount of salt in it and is bad for our health. Too much salt in our body does not let our body release excess fluids, so these fluids build up cause a lot of pressure on the heart. We release fluids through bathroom use and sweating, so if there was a lot of salt in our system, these extra fluids would not be released.

I point out that most salt in the ocean is from rivers dissolving salts from rocks and soil and carrying them to the ocean. Ocean waves that crash against rocks also cause salt to become part of the ocean because they erode those rocks away and they dissolve into the ocean like the rocks from rivers. I continue explaining that the salinity in waters varies in oceans. Ocean waters in colder areas have less salt than oceans in warmer areas because ocean waters in warmer climates evaporate more quickly and leave more salt behind. 

Elaborate / Evaluate

20 minutes

Applying What We Learned

Now that we have identified some differences between saltwater and freshwater, I engage students in an investigation to further examine how the density of water affects objects in it.  

I hand out a task card and instruct students to place it in their interactive notebook. I review their investigation before they take part in it. I explain that they are using a hard-boiled egg to demonstrate the effects of freshwater and saltwater. An egg is placed into a container with freshwater and another egg in a container with saltwater. I have them make observations about how the egg reacts to each water and note them in their interactive notebook.

eggs float

Guided Discussion

After making observations, we discuss how the salt water affected the egg and why it happenedI ask students: "What affects whether something will sink or float in a liquid." I want them to make the connection that something sinks or floats depending on its density compared to the density of the liquid. 

We conclude that the freshwater is less dense than the salt water and that the egg is denser than the freshwater, which is why it sinks to the bottom. On the other hand, the saltwater is denser than the egg, which is why the egg floats in the saltwater. All of this leads to a general understanding that the salinity of the ocean makes it denser than freshwater.

Real World Connection

I share with students that it is important to know about the density of the ocean because it has a role in how ocean currents move. I display a map illustrating ocean currents moving in and out of areas.


I explain, that ocean currents from the polar regions carry colder waters, while currents near from areas closer to the equator carry warmer waters. I point out that there is varying density levels within all the oceans and seas, so denser waters tend to be colder and will sink below waters with less density, which tend to be warmer. When dense waters sink to the bottom, more water moves in to replace it. This creates a current.  Eventually that water becomes cold and sinks leaving room for new water to move in. I explain that this cycle is continuous.

Exit Ticket

Before they leave, I hand them a 3-2-1 exit ticket. They are noting 3 ways freshwater and saltwater differ, 2 ways how salt becomes parts of the ocean, and 1 reason we cannot drink salt water.  It is an opportunity for students to reflect on what they learned from today's lesson.

I collect them at the end of class and use them as a formative assessment.