Day 2 - Fresh Water vs Salt Water

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SWBAT compare the density of fresh water and salt water while applying the scientific process.

Big Idea

Students write conclusions about salt water and fresh water and compare which one is more dense.

NGSS Connections and Class Preparation

30 minutes

Students continue their investigation on how objects float differently in salt water and fresh water. Observations and further testing are used to summarize their findings. I will access this learning (properties of salt and fresh water and how they interact with each other) when students look at river mouths and erosion in future lessons.

Cross-cutting Concepts - Appendix G

Patterns XC-P

Students consider how objects float when placed in salt water and fresh water; noting the pattern that objects tend to float higher or float in salty water versus fresh water. This pattern is clarified when students organize their data.

Science Practices

Planning and Carrying Out Investigations - SP3

Students compare how objects float in fresh and salt water.

Analyzing and Interpreting Data - SP4

Students record and review data to answer the question: Do objects float differently and fresh water and salt water and how?

Using Mathematics and Computational Thinking - SP5

Students use quantitative descriptors to indicate how much of an object was above or below the water surface.

Constructing Explanations and Designing Solutions - SP6

Students write a conclusion, noting if their hypothesis was correct and providing possible reasons for the results they received.

Lesson Preparation

Check that objects from 'Day 1' are dry and accounted for in each bag

Have the 'class chart of observations' from wrap up on 'Day 1' available

- Prepare a chart to summarize the teams' observations

- 12 clear containers for water; 1 / team

- 6 larger water containers; one per table (so students can access water to set up the lab) I poured water in the containers earlier in the day and set the team containers with water on student desks while they were at lunch.

- 12 baggies with salt

- 12 basters (to layer fresh water on top of salt water; see section 'Sharing Conclusions and More!')

- pitcher of water colored with food coloring

Material for Each Lab Team

Placed in a baggie:

- measuring spoons and cups

- 2 containers of the same size for water and salty water (if testing salt and fresh water on the same day; otherwise only 1 container is needed)

- salt

- small objects day 1 materials to test in the water

Question for the Day

5 minutes

I start science with a question, usually written on the board.  This provides an opportunity for students to consider today's topic before the lesson has officially begun. 

Students know that when they return from lunch, we meet on the rug to read our 'science question for the day'. I established this routine with the kiddos to keep transition time short and effective and redirect student's attention back to content while allowing time for focused peer interaction

I direct students to sit with their lab partner to discuss the question.

Question for the Day: What do you think will happen when you place the objects in the salt water?

This question connects to last week's lab. I want to get the kiddos thinking about the lab so that we can efficiently pick up where we left off.

After 1-2 minutes, I signal students' attention and call on volunteers to share their partner's answers.

Expecting students to share what their partner said promotes active listening.

"Today you will find out what happens when you place the same objects that you used for fresh water in the salt water."

"What part of your lab booklet will we need to review to prepare for today's lab? Right, the procedure."

I project lab page 3 on the document camera. "We will go through these steps together to set up your salt water, and then you will test the objects in the salt water and take observations."

"Please return to your seats and take out your lab booklet. Then choose one of you to go to the back table to pick up a material bag."

Procedure to Summarizing Results

20 minutes

Observations About Salt Water

Page 3 - Procedure

After teams have their materials, I direct students to turn to page 3 in their lab booklet. Volunteers read the directions and provide time for students to prepare their salt water.

After students measure and stir in the salt, I direct students to turn to the next page in their lab book.

Page 4 - Observations

On page 4 we review how their data/observations will be recorded. I point to the word bank that students created last week.

I drew pictures to go with the water line at, above, below, to help my visual and spatial learners. 

"It is important that we make careful observations and record as much detail as we can. Remember  to write where the water line was on the object, above, below or at the mid way line on the object. "

"You will have 15 minutes to complete your observations."

I walk around the room to check that students are taking accurate observations and to ask questions about what they notice or if they observed anything surprising.

After about 15 minutes or when most teams have tested the objects and written their observations on page 4, I signal students to the rug and ask them to bring their lab booklet. 

By calling students to the rug, students will not be as easily distracted by the material on their desk.

Page 5 - Observation Results

Let's summarize our salty water observations like we did with the fresh water to see if we notice any patterns about the 2 sorts.

Teams share their observations which I record into 1 of 4 categories: sank, water line below the midway line, water line above the midway line, water line on the mid way line. The sort is done on an interactive white board.

After all observations are noted, I ask students what they notice about the observations. Students were most surprised that the soap floated with the salt water.

Next I placed the fresh water class observations next to the salt water observations and ask students, "What do you notice about the 2 sorts?"

"Which water was able to float more items? Which water made the items float higher?

I write the following sentence frame on the board: "After looking over our observations we noticed that ..." Please turn to your team partner and take turns complete this sentence frame. After each of you have shared, give each other a high five and return to your desk to write your results on page 5 in your booklet."

Many students struggled to summarize all the objects, so I directed students to write just about their soap. 

While students are writing their results, I pick up and discard the salt water (if students are not going to layer salt water and fresh water).

When I see that students have completed writing their results, I signal for students attention.

"After scientists take observations, and write the results, they write their conclusion.  Let's look at a scientist's conclusion to see what elements should be in a conclusion.


Identifying Key Elements of a Conclusion

20 minutes

Using a mentor text to consider the elements of a  scientific conclusion:

I refer to Lucy Caulkins: Lab Reports and Science Books to help structure the writing component for this section

I project a 'student mentor conclusion'. The students read the text with me. Then I ask what elements they noticed in this students' conclusion.

I write student answers on the board and scaffold the discussion to include the following elements:

 1.  The scientist said whether their hypothesis was right or wrong and discussed their results.

 2.  Asked questions about the data (why?)

 3.  Gave some POSSIBLE explanations - maybe use ideas from other experiences, experiments or resources

As the above ideas are developed. I write key words on the board to help students recall the elements that should be in their conclusion. Later I create a 'Conclusion Anchor Chart' based on what was developed today.

"Now it is your turn to write your conclusion, remember to include these 3 elements in your conclusion. Please turn to pg 6 in your lab booklet."

Page 6  - Conclusion

I start the class out together. "The first sentence tells the reader if your hypothesis was right or wrong. Turn to page 2 in your booklet and read your hypothesis. Then write whether or not your hypothesis was correct. Remember to include a sentence about your results and/or observations."

"Look at our sorts, what are some 'why' questions you could ask about the data?  I call on volunteers to share their questions and write these on the board to help scaffold the lesson for students who may not be sure about what question to write."

"Now it is your turn to write a question about your data. Your question may start with 'why' or 'I wonder'."

"Finally, you are to write your possible explanation to the question you wrote. After you have completed your conclusion, please bring your booklet and meet me on the rug."

This would be a natural place to end the lesson. Students could share their conclusion with each other on the rug. The next section could be introduced during a 'science mini-lesson' during writer's workshop as a way to segue into a review of writing a conclusion. 

Sharing Conclusions and More!

25 minutes

I signal teams to the rug and call on volunteers to read their conclusions. We applaud our hard work and well crafted conclusions.

I planned this section as a hands-on lab, but chose to do it as a demonstration instead, due to time constraints. The students were engaged and excited with the results.

Explorations with Fresh and Salt Water

"Scientists did you notice when you wrote your conclusion you had questions about your results? When that happens, scientists will research or do other experiments to learn more about their topic and will add this information to their conclusion."

"Our class has more information we could add to our conclusion, because we have a tool that measures salinity. What is that tool called?"

In our classroom we have a 'touch tank' and one of the jobs is to measure the salinity of the tank with a hydrometer and note it on our salinity chart.

A hydrometer can be purchased at an aquarium store. It is not necessary to use a hydrometer for today's lesson. I mention it because the students are familiar with it and it ties in with today's lesson.

I demonstrate how the hydrometer acts differently with fresh and salt water. In 2nd grade terms I explain that salt water is more dense than fresh water. "In one cup of salt water there is water and dissolved salt, but in water there is only water. There is more 'stuff' in the salt water, making it denser, it has more mass. If something has less mass than the salt water, it will not sink.

"Remember when we measured the mass of the scientific instruments (link to lesson)? We measured the mass by using a balance scale. So if we used a scale to measure the same amount of salt water and fresh water, which do you think would be more dense? Why?"

I connect this lesson to a prior lesson, to help build the schema for mass, density and how we measure for it. I expose students to these concepts which they will explore in more depth in later years.

"What would happen if I carefully poured salt water into the fresh water? Turn to your neighbor and share your hypothesis."

Next I demonstrate what will happen referring to the scientific process terms.

I use food coloring to dye a pitcher of fresh water, pour some of the fresh water into a container  and then use the baster to move the salt water into the fresh water container. I carefully squeeze the baster. Students share observations.

I chose to demonstrate with the salt water since it will sink and if you are not careful with the baster the different waters could mix, making for unclear observations. It will be easier for students to lay the fresh water on top.

Next I ask students what would happen if they moved fresh water into the salt water.  I explain that they will get do their own test to see what will happen. I remind students that they cannot mix the water or shake the glass otherwise they will mix the 2 waters together and the salt will be distributed to the fresh water too. I demonstrate how to carefully place the baster in the water.

I have a pitcher of dyed fresh water. I bring this to each team so that they can remove some of the fresh water with a baster. Teams use their of salt water that they used in the lab earlier.

After students observe what happens when fresh water is placed in the salt water container. I direct teams to add to their conclusion. "Describe what happened and which water is denser." I call on volunteers to share the sentence they added to their conclusion. Then check for understanding:

"Raise your hand if salt water is more dense than fresh water." I mentally note how my class responded and will address misconceptions in future lessons.

If time, and/or interest, I will show the egg experiment that illustrates the difference of density between salt and fresh water. I chose to have students test a number of objects, to develop their sense of close observations and to use more precise language to capture what they are seeing. 

After Information is Added to the Conclusion:

Students pour fresh water into the potted plants and the salt water in the outside drain, dry off objects and return everything to the bags. I collect the lab books. I will use a rubric to assess student work based on the NGSS standards.