This is the fifth and final regular Investigation of the LabLearner CELL Density. In this Investigation, students will explore the density changes of solutions of different salt concentrations.

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A. Begin the PreLab by reviewing some of the concepts that the students have encountered in the CELL thus far.

Sample discussion points are as follows: 

• A liquid will float on the surface of a different liquid with a greater density.

• A liquid will sink to the bottom of a different liquid with a lower density.

• A solid will float on the surface of a liquid with a greater density.

• A solid will sink to the bottom of a liquid with a lower density.

• The formula for density is mass divided by volume.

• The density of matter depends on how far apart the particles are spaced from one another.

• Volume can be expressed as milliliters or cm2.

• The density of a specific type of matter is a constant.

• The density of water is 1 gram/milliliter.

• The densities of different types of matter are different.

• The densities of gases are less than the densities of liquids.

• With few exceptions, the densities of solids are greater than the densities of liquids.

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B. Encourage students to use the knowledge they have gained from the previous Investigations by engaging them in the following activity.

1. Tell students that they are going to read a journal entry.  Students may read the story independently, in groups of three, or as a class.  Instruct students to find the story in Problem 1 of their Student Data Record.

2. READ:

Dear Journal,

I just got back from a day at Lake Willow Tree.  My dad told me that this is a freshwater lake.  While we were there, we rented a canoe and went swimming and fishing.  Around lunchtime, my family packed a lunch and set off in the canoe.  I was scared that we were too heavy and the canoe would sink, but it floated along very smoothly.  My mom found a shallow part of the lake and dropped the anchor to stop the canoe from floating away. My sister, Jill, jumped right into the water and started swimming.  I am not a great swimmer, so I just stuck my toes over the side.  After a while, I started to get hot.  I decided it was time to go swimming so I hopped into the water.  Boy, it was hard to stay above the water!  I started flapping my arms and kicking my legs, but I kept sinking.  My mom threw a life preserver to me to help me float.  My sister started to laugh when she saw me trying to swim.  She yelled, “Jack, just stand up!”  I put my legs down and stood up and saw that the water only came up to my belly button.  I decided it was a good idea to spend the rest of the day fishing.  I got lucky and caught three fish!

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3. After students have read the story, ask them the following questions:

a. Ask students: Why did Jack have difficulty floating on the surface of the water? Student answers may vary.  Students should indicate that the density of Jack’s body was greater than the density of the water.

b. Ask students: Why do you think the life preserver was able to float on the surface of the water? Student answers may vary.  Students should indicate that the density of the life preserver was less than the density of the water.

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4. Tell students that they are going to now read a second journal entry.  Students may read the story independently, in groups of three, or as a class.  Instruct students to find the story in Problem 2 of their Student Data Record.

5. READ:

Dear Journal,

My family and I just got to Ocean City, New Jersey yesterday.  It is my first time at the beach.  I was so excited to swim in the ocean today.  I have been taking swimming lessons at home, but I am still not a great swimmer.  I decided to stay close to the shore in the shallow part of the ocean.  I was having such a great time in the water that I did not notice a big wave forming behind me.  When it hit, it knocked my feet right out from under me!  I went under the water so quickly that I didn’t have time to close my mouth.  I was so surprised at how salty the ocean water tasted.  As the day went on, I started to go further into the ocean because I think my swimming has gotten better.  I noticed that I did not have to flap my arms and kick my legs as much as I did when I was in Lake Willow Tree.  My body floated a lot easier than it had before.  The best part of the day was when I found a starfish on the beach.  I wanted to keep it my sand bucket, but my sister Jill convinced me to set it free.  I can’t wait to see what I find tomorrow!

6. After students have read the story, ask them the following questions:

a. Ask students: Think about your experiences from the Lab.  Why do you think Jack found it easier to float in the ocean than in the freshwater of Lake Willow Tree? Student answers may vary. Students should indicate that ocean water and freshwater are two different types of liquids and therefore, may have different densities.

b. Ask students: Do you think the density of ocean water is greater or less than the density of freshwater? Student answers may vary.

c. Tell students that they will be able to answer these questions after completing the Investigation Five Lab.

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C. Tell students that in the previous Investigations they studied the densities of solids and liquids.  Explain to students that in Investigation Five, they will study the densities of solutions.  A solution is a mixture of solids and liquids.  Begin a discussion to review the terms solutions, solutes, and solvents.  Use the example of a solution that consists of a solid solute and a liquid solvent.

Ask students: What is a solution? Student answers may vary.  After students have attempted to define the term solution, encourage them to refer to their Scientist’s Glossary to find the definition. 

1. Use the following questions to continue your discussion:

a. Ask students: Have you ever added a powdered drink mix to water to make a flavored soft drink? Student answers may vary.

b. Ask students: Have you ever added salt to water to make saltwater and used it as a gargle to make your sore throat feel better? Student answers may vary.

c. Ask students: What happened when you mixed the powdered drink mix or the salt with the water? Student answers may vary.  Students should indicate that both the powdered drink mix and the salt dissolved in the water.

d. Ask students: When the powdered drink mix or the salt is completely mixed with the water, does any of the solid remain in the liquid? Student answers may vary.  Students should indicate that neither of the solids is visible in the water after completely mixing.  

Encourage students to realize that all solutions are uniform mixtures of a solid and a liquid since the solid is completely dissolved.  Solutions may be colored, like the soft drink or uncolored like the saltwater.  However, all solutions are transparent since there is no visible solid.

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2. Explain to students that they will now discuss the two components of solutions:  solutes and solvents.  Use the examples of a powdered drink mix and saltwater in this discussion.

a. Ask students: Which solids were dissolved to make each of the two solutions? Students should indicate that the solid dissolved to make the flavored soft drink is the powdered drink mix and the solid dissolved to make the saltwater solution is the salt.

b. Explain to students that the solids in these two solutions are solutes.  A solute is the component of a solution that is present in a lesser amount.  Encourage students to refer to their Scientist’s Glossary to find the definition of solute.

“Solute: The substance that changes its state when a solution is created or that is in the smallest amount in a solution.”

c. Ask students:  What liquid was each solute (powdered drink mix and salt) dissolved in? Students should indicate that each solute was dissolved into water.

d. Explain to students that the liquid in these two solutions, water, is the solvent.  A solvent is the component of a solution that is present in the greater amount. Encourage students to refer to their Scientist’s Glossary to find the definition of solvent.

“Solvent: The substance in a solution that does not change its state when a solution is created or that is present in the largest amount.”

e. Ask students:  When you make a flavored soft drink, is the solid or the liquid in the greater amount?  The lesser amount? Student answers may vary.  Students may or may not indicate that the solid is in the lesser amount and the liquid is in the greater amount.

f. Ask students:  When you make saltwater, is the solid or the liquid in the greater amount?  The lesser amount? Student answers may vary.  Students may of may not indicate that the solid is in the lesser amount and the liquid in the greater amount.

g. IMPORTANT NOTE: Explain to students that in a solution consisting of a solid and a liquid, the solute is always the solid and the liquid is always the solvent.

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D. Engage students in the following role-play (or simply follow this and the following two slides) to show how dissolving lower or higher amounts of a solute in a solution will change the solution.  Explain to students that the amount of a solid that is dissolved in a specific volume of liquid is called concentration.  Increasing or decreasing the amount of dissolved solute in a solution will change the concentration of the solute.

1. Select ten students and ask them to write “solvent (water)” on a piece of paper.  Tell the students that they will represent the liquid particles in a solution.  Ask them to stand two arm lengths apart in one area of the classroom.

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2. Select six students and ask them to write “solute (salt)” on a piece of paper.  Tell the students that they will represent the solid particles in a solution.  Ask three of these students to stand within the group of students representing solvent particles.

3. Explain to students that the concentration of this solution would be described as three solute students within the ten solvent students.

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4. Ask the remaining three “solute” students to also join the group of students representing solvent particles.

5. Ask students:  How would you describe the concentration of this solution? Student answers may vary.  Students may indicate the concentration is six solute students within the ten solvent students.  

Encourage students to realize that the concentration of the solution has been increased.

6. Tell students that the concentration of a solution is usually written as grams of a solute that is dissolved in a certain volume (milliliters) of a solvent.  As the mass of the solute decreases, the concentration of solute decreases.  As the mass of the solute increases, the concentration of solute increases

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E. Ask students to think about what a Particle Diagram for a solution would look like.  Use the following questions to facilitate a discussion.

1. Ask students: How would you describe the particle diagram for a gas? Students should indicate that the particles in a gas are spread out with a lot of space between individual particles.

2. Ask students: How would you describe the particle diagram for a liquid? Students should indicate that the particles in a liquid are closer together than in a gas.

3. Ask students: How would you describe the particle diagram for a solid? Students should indicate that the particles in a solid are very close together.  They have almost no space between them.

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4. Ask students: You can make a solution by dissolving a solid into a liquid.  What do you think the particle diagram would look like for this type of solution? Student answers may vary.  

Encourage students to realize that the solution would have a mixture of liquid and solid particles as shown on the slide.

F. Explain to students that during Lab, they will investigate the densities of 0%, 20% and 45% salt solutions in order to answer the following questions:

• What happens to the density of water when a solute is added?

• How can you determine the density of a solution?

• How does the density of a solution compare to the density of a solid object?