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Salt Water Density (Advanced)
Previous knowledge:
 
As the amount of dissolved salt increases, a salt water solution becomes denser.
 
In this activity, you will prepare a set of 5 salt water solutions.  In order to distinguish them easily, each solution will have a different color. After preparing the solutions, you will experiment with them to study how solutions of different densities can maintain separate layers.   Using this knowledge, you will determine the % of salt in a sample of sea water. 
 
Tools

Prepare a set of 5 salt water solutions of different densities, each tinted a different color, using
 
    • 5 plastic cups
    • permanent marking pen
    • salt
    • water
    • electronic balance
    • food coloring
 
Investigate how the solutions of different density can maintain separate layers using
 
    • five test tubes
    • Test tube rack
    • Hollow glass rod , 15 cm in length
    • Beaker to act as a container for waste
    • the prepared set of salt water solutions
 

In the final activity, you will need to determine the % of salt in ocean water.  You will need
 
    • a test tube
    • ocean water
 

Solution Preparation
 
    % by mass = (mass of salt / total mass of solution) x 100

Example: Prepare 100.0 grams of 1.0% salt water.
 
1.0% by mass = (X grams of salt / 100.0 grams of solution) x 100

X = 1.0 gram of salt
 
To make this solution, you will need to add 1.0 g of salt to just 99.0 g of water. Remember that the total mass of the solution is 100.0 g.

You will need to prepare 5 different solutions. You might try making up 0%, 1.0 %, 2.0%, 3.0%,  and 4% salt water. Use food coloring to tint each solution a different color.

Once the solutions have been made, “stack” two different colored solutions in the glass rod. If it doesn’t work and the solutions mix, try reversing the order. Investigate until you have a solid understanding of how the % of salt is related to the density of the solution. Make sure you are able to stack all 5 layers in the glass rod.
 

Part II
 
A sample of water was obtained from the Atlantic Ocean. Determine the density of the ocean water. Justify your position using experimental evidence. You should be able to include ocean water in the stack you assemble in the glass rod. Show your teacher when you have accomplished this.

Teacher initials ______________________
 
 
 
Conclusions
 
Q1. If you have ever ordered a soft drink at a fast food restaurant, you may have played with the idea of picking liquid up with the straw. By placing your finger over the end of the straw, you can lift some of the drink up in the straw. As long as your finger remains on the straw, the liquid doesn’t fall out. Why is this?

Q2. In your experiment, what happens when a dense liquid is placed above a less dense liquid? Explain.
 
Q2. What happened when a less dense liquid is stacked above a liquid with greater density? 
 
Q4. With care, you can “stack” all five solutions in the glass rod.  Assuming you could maintain the seal, would the 5 layers stay separated forever or would they eventually mix to create a uniform distribution? When explaining this, be sure to recall that molecules are in constant motion. 

Q5. Draw a diagram showing the sequence of the five solutions, from least to most dense, stacked in the glass rod. Label the % salt in each layer.
 
Q6. Make a graph of mass of salt (x-axis) versus % salt (y-axis).  Draw the line of best fit. On the graph, indicate the region of the line corresponding to the ocean water you analyzed.

Bonus:

People find that swimming in the Dead Sea is difficult because they tend to “bob” at the surface like corks. Do an internet search. What is the % of salt in the Dead Sea? 
 
Why are people more buoyant in the Dead Sea than in the Atlantic Ocean?



E Saylor

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