Lesson 1: The Hydrosphere


Activity 1: Making Density Solutions

Materials: blue food coloring (kit), graduated cylinder (kit), green food coloring (kit), ice cube tray (kit), measuring cups and spoons, pot, red food coloring (kit), salt, small clear cups (kit), yellow food coloring (kit)
In the reading for today you learned about density and salinity — important characteristics of water. In later lessons, you will learn about other concepts of water, such as cohesion and polarity. Understanding these concepts is important for being able to understand the relationships between living and non-living things and the water resources found on the planet.

In this lesson, you will conduct an investigation about the density (ratio of mass and volume) of different solutions. A solution is made when two or more substances are mixed together. In this activity, you will make solutions using salt and water.

Density — Making Solutions

Today you will create a 100% saltwater solution and then use that solution to make 25% and 50% saltwater solutions. You will create ice cubes of each solution for use in tomorrow's investigations.

You will need approximately 2 cups of water and 1/3 cup of salt. Follow these steps to create your 100% saltwater solution:
  1. Put approximately 2 cups of water into a pot on the stove. Turn the heat on to medium. You want the water hotter than hot tap water, but it should not be boiling; if the water begins to boil at any point, just turn the heat down.
  2. Stir one tablespoon of salt into the water until it dissolves completely.
  3. Continue stirring salt into the water, one tablespoon at a time. Eventually, the water will not be able to dissolve any more salt. When this happens, you will have a 100% saltwater solution.
  4. Turn the heat off and let the water cool for about 10 minutes.
  5. As the solution cools, you will begin to see undissolved salt crystals in the water. Pour off the solution into a transparent cup, being careful to not pour any of the solid crystals into the cup. As the solution continues to cool, you may notice more undissolved salt crystals; if this happens, you can again pour off the solution into another container, leaving the solid crystals behind.
  6. Label this container "100% Salt Water Solution" and set it aside on a kitchen counter or somewhere where it will not be knocked over. You will be using the solution from this container for Activities 1, 2, and 3, so do not throw this solution out until you have finished with all three activities.
Once you have made your 100% saltwater solution, you will use it to make a small amount of a 25% and a 50% salt solution. Before you begin, take four clear cups and label them "Tap Water," "25%," "50%," and "100%." For this part of the activity, you will be making two small ice cubes of each solution, so you won't need a large amount of each solution.
Salt Solutions for Ice Cubes
Salt PercentageInstructionsFood Coloring
0%Pour approximately 40 milliliters of tap water into the "Tap Water" containerAdd 2 drops of green food coloring and mix well.
25%Mix 10 milliliters of 100% solution with 30 milliliters of water, and put this solution in the "25%" container.Add 2 drops of blue food coloring and mix well.
50%Mix 20 milliliters of "100% Salt Water Solution" with 20 milliliters of water, and put this solution in the "50%" container.Add 2 drops of yellow food coloring and mix well.
100%Pour approximately 40 milliliters of "100% Salt Water Solution" into the "100%" container.Add 2 drops of red food coloring and mix well.

Density: Making Ice Cubes

You will now make ice cubes to use in one of tomorrow's experiments:
  1. Pour enough of each solution into an ice cube tray to make two ice cubes from each solution (two green ice cubes, two blue ice cubes, etc.) for a total of eight ice cubes.
  2. Put the ice cube tray in the freezer and leave it overnight. If some of the solutions do not freeze completely overnight, that is okay. Salt water found in the ocean freezes at 28 degrees Fahrenheit. As the concentration of saline increases, the freezing temperature will decrease. There is no need to give the saline solutions more time to freeze.
  3. You can pour out the remaining colored solutions, but keep your four labeled containers ("Tap Water," "25%," "50%," and "100%"); you will use them to mix more solutions for Activity 2.