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What You Need:
- 4 Identical Glasses or Cups
- 4 Colors of Food Coloring
- 2 Straws
Fill each glass with water. Put several drops of food coloring in each glass – one color in each. You will have one cup that is red, one that is blue, one that is green and one that is yellow. Now take the straw and put one end of it in the red glass so that one inch of the straw is covered with water. Put your thumb over the other end of the straw and pull the straw out of the water. See how the red water is still in the straw?
Now, with your thumb still covering one end of the straw, submerge the red end of the straw into the blue cup until two inches of the straw is covered with blue water. Slowly lift your thumb and then put it back on the end of the straw again. Draw the straw out. What happens to the blue and red water in the straw?
Now add 1 teaspoon of salt to the red glass, 2 teaspoons of salt to the blue glass, three teaspoons to the green glass and 4 teaspoons to the yellow glass. Repeat the experiment until you have four inches of water in the straw. What happens to the four colors? Did fresh water behave differently than salt water?
When you added salt to the water you increased the density of the water. Density is how close together the molecules of a substance are or how much mass a substance has in a given space. If you have one cup of jelly beans and one cup of marshmallows…the jelly beans have more mass…there is more “stuff” compacted into the cup. The marshmallows are mostly air. If you put each of those cups in a microwave to melt…the sugar and water that makes up the jelly beans would almost fill the cup to the top. The sugar and water that makes up the marshmallows would only fill the cup a little bit because marshmallows have less mass, they are mostly made of air. A cup of salt water has more density than a cup of plain water. The more salt you add to a cup of water, the more density it has.
Each of the four colors of salt water have different amounts of salt. The colors with more density are heavier and will sink to the bottom when all four colors are mixed together.
- Steve Spangler Science: Density Experiments 
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- ZOOM Activity:Water Density 
More IndyPL Experiments about Density:
Words to Know:
Density – How closely packed together the molecules of a substance are.
Mass – How much matter fits in a given space.
Matter – Stuff
Weight – A measure of the force of gravity on an object. Materials with more density weigh more.
Volume – How much space a substance takes up.
*****The confusing relationship between weight and mass: On earth, a bowling ball can weigh about 10 pounds. If you take that same bowling ball to the moon it will weigh much less because the graviational pull of the moon is less than the gravitational pull on the earth. Weight is a measure of gravitational pull. So the weight of the bowling ball, or anything else, changes depending on where you weigh it. The mass of the bowling bowl does not change no matter where it is. The bowling ball has the same amount of mass on earth as it does on the moon or anywhere else you take it.