Density – Buoyancy

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Every object on earth is made of atoms and gravity pulls these atoms to the earth. You can measure the pull of gravity on an object – we call that measurement weight.

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. Materials with more density weigh more. A cup of jelly beans weighs more than a cup of marshmallows.

For an object to be bouyant, or float, it must have less density that what it is floating in, or it has to have something attached to it that helps it float – like you with a life jacket on.

You Will Need:

• Drinking Glass
• Clear Soda
• Water
• Ten Raisins

Fill one clear glass up with water and drop in five raisins. Fill another clear glass up with clear soda like sprite or 7up. Drop in five raisins. What happens when you drop the raisins in? What a few minutes – now what is happening to the raisins in each glass? Can you guess why the raisins are behaving differently?

Raisins are heavier than the water in the drinking glass. The raisins are also heavier than the soda in the drinking glass. At first, both sets of raisins sink to the bottom of the glass, they don’t float.

But the soda has little air bubbles in it – the carbination. These bubbles are attracted to the rough surface of the raisens and stick there. When there are enough of these little carbonated balloons (the bubbles) stuck to the raisins the bubbles lift the raisins to the surface making the raisin float, like little temporary life jackets. When the bubbles pop and the gas inside them escapes into the air…the raisins don’t have anything to help them float anymore and they sink to the bottom of the glass again.

• PBS Kids: Fetch Float My Boat
• NOVA Online: Buoyancy Basics
• Highlights: What Makes People Float?
• Science Bob: The Magic Ketchup Experiment
• Kids Science Challenge: That Sinking Feeling
• Fetch Potion Commotion
• ZOOM Activity: Water Density
• Activity TV: Double Density
• Virtual Mass, Volume and Density Lab
• ZOOM Dancing Raisins
• ZOOM Dancing Raisins 2

Here are some books to help you learn about bouyancy:

• Google Preview: Science Experiments That Surprise and Delight: Dancing Raisins (pages 6-7)
• Google Preview: Science Experiments That Fizz and Bubble: Bubbling Blobs (Pages 16-17)

Science Experiment Idea: Try putting other small objects in soda to see if the bubbles will attach to them and help them float to the surface of the soda. Try a penny, a toothpick, a peanut, a skittle. Can you find something that the bubbles will float to the surface like the raisin?

Words to Know:
Density – How closely packed together the molecules of a substance are.
Bouyancy – Ability to float or rise to the top of a liquid or gas.
Float – To sit near the surface of a liquid or gas, to not sink.
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.

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