Every object on earth is made of atoms. Gravity pulls these atoms to the earth. You can measure the pull of gravity on an object. We call that measurement weight.
A molecule is a group of atoms bonded together. Density is how close together the molecules of a substance are or how much mass a substance has in a given space.
For example, if you have one cup of jelly beans and one cup of marshmallows, the jelly beans have more mass because there is more "stuff" compacted into the cup. The marshmallows have less mass because the molecules of marshmallows or NOT close together. Marshmallows are mostly air.
If you put each of those cups in a microwave to melt the jelly beans and the marshmallows, the sugar and water molecules that make up the jelly beans would almost fill the cup to the top. The sugar and water molecules 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 buoyant, 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.
Try it at Home! You Will Need:
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 carbonation. 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. The bubbles are 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.
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, or a skittle. Can you find something that the bubbles will float to the surface like the raisin?
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An introduction to how boats and ships are made and why they can float.
Instructions for building several kinds of boats to build with your legos. Try a few and float them in the sink. Do some float better than others? Why?
A clever story about a goat and a chicken who learn about density by trying to cross a moat in a barrel. Which barrel floats the best? An empty one? A full one? Or a half-full one?