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Friday, August 20, 2010

All About Density: Layered Liquids and Sinking Ice

Ice floats because it is less dense than water: if you weigh a cube of ice, and weigh exactly the same volume of water, the ice will weigh a little bit less. But you can make ice sink by putting it in a liquid that is less dense than water. Here are some fun experiments with density.

Objects from top to bottom:
Styrofoam, wood, pecan, 
cherry tomato, blueberry, 
chocolate chip, steel bolt, 
hard candy
Ages 5 and up; Download Printable PDF

Experiment 1:  Layered Liquids

You will need:
clear plastic cups
newspapers (to contain spills)
corn syrup
vegetable oil
food coloring
objects to test (cherry tomatos, grapes, chocolate, hard candy, small metal objects, beads, bits of Styrofoam...)

Pour corn syrup into a plastic cup until the cup is about a quarter full. In another cup, mix water and food coloring (don't make the liquid too dark- you want to be able to see through it).  Slowly pour the colored water over the corn syrup until the cup is about half full. Then slowly pour vegetable oil over the water layer until the cup is three quarters full. The liquids should remain in three separate layers. The corn syrup and water will eventually mix with the water, but if you pour carefully, this will take time.

Carefully drop objects such as bits of styrofoam, wood, nuts or bolts, cherry tomatos, and grapes into the layered liquid. List the objects and liquids in order of increasing density (from top to bottom).

What’s Happening?
Think about how the compositions of the solids compare to the composition of the liquids in the glass.  Hard candy is mostly sugar, and has a density near that of corn syrup, which is also mostly sugar. So hard candy sits in the corn syrup layer. Nuts a high fat content, and thus stay in the oil layer. Chocolate chips contains both sugar and oil, and have a density intermediate to that of water and corn syrup. Blueberries and tomatoes are mostly water, and have densities similar to that of water. Wood and styrofoam are filled with pockets of air, and float in oil. The steel bolt is quite dense, and sinks in all the liquids.

Experiment 2:  Sinking Ice

You Will Need:
vegetable oil
ice cubes

Fill one cup with water, and another cup with oil. Drop an ice cube into each cup.  Does it float or sink?

What's happening?

The simple explanation: Ice is, of course, frozen water. But in ice the molecules form a crystal lattice that spaces them further apart than in liquid water, so ice is less dense than water. Ice is denser than oil, however, and sinks in this liquid.

The hard-core explantation: Scientists define density as mass per unit volume (density = mass/volume). Volume is an easy concept to understand: it is the amount of space something occupies. Kids, for example, are typically smaller than grownups and occupy less space. So, you could say that the volume of a kid is less than the volume of a grownup.

Mass is a bit trickier.  We measure mass using a scale, but what are we really measuring?  A chemist might say that we are measuring the amount of matter in a sample: the more matter a sample has, the higher the mass. Imagine you are holding a brick and a Styrofoam block of exactly the same size and shape.  The volume of each is the same.  But the brick has a higher mass than the Styrofoam block because it contains more matter; it is therefore has a higher mass.

A physicist defines mass a little differently.  According to a simplified physical definition, mass is a measure of the acceleration a body will experience when a given force is applied.  This sounds pretty complicated, but in our everyday experience, the only important force is the force of gravity.  A brick feels heavier to us than a Styrofoam block because the Earth is pulling on it with more force. In space, there would be no difference in how heavy each one felt. When we use a scale, we are actually measuring the pull of the Earth on an object; in the absence of gravity, a scale would not work.  So, next time you pick up a brick, think about how hard the Earth is pulling it away from you.

Did you Know?

The Great Salt Lake is very salty, and people who swim in it float higher than they do in fresh water.  Do you think salt water is more or less dense than fresh water?


  1. Okay that is way too cool beyond words.

    1. okoch cheje lski moki oki chaki ka ka le ki nochi ocheeka