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Do Objects Float Better in Salt Water Than in Fresh Water?

Will a bottlecap really float in salt water?

Students will learn about density and surface tension with this printable science activity. They compare how well objects float in fresh water compared to salt water.

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Physics (644)

Density (9)

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If you've ever floated in the ocean, you may have noticed that it's much easier to do so than it is to keep afloat in a swimming pool.

If you had to guess why that's the case, what would you say? The answer is just one word-salt.

When salt is dissolved in water, as it is in ocean water, that dissolved salt adds to the mass of the water and makes the water denser than it would be without salt.

Because objects float better on a dense surface, they float better on salt water than fresh water. The denser the salt water, the easier it is for objects to float on top of it.

You could make a science fair project out of this concept by measuring different amounts of salt into a specific amount of water and testing how well different objects float.

Density is defined as mass per unit volume. If two objects are the same size, say a paper plate and a ceramic plate, we say that the ceramic plate is denser than the paper plate. The ceramic plate has the same volume, but much more mass.

A suggested method is to use five containers that are all the same size and shape. Put the same amount of water into each container. Use the first container as your control, and do not add any salt to it. Add 1 teaspoon of salt to the second container, two teaspoons to the third container, and so on.

Locate some objects that barely float in water, such as a paper clip, a small plastic ball, and a pen. Place the objects, one at a time, in the first container and observe how long they float in the water. Dry off each object and place it into the other containers in -the same manner, observing carefully how long they remain afloat in the water.

Run three trials for each object in each container, recording all your information carefully and then graphing it.

There are hundreds of science fair projects dealing with physical science topics. Use your imagination to try to think of others you may enjoy doing.

Excerpted from

The Complete Idiot's Guide to Science Fair Projects
The Complete Idiot's Guide to Science Fair Projects
Nancy O'Leary and Susan Shelly
This book contains great information for different kinds of science fair projects.


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