Saltwater is denser than freshwater because the dissolved salt particles increase the mass of the water without adding to the volume. This allows objects to float better in saltwater than in freshwater. An object sinks when it cannot displace the same amount of water equal to its weight--which is why boats float. A science project can be designed to compare similar or identical objects in freshwater and saltwater. Objects from around the house can be used for the project.
Things You'll Need
- 2 large buckets
- Masking tape
- Measuring cup and spoon
- Aluminum foil
- Plastic bottles
- Iron nails
- Hammer (optional)
Fill two buckets with one quart (32 oz.) of fresh water and label the outside of one bucket "Freshwater" and the other "Saltwater."
Add one cup of salt to the saltwater bucket and stir.
Cut aluminum foil into four sheets of equal size, such as 8 inches by 8 inches.
Fold two sheets of foil in half twice to create two squares, such as 4 inches by 4 inches each, and label them "A1."
Crumple the other two sheets of aluminum foil into very compact balls (use a hammer if needed). Note these items in your project log as "A2."
Fill each plastic bottle with one cup of water and label the outside of each bottle "B."
Place an aluminum square (A1) in each bucket and note whether they sink or float. They should float.
Note in your project log that the location of A1 is at "0" (which is the surface of the water) in freshwater and saltwater and remove the items.
Place an aluminum ball (A2) in each bucket and observe whether each sinks or floats. One should sink in freshwater and the other should float in saltwater.
Measure the depth of each object in the water from the water line and mark this as a negative number in your project log. For example, "the location of A2 in Fresh is -4" because it sunk 4 inches and "the location of A2 in Saltwater is -1" because it didn't float on the surface, but was submerged 1 inch from the top.
Write your observations in your project log and remove the items from the buckets.
Carefully place a water bottle in each bucket and observe that the bottle floats, but at varying depths in each bucket.
Measure the location of the bottom of each water bottle from the surface of the water.
Note your observations in your project log and remove the items from the bucket.
Drop an iron nail (noted as object "C" in your project) in each bucket and observe whether it sinks or floats.
Measure the depth of the object from the water line and write your observations for object "C" in your project log.
Tips & Warnings
- Consider using pairs of objects for further comparison, such as an egg, pen or marble.
- Take pictures of each object in each bucket to document your scientific process.
- Consider adding additional buckets containing different amounts (concentrations) of salt to compare buoyancy. Label each bucket "Salt 2" (for 2 cups of salt), "Salt 1" (for 1 cup of salt), "Salt 0.5" (for 1/2 cup), and so on.
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