Learning Lesson: Salt 'n Lighter


OBJECTIVE Show that as the salinity increases the density also increases.
OVERVIEW Fresh eggs, which are less dense than saltwater, will float, but will sink in freshwater.
TOTAL TIME 3 minutes as a demonstration, 10 minutes if students perform the demonstration.
SUPPLIES 3 fresh eggs
3 - 1.5 liter beakers (or quart jars)
6 ounces of salt.
PRINTED/AV MATERIAL None
TEACHER PREPARATION None
SAFETY FOCUS Turn Around, Don't Drown®


Background

Just as air can have different densities, water can have different densities as well. As the salinity of water increases, the density increases as well.

Procedure
  1. Fill each beaker with one liter of tap water (or each quart jar with one pint of water).

  2. Add 35 grams of salt to one beaker and 290 grams of salt to a second beaker (½ ounce of salt to one quart jar and 4½ ounces of salt to a second jar).

  3. Ask the students to speculate in which water solution will the eggs float.

  4. Place an egg in each solution and observed which egg floats.
Discussion

Fresh eggs are more dense than fresh water and therefore will sink. However, as the salt content increases in water, it becomes more dense. The egg will float in the two beakers with the added salt. This happens because the added salt makes the water heavier than the egg causing the egg to float.

The solution with the 35 grams of salt represents the salinity of the oceans. The solution with 290 grams of salt added represents the salinity of the Dead Sea. As the salinity increases, the density increases as well. The egg in the beaker with the most salt should float higher that in the other salty solution.

The increased density of the salty water actually increased the weight of the water. An egg will be buoyant (float) if the weight the egg is less than the weight of the water it displaced. The egg sinks if it weights more than the weight of the water that was displaced.

Ships float for the same reason. Their actual weight is less than the weight of the water that is displaced. Since the weight of the water weighs more, the ships floats. The following are sizes of large ships.

Ship Year Type Owner Length
(Feet)
Width
(Feet)
Weight
(Tons)
Titanic 1912 Liner White Star Line 883 92 46,328
Queen Mary 1934 Liner Cunard 1,019 119 81,237
Bismark 1939 Battleship Germany 880 120 50,000
Missouri 1944 Battleship United States 887 108 58,000
Enterprise 1962 Aircraft Carrier United States 1,101 133 89,600
Ronald Reagan 2003 Aircraft Carrier United States 1,092 134 101,000
Queen Mary 2 2004 Liner Cunard 1,132 135 149,000
Freedom of the Seas 2006 Liner Royal Caribbean 1,112 127 160,000
Oasis of the Seas 2009 Liner Royal Caribbean 1,184 184 220,000
Knock Nevis 1981 Supertanker First Olsen Ltd 1,504 226 647,000

Live Weatherwise

Turn Around, Don't Drown
During periods of very heavy rainfall and flash flooding is occurring, many people risk their lives by driving through flooded roads. People erroneously think their "heavy" vehicle will keep them on the road.

Look once again at the size and weight of the ships in the table above. If these vessels float what would make a person think their puny 2 ton vehicle will not float?

When is comes to flooding, just "Turn Around, Don't Drown". Either find an alternative route to your destination or wait until the water subsides. It is not worth the risk to attempt a crossing of a flooded road!

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