National Weather Service United States Department of Commerce

Learning Lesson: Salt 'n Lighter

Overview

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. Fresh eggs will float saltwater, but will sink in freshwater. This will show that as the salinity increases the density also increases.

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®

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, if any, 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 water's salt content increases, it becomes denser. The eggs float in the two beakers with the added salt.

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. The egg in the beaker with the most salt should float higher than the egg in the less salty solution.

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

Ships float for the same reason. Their physical weight is less than the weight of the water that is displaced. Since the weight of the water is greater, ships float.

Sizes of some large ships and the weight of the that is displaced
Ship Year Built 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
Madrid Maersk 2017 Container ship Maersk Line 1,309 192 214,000
Harmony of the Seas 2016 Cruise ship Royal Caribbean 1,188 215 227,000
Mont* 1981 Supertanker Amber Development Corp. 1,504 226 647,000
*a.k.a. Knock Nevis, Jahre Viking, Happy Giant, Seawise Giant

Building a Weather-Ready Nation

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!