Teacher Demonstration 19
The Mysterious Egg
Materials:
• Large drinking glass, jar or beaker
• Water in bottle or jug
• Spoon
*
• Egg
• Cooking salt (not table salt)*
• Food colour (optional)
Table salts contain a non-water soluble anti-caking agent resulting in a cloudy solution – check
packaging ingredients list if unsure
Instructions
This classic science trick demonstrates the effect of fluid density on the buoyancy of an
object. An egg sinks in fresh water but floats in salty water. You can demonstrate the
whole process or start by showing the final result and give students the opportunity
to figure out what is going on. You can also use this demonstration to discuss the
fascinating properties of the Middle East’s hyper-saline Dead Sea.
1 Half fill a large glass, jar or beaker with water. Add an
egg and you’ll notice that it sinks
2 Add a tablespoon of salt and stir gently. If the egg
doesn’t float after all the salt has dissolved, add
another teaspoon of salt.
Note: if the solution remains cloudy, the salt you are
using contains anti-caking agent
3. The egg floats because the density of salty water is
greater than the density of fresh water.
Teacher Demo 19 – Mysterious Egg | Page 1
© 2006 Ruben Meerman | ABC Science Online
4. Gently pour fresh water over the back of a spoon and
onto the salty water. This prevents the fresh and salty
water mixing.
5. The result is an egg floating curiously in the middle of
a glass of water.
6. To add to the mystery, push the egg to the bottom
and then quickly remove the spoon. The egg will bob
above and below its final resting place in the centre
of the glass several times.
7. When you have discussed the demonstration and
students are aware of the salt, add a few drops of
food colouring and gently stir the top layer of water.
The food colouring will not descend and mix into the
saltier water below demonstrating that fresh water
floats on salty water.
Safety notes
There are no specific hazards associated with this demonstration however, teachers
should always conduct a risk assessment of any science activity based on the
individual circumstances of their students and classroom environment.
Teacher Demo 19 – Mysterious Egg | Page 2
© 2006 Ruben Meerman | ABC Science Online
Teacher notes
Why things float
Substances
A substance (eg oil, chocolate, styrofoam, wood) will float if it is less dense than or
sink if it is more dense than the surrounding fluid.
Objects
An object (eg egg, boat) will float if it weighs less than or sink if it weighs more than
the volume of fluid it displaces.
Fluid density and buoyancy
Density is a measure of the weight of a substance for a given volume. One litre of
water weighs exactly one kilogram. One litre of cooking oil weighs about 900 grams,
one litre of seawater about 1.03 kilograms, and one litre of mercury weighs 13.6
kilograms. As a result, objects are a little less buoyant in oil, slightly more buoyant in
seawater and much more buoyant in mercury.
The Dead Sea
Normal seawater contains about 35 grams of salts per litre on average. The Dead Sea,
which lies between Jordan and Israel however, is a landlocked lake which is much
saltier than the open oceans. The surface water contains about 300 grams of salt per
litre – almost ten times more than the oceans. This allows bathers to float on their
back and even read the newspaper! But you can enjoy the same experience in the
flotation tanks offered at many relaxation centres. They’re filled with hyper-saline
water so you float like you would on the Dead Sea.
The Dead Sea is currently at risk of drying out because the water that used to flow
into it has been diverted for domestic, industrial and agriculture use. Some
environmentalists warn that it will disappear by 2050 if its level continues to drop at
the current rate.
Students will find this a fascinating topic to investigate further. Most encyclopedias
provide a detailed account of the Dead Sea’s chemical composition, geography,
history and religious significance and there are numerous websites with similar
photographic and video content on the world wide web. Some suggested links are
provided below:
Wikipedia article
http://en.wikipedia.org/wiki/Dead_Sea
Dead Sea ‘to disappear by 2050’ - BBC News Article
http://news.bbc.co.uk/2/hi/middle_east/1472842.stm
Teacher Demo 19 – Mysterious Egg | Page 3
© 2006 Ruben Meerman | ABC Science Online