What it shows
A fresh egg sinks in plain water but floats once enough salt is dissolved, because dissolving salt makes the water denser than the egg. Counting the spoons needed turns a trick into a measurement of density.
The science
An object floats when the liquid it is in is denser than the object. An egg is slightly denser than plain water, so it sinks. Each spoon of salt dissolves and adds mass to the water without adding much volume, so the water gets denser spoon by spoon. When the water becomes denser than the egg, the upward push of the water is enough to hold the egg up. This is why people float so easily in the very salty Dead Sea.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- One fresh egg
- A tall transparent glass or jar
- 2 cups of water
- Table salt, about 12 teaspoons
- A teaspoon and a stirrer
- Sugar, about 30 teaspoons, for a second test (optional)
- Notebook
Safety
For this project: Handle the egg gently and wash hands after touching it. Do not drink the salty water.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
Step by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fill the glass two thirds with water and lower the egg in with a spoon; it sinks to the bottom.
Take the egg out, add one teaspoon of salt, stir until it dissolves, and put the egg back.
Note whether the egg sinks, hovers or floats, and record it against the spoon count.
Keep adding one spoon at a time, stirring fully each time, until the egg just floats with its top at the surface.
Write down the number of spoons; this is the point where the water became denser than the egg.
Add two more spoons and watch the egg rise higher out of the water.
Now slowly pour plain water down the side of the glass over the salty water; the egg hangs in the middle where the two layers meet.
Repeat the whole count with sugar in fresh water and compare how many spoons were needed.
What you should see
The egg floats after roughly 8 to 12 teaspoons of salt in two cups of water (the exact number depends on the egg), hangs at the boundary between salt water and fresh water, and sugar needs far more spoons than salt to float it.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01Why does the egg sink in plain water?
The egg is denser than plain water.
02What does adding salt change?
It makes the water denser, until the water is denser than the egg.
03Why does the egg hang in the middle when fresh water is poured on top?
It floats on the dense salt water below but sinks through the lighter fresh water above.
04Where in the world do people float without trying?
In the Dead Sea, which is extremely salty.
Take it further
Test an old egg and a fresh egg in plain water; an old egg floats because air has collected inside it.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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