How can you tell a fresh egg from a rotten one?
It’s easy. Put them in a bowl of water.
The fresh egg will sink but the rotten one will float.
When an egg goes rotten, gas is made and trapped inside.
The gas has a lower density than water and this allows the rotten egg to float.
But you can make a fresh egg float by dissolving plenty of salt in the water. The salt makes the water more dense than the egg.
Now the egg will float, or become buoyant.
What is the water is only slightly salty?
The egg might float or sink, depending on how salty the water is.
The fresh egg is more dense than the water and so it sinks.
The rotten egg has lots of gas.
It is less dense than the fresh egg and so it floats.