Making an egg float
Drop an egg into a glass of water and stir in salt, a spoonful at a time, until the egg floats. Then try an old egg, which floats in plain water, and find out why cooks use this to test eggs.
Core concept
Dissolving salt makes water denser; an object floats when the liquid is denser than the object, so an egg floats once the salty water is denser than the egg
Topic: Chemistry in the kitchen
Whether something floats or sinks depends on its density: How much mass is packed into a space. Things less dense than water float. All key words: how much mass: How much matter something has, measured in grams or kilograms. It is the same everywhere. All key words is packed into each cubic centimetre. Pure water has a density of 1 g/cm³. A fresh egg is a little denser, about 1.03 g/cm³, so it sinks. When salt dissolve: To mix into a liquid so completely that you can no longer see the pieces. All key words, it adds mass to the water without adding much volume: How much space something takes up, measured in millilitres or cubic centimetres. All key words, so the water becomes denser. Once the salty water is denser than the egg, the egg floats. This is why swimmers float more easily in the sea, and even more in the very salty Dead Sea. An egg also changes as it ages: water slowly escapes through its shell and an air pocket grows inside, so an old egg is less dense and can float even in plain water.
What you will learn
- Explain floating and sinking using density.
- Show that dissolving salt makes water denser.
- Use density to tell a fresh egg from an old one.
Before you start
- Know that density = mass ÷ volume.
Materials
- A tall glass with 500 mL of water
- Salt and a spoon
- A fresh egg and an old egg
Equipment
- A kitchen scale (optional)
Procedure
- Record a fresh egg in plain water (no salt).
- Add salt, 10 g at a time, until the fresh egg floats, and record.
- Try an old egg in plain water and record.
How this simulation is simplified
- Each gram of salt per litre of water raises its density by about 0.0007 g/cm³ (a good guide for weak brines).
- Egg densities: fresh about 1.03 g/cm³, a week old about 1.01 g/cm³, old about 0.99 g/cm³.