Monday 30 April 2012

How to make a Fresh Egg Float

How to make a Fresh Egg Float

Most of us know that a fresh egg will sink and a rotten egg floats but is it really possible to make a fresh egg float?  Yes it is!

We can experiment with this and all you will need is:

  • 2 fresh eggs
  • 2 clear large plastic cups
  • Table salt
  • Water
  • Pencils/pens (optional)
  • Paper (optional)
  • Science Journal (optional)
Fill both of your plastic cups with tap water.

Add salt to one of the cups and with pen and paper label the cups so you know which is which. Stir the salt in well and wait for it to completely dissolve, you will know when this has happened because the water will turn from cloudy to clear.


If you want, you can double check that you have a fresh egg by placing it in some fresh water (out of the tap will do) before putting it into your salt water.

Write in your Science Journal at this stage.  Ask the children if they would like to write hypotheses of what you think will happen to the egg in the fresh water and what will happen to the egg in the salt water.  If the child/ren don't want to write that's fine, just talk about it. STEP FOUR
Plop an egg into the water without salt.  What happened?  If you are using a science journal you can write down your observations or again just talk about it.  Keeping it fun is the most important thing after all!


Now plop (I just like the onomatopoeic sound) in the salt water now.  What has happened this time?  It should float if there is enough salt in your water.  Write down this observation.

So what has happened?  How does it work?
The reason why the egg in salt water floats is because salt water is denser that fresh water.   
In order for something to float, the buoyant force has to be greater than or equal to the weight of the object. The buoyant force is equal to the weight of the liquid that is displaced by the object.
To figure out the weight of the water displaced you multiply the density of the water by the volume and then multiply that by gravity.
The density of the fresh water is less than the density of the salt water, therefore the weight of the water displaced will be greater in the case of the salt water,hence resulting in a greater buoyant force.

The more salt in water the more buoyant an object becomes as it is the salt that makes the water denser.  You might want to google about the dead sea, or the sea of salt as it is also known, it has the most salt content in any lake or sea.

The density of the salt water is greater than the density of the egg.  objects, like an egg, sink if their own density is greater than the density of whatever they are trying to float in. Items float to the top if their density is less than the density of what they are floating in, and items hang in the middle if the densities are the same.
Other things you can try... 
You can try adding salt bit by bit.  How much salt to water ratio do you need for the egg to float?
In your science journal as the child to draw  what they see if they don't feel up to writing.
Other posts you might like if you enjoyed this and want to learn more...

No comments:

Post a Comment