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Under pressure


NOTE TO PARENTS: One of the eggs is just to break and look at uncooked so can be used afterwards. The other is for use in the later experiment but if the bottle is clean should remain edible!
You will need:
  • gloves (marigolds?) to protect against iron wool and hot water!
  • 2 medium-sized hen's eggs
  • small pan for boiling one of the eggs (glass sided pan would be brilliant but not essential)
  • access to cooker/hob/burner!
  • access to sink/bowl and (v)hot water-probably the kitchen sink is best
  • bottle with tube ( as shown on the video)
  • wire ( iron/steel) wool
  • stop clock/phone
  • pencil and paper/pad
  • glass bottle with 3cm diameter mouth
  • a half used tealight
  • a teaspoon
  • cocktail stick or similar (e.g a used matchstick)
  • 5 drops washing up liquid

Feeling the pressure

The egg gets drawn into the bottle because of the pressure of the air. The air inside the hot bottle cools and contracts. As a result the air pressure inside the bottle falls. The bottle is sealed by the boiled egg placed in its mouth. The air (atmospheric) pressure outside the bottle is higher than the pressure inside and so the egg experiences a force and is slowly pushed into the bottle by the atmosphere.

 What we did..

  • Put water in a small saucepan so that it is about 2/3 full.
  • Float a half used tealight on the water
  • Heat the pan on the cooker until the water boils.
  • Observe the water and the tealight carefully.
  • Keep the water boiling ( simmering ) and place an egg in the water for six minutes.
  • use a small spoon to add a litle of the egg white to the hot water
  • Carefully remove the tealight and pour the molten wax into a tray of cold water.
  • after 6 minutes remove the egg. Cool it under the cold tap and peel it carefully
  • Heat your glass bottle by submerging in very hot water (CARE) - wear gloves 
  • After 3 or 4 minutes remove the (HOT) bottle from the water and empty it. 
  • Immediately place the egg in the top of the bottle.
  • Cool the bottle with the cold water tap if required. OBSERVE , DESCRIBE, EXPLAIN

 What we found out..

  • Small bubbles appear on the inside surface of the pan soon after the heat is applied.
  • After a few more minutes larger and more frequent bubbles appear 
  • As the water heats up the wax in the tealight becomes liquid and transparent
  • When the liquid wax is poured into cold water it solidified instantly 
  • When the egg was placed in the mouth of the bottle it started getting forced down and eventually got broken and fell into the bottle.

 The tekkie bit..

Forever blowing bubbles 

When water is heated ( even if just left out in a glass for a few hours) the small bubbles which appear are bubbles of dissolved oxygen. Gases become less soluble when the temperature  increases. This means that boiled water contains much less dissolved oxygen and it is why fish tanks and aquaria need to be aerated to keep the fish healthy.

As water is heated further - bubbles of water vapour begin to form and rise to the surface. Boiling happens when the pressure of the vapour in the liquid is equal to the atmospheric pressure ( at sea level this happens at 100'C. Water boils at a lower temperature at high altitude ( where the atmospheric pressure is lower). Water in a pressure cooker achieves a much higher temperature than ordinary boiling water because the pressure inside is increased but having a gas tight lid.

Reversible or permanent ?

Egg white undergoes an irreversible/ permanent change when heated by placing in hot/boiling water. This is because the molecules in egg protein are very complicated and their shape is distorted by the heat. The egg white is said to have been denatured. This is a permanent ( chemical) change.

When wax is heated it melts. When molten wax is poured into cold water - it solidifies instantly. This is an example of a reversible ( physical) change.


Comments 1

Guest - Mother Miller on Friday, 15 May 2020 08:41

Shush! Part of an egg got sucked into Molly's special water bottle....but we kept that information to ourselves because otherwise there may have been severe consequences within the household...

Shush! Part of an egg got sucked into Molly's special water bottle....but we kept that information to ourselves because otherwise there may have been severe consequences within the household...
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Friday, 26 February 2021

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