lasr disk demo

Relevant Resource questions :

Concepts demonstrated :

• Phase transformation
• Latent heat
• PV = nRT

Equipment needed :

• syringe and warm water OR
• Round flask
• retort stand and flask mount
• bunsen burner and matches
• "Venturi" vacuum method needs a tap with running water, thermometer

• Connect as shown
• Run tap to create a lower pressure in the flask
• Heat the flask and measure the temperature at which the water starts to boil

Physics involved :

Explain boiling - discuss vapour pressure.

How to use the Demonstration :

• Create a lower pressure and start to heat the water.
• Monitor temperature versus time until the water starts to boil.
• Discuss the lower boiling point because of the reduced pressure.
• It may be good to start with POE on the syringe of warm water (see Things to Do below).
• What do students expect to happen when pressure is reduced in the syringe ?
• Get them to write something down.
• Then show them the demonstration (hidden until now) and develop.

• Boil water vigorously in a round-bottomed 1l flask for about a minute to drive out the air.
• Remove the flame and quickly insert a tight rubber stopper.
• Invert the flask, supporting it in a clamp or ring. Submerge the stopper in a beaker of water to prevent the entry of air.
• The water in the flask may be made to boil repeatedly by pouring cold water over the outside or by applying ice to it.
• A thermometer may be used to show that the temperature of the boiling water is well below 100 deg C.

• The other extreme of this is the old pressure cooker used at home in which an increased pressure could increase the temperature at which the water boils.
• Heat pipe - water in sealed glass under reduced pressure.

• The simple demonstration with warm water in a syringe would be good as an accompanyment to this demonstration.
• Monitor temperature as a function of time on a computer using a data taker and explore this idea.

• This is a good demonstration but should be improved
• The "venturi" method of making a vacuum is a complication.