Cool Nitrogen

AIM:

To introduce change of state due to change in temperature.

CURRICULUM LINKS:

KS2: Sc3 Materials and their Properties, Changing Materials. 2 (b) - (d)

TIME:

15 minutes.

MATERIALS REQUIRED:

• 8 - 10 litres of liquid nitrogen - depending on the weather / temperature
• 1 kettle full of freshly boiled water
• 1 plastic bottle
• 1 balloon
• 1 rose (other types of flower aren't as good)
• 1 rubber glove (preferably marigold)
• 2 rubber extra bouncy balls
• 1 egg
• Various soft fruit
• Hammer
• Washing up liquid
• Glycerol
• Tongs
• Heat proof / protective gloves
• 6 inch petri dish
• Cling film

METHODS:

• Prepare soap bubbles in advance by mixing 2 caps of washing up liquid and 5 caps of glycerol in a large tray (approx. 30 x 40 cm, 7 cm deep) with approximately 1.5 inches of warm water
• Begin the demonstration by introducing the concept of temperature using a card thermometer
• Pour the liquid nitrogen into an empty dewar to show the children it is a liquid
• Explain how it's normally a gas, but at room temperature it is a liquid
• Demonstrate the effect of heating and cooling gases (expansion and contraction) by putting a balloon on top of a bottle, putting the bottle in the liquid nitrogen to show contraction, and then into the boiling water showing the balloon expanding again
• Demonstrate the solid - liquid - gas transition (e.g. ice - water - steam) with an egg and a flower (water in flower freezes etc...) Note: Soft fruit can also be used.
• Demonstrate the effect of heating / cooling solids by putting a rubber glove and a rubber ball in the nitrogen and then removing them.
• Finish the demonstration by showing the children what happens when liquid nitrogen is added to soap bubbles. (Add at least 2 - 3 litres of nitrogen rapidly for best effect).

THEORY:

• Temperature is a measure of how hot or cold things are
• Changes in temperature (cooling/heating) can change the properties of different materials/objects
• Approximately 78% of the air we breathe is nitrogen gas
• When the nitrogen gas is compressed it becomes a clear liquid
• Liquid nitrogen boils at minus 196 degrees Celsius
• It looks similar to water in several ways, for example it is wet (unlike mercury). It also looks like boiling water when it is at room temperature and the vapour looks like steam on a cold day.
• Contraction of gases occurs when they are cooled as the molecules of gas which are constantly moving are slowed down, and so take fill a smaller volume
• When the gas is heated up again the molecules gain extra energy and so move around faster and so take up a greater volume
• All fruit/flowers etc contain water and so when they are frozen the water turns to ice and so these things can shatter if hit hard enough
• Other materials, e.g. rubber, exhibit a change in properties upon freezing. Rubbers become glass-like and can become brittle
• When liquid nitrogen is added to soapy water the liquid nitrogen boils furiously creating nitrogen gas which causes the bubbles to multiply. The liquid nitrogen also freezes the water and so frozen bubbles result.
• The small amount of glycerine prevents the water from freezing immediately allowing some more bubbles to form, i.e. it lowers the temperature at which the water will solidify.

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