Expt:An alternative to using compressed gas cylinders

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Introduction

Cylinders of hydrogen, oxygen and carbon dioxide are very expensive. Getting gas under pressure allows exciting demonstrations such as igniting balloons filled with hydrogen gas.

Comparing the density of gases and investigating Avogadro’s hypothesis is possible if balloons are filled with different gases. Igniting balloons filled with pure hydrogen gas is a very popular demonstration. Generating about 0.1 mole of common gases and filling balloons can be done with very basic equipment.

Apparatus and chemicals

2 L plastic fizzy drink bottle (empty) with top
Glass trough or plastic washing-up bowl (dish pan)
2-hole bung to fit bottle fitted with glass delivery tubes
1-hole rubber bung fitted with glass delivery tubes
Plastic or rubber tubing with adapters
Spring clips to close the rubber tubes
Nichrome wire
Side-arm flask, 250 cm³
Delivery tube
Tap funnel with bung to fit side-arm flask.
Hydrochloric acid, 1 mol dm^-3, 250 cm³ (Irritant, Refer to CLEAPSS® Hazcard 47A)
Magnesium turnings, 4 g, (Highly flammable, Refer to CLEAPSS® Hazcard 59A)
Clamp stand
Boss head
Clamp
Cotton thread
1 metre wooden rule
Splint
Sticky tape
Balloons (see note 3)

Technical notes

The description provided is for pressurising hydrogen gas, but the same method can be used for other gases.
Hydrogen is extremely flammable and explosive if mixed with oxygen. All flames should be extinguished during the preparation process.
If the balloon is inflated with air first to stretch it, it will fill more easily with hydrogen.

Procedure

HEALTH & SAFETY: Wear eye protection and ear protection. Direct audience to cover their ears.

Part 1

a Prepare the 2-hole bung with delivery tubes and clips as in the diagram. Wiring up the rubber tubes with nichrome wire will prevent them being pushed off under pressure.

b Tape a splint to the 1 metre rule.

c Half fill the trough with water.

d Completely fill the bottle with water from the tap and screw on the top.

e Invert the bottle over the trough and remove the top. (Water will come out and the bottle will deform at this stage. This does not matter; It will reform.)

f Place the magnesium turnings in the flask, fit the delivery tube and put it under the water.

g Fit the tap funnel and run hydrochloric acid to the flask until the level is close to the side-arm.

h Allow the reaction to run for 30 seconds at least to flush air out of the flask and delivery tube.

i Put the bottle over the delivery tube and fill it with hydrogen gas. The bottle will reform as the water is displaced with hydrogen gas.

j When all the water has been displaced, keeping the neck of the bottle under water and fit the 2-hole bung.

k Take the bottle out of the water and stand it on its base.

Part 2

a Fit a balloon to the 1-hole bung.

b Fit the other delivery tube to a laboratory water tap.

c Slowly run water into the bottle and the balloon will inflate as the water displaces the hydrogen.

d When the balloon has been filled with hydrogen gas, tie off the balloon.

e Tie a cotton thread to the balloon and allow it to float in the air.

f Remind audience to cover their ears.

g Light the splint and ignite the balloon.

Reference

This experiment was written by Mike Thompson on behalf of the RSC.

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