Theme: Our Earth Resources
Learning and Teaching Activities
Demonstration on cracking of medicinal paraffin
Teaching Tips
Experiments provide the opportunity for students to get the first-hand information and carry
out scientific investigation. However, as the generated petrol has strong pungent smell and
the gases emitted are regarded as air pollutants. This excise is a demonstration. Still students
and teachers need to take proper precautions during practical.
Safety precautions
Be awake.
Follow instruction and do not disturb others.
Keep calm if there is an accident. Escape to safe place where report and seek help can
immediately be practised.
Distribute the worksheet before the practical so that students can prepare for the practical.
Objective: Learn the production of petrol and its uses. Know the safety measures in scientific
investigation.
Introduction
The demand for petrol exceeds the production that generated by fractional distillation of
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crude oil. Cracking of heavy fractions of petroleum could synthesize petrol to meet the
demand of modern world. This experiment is to model the industrial cracking process in a
teaching laboratory.
Materials
 Boiling tube  Delivery tube  Test tube
 Stopper
 Bunsen burner
 Water trough  Stand
 Glass wool
 Liquid paraffin
 Porcelain
chips
Safety Precaution
1.
The experiment should be carried out inside a fume hood.
2.
Wear eye protection throughout the experiment.
3.
Beware of the sucking back in water trough, lift the apparatus out of water using stand
when heating stops.
4.
Make sure the apparatus especially the delivery tubes are not blocked.
5.
Paraffin is inflammable.
Procedures
1.
Set up the experimental apparatuses as shown in Figure 1. Fill three test tubes with water
and invert them in the water trough.
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glass wool soaked with
liquid paraffin
Figure 1. Experimental set up of cracking of medicinal paraffin
2.
Strongly heat the porcelain chips (catalyst for cracking) by a Bunsen burner for 5
minutes.
3.
Heat the glass wool containing liquid paraffin. Try to make a steady stream of gas
bubbles.
4.
Collect tubes of the gases obtained from cracking; discard the first one.
5.
Stopper the remaining two tubes of gases.
6.
Examine the smell of the glass collected.
Observation
Discussion
1.
Smell the gases produced by cracking of medicinal paraffin. Describe the smell.
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2.
The first test tube of gases obtained should be discarded. Suggest reason(s) for it.
3.
Compare the flammability of the medicinal paraffin with gaseous product. (Hint: Use
glowing splint and lighted splint respectively).
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Suggested answers (Accept alternative and correct answers)
1.
Smell the gases produced by cracking of medicinal paraffin. Describe the smell.
The smell is similar to that of liquefied petroleum gas.
2.
The first test tube of gases obtained should be discarded. Suggest reason(s) for it.
It is because the first tube of gases obtained contains the gases in the boiling tube
(contain mainly oxygen and nitrogen).
3.
Compare the flammability of the medicinal paraffin with the gaseous product. (Hint:
Use glowing splint and lighted splint respectively).
The medicinal paraffin is not flammable and gives out black smoke in combustion.
The gaseous product is flammable, flame color is yellow – blue, and do not gives out
black smoke.
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