Demonstration or class practical

This experiment simulates the industrial fractional distillation of crude oil in the laboratory.

Lesson organisation

Teachers will need to assess whether this activity can be done as a class experiment or as a demonstration.

The experiment time depends on the age and experience of the students. A year 7 (or equivalent) group needs about 50 mins and a year 12 (or equivalent) group about 30 mins.

Apparatus

Chemicals

Eye protection

Bunsen burner

Heat resistant mat

Side-arm hard-glass test-tube (Note 1)

Bent delivery tube and rubber connection tubing

Small sample tubes (20 mm x 5 mm) minimum size (small test-tubes can also be used), 4

Thermometer 0–360°C with cork to fit side-arm test-tube

Teat pipette

Beaker (100 cm3)

“Hard glass” (borosilicate) watch glass

Mineral or ceramic fibre

Wooden splints

Crude oil substitute (HIGHLY FLAMMABLE and HARMFUL), about 2 cm3 (Notes 2 and 3)

Refer to Health & Safety and Technical notes section below for additional information

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection. 

Crude oil substitute (HIGHLY FLAMMABLE and HARMFUL) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

1 Side-arm boiling tubes produce more consistent results than boiling tubes fitted with bungs with two holes, one for a thermometer and one for a delivery tube.

2 Real crude oil contains more than 0.1% benzene, which is carcinogenic, which makes it unsuitable for use in schools.

3 It is important to try the experiment beforehand. It may be necessary to add an additional low boiling point fraction to the ‘crude oil’ mixture  – eg cyclohexane – to obtain something below 70 °C.

4 This is quite a messy experiment. If it is done regularly, it is probably best to keep sets of apparatus – apart from the thermometer and watch glasses – dedicated to the experiment. This is because it is difficult to get clean, and it still works if oil residues are present.

Procedure

a Place about a 2 cm3 depth of ceramic fibre in the bottom of the side-arm test-tube. Add about 2 cm3 of crude oil alternative to this, using the teat-pipette.

b Set up the apparatus as shown in the diagram, with one addition for the first fraction: ensure you place a beaker of cold water around the collecting tube. The bulb of the thermometer should be level with, or just below the side-arm. Heat the bottom of the side-arm test-tube gently, with the lowest Bunsen flame. Watch the thermometer.

When the temperature reaches 100°C, replace the collection tube with another empty one. The beaker of water is no longer necessary.
Collect three further fractions, to give the fractions as follows:

Room temperature to 100°C

2 100–150°C

3 150–200°C

4 200–250°C

c A black residue remains in the side-arm test-tube. Test the four fractions for viscosity (how easily do they pour?), colour, smell and flammablility. 
To test the smell, gently waft the smell towards you with your hand. 
To test for flammablility, pour onto a hard glass watch glass and light the fraction with a burning splint.

d Keep one set of fractions and see that they combine to form a mixture very like the original sample.

Teaching notes

The fractions increase in viscosity with boiling temperature and should become more coloured as the temperature increases. With some artificial mixtures, the difference in colour can be difficult to observe.

The descriptions of smells vary from student to student, but students can be encouraged to liken them to familiar smells – eg ‘like lubricating oil’.

The samples become increasingly difficult to burn and burn with increasingly smokey flames.

This experiment forms an important part of understanding how we obtain chemicals from oil.

Health & Safety checked August 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Weblinks

Students can be encouraged to find these themselves – there are many.

Wikipedia - Fractional distillation

Page last updated September 2016