Demonstration

A piece of paper (or an old £5 or £10 note) soaked in a mixture of ethanol and water is ignited. The ethanol burns but the paper does not.

Lesson organisation

This is a demonstration experiment which can either be used for fun as part of a public event or in a class to stimulate discussion of the conditions required for combustion.

Apparatus Chemicals

The quantities given are for one demonstration.

The teacher requires:

Eye protection

Bunsen burner

Pair of tongs

Heat-resistant mats, 2

Beakers (250 cm3), 3

Paper, e.g. filter paper (Note 1)

Ethanol (HIGHLY FLAMMABLE) or Industrial Denatured Alcohol (IDA) (HIGHLY FLAMMABLE, HARMFUL), 75 cm3

Sodium chloride (common salt), about 1 g

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.

Ethanol, C2H5 OH(l) (HIGHLY FLAMMABLE) - see CLEAPSS Hazcard .

Industrial Denatured Alcohol (IDA) (HARMFUL, HIGHLY FLAMMABLE) - see CLEAPSS Hazcard

Sodium chloride, NaCl(s) - see CLEAPSS Hazcard

1 Prepare some pieces of absorbent paper, e.g. filter paper, about the size of a £5 note.

2 Place about 50 cm3 of water in one beaker, a similar volume of ethanol in a second beaker, and a mixture of 25 cmwater and 25 cmethanol in the third beaker. Add a little (about 1 g) of sodium chloride to the third beaker and stir until it has all dissolved. Label the beakers.

Procedure

a Label the beakers.

b Place the Bunsen burner on the heat-resistant mat and adjust it to give a yellow flame. Ensure that the beakers of ethanol, water, and the ethanol-water mixture are a safe distance (2 m) away from the Bunsen burner.

c Using the tongs, soak one piece of paper in the water in the first beaker. Allow the paper to drain. Try to ignite it by holding it in the Bunsen flame for a few seconds. It does not ignite.

d Soak a second piece of paper in ethanol and use the tongs to hold it in the Bunsen flame just long enough for it to ignite. Take care to drip as little alcohol as possible on the bench between the beaker and the Bunsen burner. The alcohol on the paper ignites easily and sets fire to the paper, which burns away.

(If the alcohol in the beaker does ignite by accident during the demonstration, it can be easily and safely extinguished by covering the beaker with a heat-resistant mat.)

e Soak the third piece of paper in the alcohol-water mixture and use the tongs to hold it in the Bunsen flame just long enough for it to ignite. Swiftly remove the paper from the Bunsen flame and observe as the alcohol burns with a yellow flame, but the paper does not burn. The paper will still be wet with water after the alcohol has burnt away.

Teaching notes

A wealthy and/or confident demonstrator can start this experiment with a £5, or even higher value, note and the alcohol-water mixture! More amusement can be added if a member of the audience ‘with money to burn’ can be persuaded to part with the money. It is important to use a yellow Bunsen flame, and to only hold the paper in the flame long enough for it to ignite, to prevent the note from burning. The demonstrations with ordinary paper and the other liquids could then follow to provide an explanation.

The water in the alcohol-water mixture evaporates as the alcohol burns, keeping the temperature of the paper below its ignition temperature (approximately 230°C). Converting this temperature to the Fahrenheit scale can also allow for a discussion of the use of this scale and for the ‘keen’ science fiction fans a discussion about the  Novel and Film ‘Fahrenheit 451’. See Fahrenheit 451 .

The flame from the paper soaked in alcohol alone should be visible but the flame from a burning alcohol-water mixture is often difficult to see. This is why the sodium chloride is added, to give an orange-yellow colour to the flame. The demonstration looks even more impressive in subdued lighting.

Health & Safety checked, 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

Page last updated October 2015

Downloads