Demonstration and class practical

This experiment involves preparing and looking at the properties of hydrogen chloride, especially the demonstration of its great solubility in water (fountain experiment), forming hydrochloric acid.

Lesson organisation

Preparation of hydrogen chloride gas on a large scale must be done as a demonstration by a teacher.

Investigating some of the properties of the solution of the gas in water – that is hydrochloric acid – can be done either as a part of the demonstration or as a class experiment, unless the demonstration is only being used to show the great solubility of gases like hydrogen chloride in water.

The demonstration of the fountain experiment should take about 15mins and the class investigation of properties of hydrochloric acid, about 20mins.

Apparatus Chemicals

Eye protection (including goggles for generating and handling hydrogen chloride gas)

For the fountain demonstration (see diagram below), the teacher requires:

Access to a fume cupboard

Safety screens

Large round-bottomed flask (500–1000 cm3) (Note 1)

Bung to fit flask, with holes for a teat pipette (with detachable teat) and delivery tube as shown (Note 2)

Valve, consisting of a short length of rubber tubing and a Mohr’s clip

Glass trough or plastic bowl

Short lengths of rubber tubing

Clamp stands, bosses and clamps (Note 3)

Small glass funnel – to fit inside a 250 cm3 beaker

Beaker (250 cm3)

Conductivity testing apparatus (3 V DC supply, graphite electrodes in bung, leads and clips, torch bulb in holder)

For testing the properties of pre-prepared dilute hydrochloric acid, each group of students requires:

Test-tubes, 4

Test-tube rack

Plastic dropper pipettes

Wooden splints

Concentrated sulfuric acid (CORROSIVE), 10 cm3

Sodium chloride, 30 g (Note 4)

Universal indicator solution

Access to:

Dilute hydrochloric acid, 0.1 M

Sodium hydroxide solution, 0.1 M (IRRITANT)

Limewater (IRRITANT), fresh, a few cm3

Universal indicator solution (HIGHLY FLAMMABLE)

Universal indicator papers

Magnesium ribbon, 2 cm lengths

Small marble chips, a few

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

Health & Safety and Technical notes

Read our standard health & safety guidance

Work in a fume cupboard and wear goggles for generating and handling hydrogen chloride gas. Wear eye protection for testing the properties of dilute hydrochloric acid.

Hydrogen chloride gas, HCl(g) is generated (TOXIC, CORROSIVE) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Concentrated sulfuric acid, H2SO4(aq) (CORROSIVE) - see CLEAPSS Hazcard.

Sodium chloride, NaCl(s)  (or rock salt) - see CLEAPSS Hazcard. 

Dilute hydrochloric acid, HCl(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book

Universal indicator solution (HIGHLY FLAMMABLE) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Magnesium ribbon, Mg(s) - see CLEAPSS Hazcard.  

Limewater, Ca(OH)2(aq), (treat as IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.  

Sodium hydroxide solution, NaOH(aq), (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

1 The round-bottomed flask should be a thick-walled one made of borosilicate (Pyrex) glass and free of any flaws. The safety screens are necessary, because the sudden vacuum produced when the gas dissolves in water can cause the flask to implode.

2 The pipette inserted through the bung should have a removable rubber teat that can be easily attached once the apparatus is in place. It is important to check that the volume of water the teat bulb holds, when filled, is sufficient to ensure that drops of water emerge into the flask, when the teat is squeezed. It helps to fill the teat bulb with water beforehand, and place it upright in a small beaker ready to use.

The flask and the bung carrying the glass tubes should be dried in an oven at about 80 °C before the experiment.

3 It is very important to set up the necessary stand, bosses and clamps, safety screens and the empty trough/bowl in their correct positions on the bench in front of the class before the demonstration. Use the empty round-bottomed flask fitted with the bung and tubes (but no teat bulb on the pipette) to adjust the position of the clamps, so that the long tube and valve reaches almost to the bottom of the bowl. Make sure the flask will not topple over when it fills with water from the bowl. Do this by positioning the bowl on the base of the stand, or putting some extra weights on it.

4 Powdered rock salt can be used instead of reagent sodium chloride.

The set-up of the hydrogen chloride fountain demonstration can be seen here:



Preparing hydrogen chloride and demonstrating its solubility in water (Fountain experiment)
a Fill the bowl with tap water and add enough Universal indicator solution to make the colour clearly visible.

b Set up the hydrogen chloride gas generator in a fume cupboard. Remove the empty round-bottomed flask from its position on the bench set-up to the fume cupboard for filling. Clamp the flask in position near the gas generator.

c Purge the air out of the flask containing sodium chloride, by dripping about 1 cm3 of sulfuric acid onto the sodium chloride. A piece of damp Universal indicator or blue Litmus paper held near the end of the delivery tube turns red when hydrogen chloride starts to emerge from it.

d Position the delivery tube in the round-bottomed flask. Slowly fill the flask with hydrogen chloride, by dripping more sulfuric acid onto the sodium chloride in the gas generator. Test to see when it is full, by holding damp indicator paper near the mouth of the flask. Allow the gas to pass for a little longer, even if the indicator paper turns red.

e Stop the gas stream and remove the delivery tube from the round-bottomed flask. Refit the bung to the flask and clamp it in position over the bowl of water ready for the fountain demonstration. Ensure the valve at the end of the glass tube jet is well immersed in the water and the system is air tight.

f Carefully attach the bulb of the teat pipette filled with water to the pipette tube, without squeezing any water into the flask.

g Remove the clip from the valve. The water starts to rise slowly up the tube as the gas dissolves in it. To speed up this process, squeeze a few drops of water into the flask from the pipette.

h All the hydrogen chloride in the flask dissolves in the water drops from the pipette, creating a vacuum in the flask. Water is then sucked up at great speed from the bowl, creating a spectacular ‘fountain’ in the flask. The indicator in the water changes colour to red. The extent to which the flask fills with water depends on how much air was left in the flask. It can fill almost completely.

Properties of hydrochloric acid
If these experiments are only demonstrated, then hydrochloric acid prepared by bubbling hydrogen chloride from the generator can be used in a fume cupboard. This must be done using the method described below in steps  a-c because of the great solubility of the gas. If students are to carry out tests on the properties, they should begin at step d, using pre-prepared dilute hydrochloric acid.

a Half fill a 250 cm3 beaker with distilled/deionised water. Connect the small funnel to the gas generator with a length of rubber tubing. Clamp it upside down in the beaker, so that the funnel rim is just below the surface of the water. See the diagram below:

b Bubble hydrogen chloride gas slowly through the funnel, by dripping sulfuric acid onto the sodium chloride. If the gas flow rate is too slow, the water level rises in the funnel and falls in the beaker – until bubbles of air are sucked into funnel. This stops the water rising further. If this happens, increase the gas flow rate.

c After one to two minutes raise the funnel from the beaker and stop the gas flow. Use the conductivity tester to test the conductivity of the solution in the beaker. The lamp should light, showing the presence of ions in solution. If not, bubble more hydrogen chloride through the water and test again.

d Fill three test test-tubes to a level of one-third with the hydrochloric acid prepared above or use dilute hydrochloric acid. Add a couple of drops of Universal indicator solution to one of the tubes. It turns red, indicating the presence of a strong acid.

e Add sodium hydroxide solution slowly to the same test-tube using a pipette. Shake to mix after each addition. Continue adding until the alkali neutralises the acid and the colour changes from red, through yellow-green, to purple.

f Add a strip of magnesium to a second test-tube of acid. It fizzes vigorously, giving off a colourless gas, and dissolves to give a colourless solution of magnesium chloride. The gas can be collected by holding an inverted test-tube over the reaction for a several seconds. Test for hydrogen by applying a flame to the mouth of the inverted tube. You should hear a squeaky pop should.

g Add a few marble chips to a third test-tube of acid. They fizz, giving off a colourless gas – carbon dioxide. The presence of this gas can be proved using the limewater test.

Teaching notes

It advisable to try the fountain experiment before demonstrating it to a class, to make sure the flask can be successfully filled with HCl gas. Be careful not to disconnect the flask from the gas supply too soon, even if HCl can be detected at the outlet. Oven drying the glass apparatus beforehand improves the chances of success.

The solubility of hydrogen chloride in one volume of water at 0 °C and 1 atm is 506 volumes.

Health & Safety checked, 2016


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 April 2017