ChemistryICSE Class 10 · Hydrogen Chloride

Fountain Experiment and Making Hydrochloric Acid without Back Suction (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

The fountain experiment shows that hydrogen chloride is highly soluble in water and its solution is acidic. A flask of dry HCl is inverted over water containing blue litmus; when a few drops of water enter, the gas dissolves, the pressure falls, and water rushes in as a red fountain. An inverted funnel prevents back suction when making hydrochloric acid.

Understand Fountain Experiment and Making Hydrochloric Acid

Density: heavier than air. Hydrogen chloride has a vapour density of 18.25, greater than that of air (about 14.4). A simple test: take two gas jars of HCl, keep one mouth upwards and the other mouth downwards for a minute, then bring a rod dipped in ammonia solution to each. The upright jar still gives dense white fumes; the inverted jar gives few or none, because the heavier gas has fallen out.

Solubility: the fountain experiment.

*Apparatus.* A dry round-bottomed flask filled with dry HCl gas, closed with a cork through which passes a long glass jet tube (and, often, a dropper containing water). The flask is clamped upside down so that the end of the jet tube dips into a trough of water coloured with blue litmus solution.

*Procedure.* A few drops of water are squeezed into the flask from the dropper (or the flask is cooled slightly so a little water enters).

*Observation.* Water from the trough rushes up the tube and sprays into the flask as a fountain, and the blue litmus turns red.

*Inference.* Hydrogen chloride is extremely soluble in water: the first drops dissolve so much gas that the pressure inside the flask falls below atmospheric pressure, and the outside air pressure pushes water up. The red colour shows the solution is acidic.

The flask must be completely dry; otherwise some HCl dissolves in moisture before the experiment and the fountain is weak or absent. The same experiment with ammonia uses red litmus, giving a blue fountain.

Making hydrochloric acid: avoiding back suction. If HCl gas is passed into water through a narrow tube dipping below the surface, the gas dissolves so fast that the pressure in the tube drops and water is sucked back into the hot generating flask, which can crack.

The special arrangement: the delivery tube is attached to an inverted funnel whose rim just touches (dips very slightly into) the water in a beaker.

*How it works.* When gas dissolves rapidly and water rises into the funnel, the level in the beaker falls below the rim of the funnel. Air enters the funnel, the pressure is equalised, and the water in the funnel falls back into the beaker. The water can never rise far enough to reach the generator. The wide mouth of the funnel also offers a large surface area for the gas to dissolve quickly.

The solution formed is hydrochloric acid: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ .

Fountain Experiment and Making Hydrochloric Acid: formulas and key facts

HCl in water: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$
Vapour density and molecular mass: $V D_{H C l} = \frac{36.5}{2} = 18.25 > V D_{ai r} \approx 14.4$
Fountain experiment: summary: Apparatus dry flask of dry HCl, inverted, jet tube into water with blue litmus
Procedure squeeze a few drops of water into the flask
Observation water rushes in as a red fountain
Inference HCl is highly soluble and its solution is acidic
Inverted funnel: two advantages: Prevents back suction: water level falls below the rim and air enters | Large surface area: gas dissolves quickly
HCl and ammonia fountains compared: Hydrogen chloride blue litmus in the trough, red fountain, acidic solution
Ammonia red litmus in the trough, blue fountain, alkaline solution
Both flask must be dry; shows high solubility

Apparatus	dry flask of dry HCl, inverted, jet tube into water with blue litmus
Procedure	squeeze a few drops of water into the flask
Observation	water rushes in as a red fountain
Inference	HCl is highly soluble and its solution is acidic

Hydrogen chloride	blue litmus in the trough, red fountain, acidic solution
Ammonia	red litmus in the trough, blue fountain, alkaline solution
Both	flask must be dry; shows high solubility

How to solve Fountain Experiment and Making Hydrochloric Acid questions

Fountain-experiment questions follow the CISCE wording: setting of the apparatus, procedure, observation, inference. Answer under those four headings.

1Apparatus: dry flask, dry gas, inverted, jet tube into coloured water.
2Procedure: introduce a few drops of water.
3Observation: the fountain and its colour (red with blue litmus for HCl).
4Inference: highly soluble gas; acidic solution; pressure inside fell.
5For back suction: describe the inverted funnel just touching the water, then explain how air entering stops the water rising.
6If asked about density, use the vapour density (18.25) or the upright/inverted jar test.

Fountain Experiment and Making Hydrochloric Acid in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why a dissolving gas can suck water back

Suppose a student bubbles HCl gas from a heated flask straight into a beaker of water through a narrow glass tube, and the teacher warns that the flask may crack.

Explain the danger and describe how the set-up should be changed.

Try it first, then show the solutionSolution

HCl is so soluble that the gas in the tube dissolves very fast, lowering the pressure inside the tube and flask.
Atmospheric pressure pushes water up the tube and into the hot flask: back suction. Cold water hitting hot glass can crack it.
Attach an inverted funnel to the delivery tube with its rim just touching the water: if water rises into the funnel, the level outside falls below the rim, air enters and the water drops back.

Answer: Back suction could pull cold water into the hot flask; use an inverted funnel just touching the water.

Memory hook: A funnel's wide mouth lets air in before water can climb out.

Tricks to remember Fountain Experiment and Making Hydrochloric Acid

Short memory aids. Say each one aloud once, then use it on a question the same day.

Dry flask, wet fountain
The flask must be dry for the fountain to be wet and strong.
Use it when: Precaution in the fountain experiment.
Acid paints the fountain red
HCl turns blue litmus red; ammonia turns red litmus blue.
Use it when: Fountain colours for the two gases.
Rim on the water
Funnel rim just touches the water: no deeper, no higher.
Use it when: Back-suction arrangement.

Fountain Experiment and Making Hydrochloric Acid: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Colour of the fountain · 1 mark · MCQBoard level

In the fountain experiment with hydrogen chloride and blue litmus solution in the trough, the fountain is:
(a) blue
(b) red
(c) colourless
(d) pink

Try it first, then show the solutionSolution

HCl dissolves to form an acid.
Acids turn blue litmus red.

Answer: (b) red.

Example 2: Observation and inference · 2 marks · Short answerBoard level

State the observation and the inference from the fountain experiment with hydrogen chloride.

Try it first, then show the solutionSolution

Observation: water rushes into the flask as a fountain and blue litmus turns red.
Inference: HCl is highly soluble in water and its solution is acidic.

Answer: Red fountain; HCl is very soluble and acidic in water.

Example 3: Why the fountain forms · 3 marks · Short answerHarder board level

Explain why water rises into the flask in the fountain experiment, and why the flask must be dry.

Try it first, then show the solutionSolution

The few drops of water dissolve a large amount of HCl, so the pressure inside the flask falls well below atmospheric pressure.
Atmospheric pressure on the water in the trough pushes water up the jet tube into the flask, forming a fountain.
If the flask is wet, some HCl dissolves before the experiment, so the pressure difference is small and the fountain is weak or does not form.

Answer: Pressure drop inside the flask; a wet flask uses up gas beforehand.

Example 4: Inverted funnel · 3 marks · Short answerHarder board level

Describe, with reasons, the arrangement used to dissolve hydrogen chloride gas in water, and state two advantages.

Try it first, then show the solutionSolution

The delivery tube is joined to an inverted funnel whose rim just touches the water in a beaker.
If water rises into the funnel, the level in the beaker falls below the rim, air enters and the water falls back: back suction is prevented.
Advantages: no back suction into the generator; large surface area for quick dissolving.

Answer: Inverted funnel just touching the water; prevents back suction and speeds dissolving.

Example 5: Density test design · 4 marks · Long answerHOTS / competency

Two gas jars filled with HCl are left open for one minute, jar A mouth upwards and jar B mouth downwards. A rod dipped in ammonia solution is then held at each. (a) Predict the observations. (b) What do they show? (c) How would the results differ for ammonia gas, tested with a rod dipped in concentrated HCl? Explain using vapour densities (VD of air about 14.4).

Try it first, then show the solutionSolution

(a) A: dense white fumes; B: few or no fumes.
(b) HCl (VD 18.25) is heavier than air: it stays in the upright jar and falls out of the inverted one.
(c) Ammonia (VD 8.5) is lighter than air: it would escape from the upright jar and stay in the inverted one, so jar B would give fumes and jar A few or none.
Reaction for the fumes in both cases: $N H_{3} + H C l \to N H_{4} C l$ .

Answer: HCl: fumes from the upright jar only (heavier than air). NH₃: fumes from the inverted jar only (lighter than air).

Example 6: How much gas filled the flask? · 3 marks · Short answerHOTS / competency

A fountain experiment flask holds 448 cm³ of HCl gas at S.T.P. After the fountain, all the gas has dissolved. (a) How many moles of HCl dissolved? (b) What mass of sodium hydroxide would neutralise the acid formed? (c) What mass of calcium carbonate could this acid dissolve? (H = 1, Cl = 35.5, Na = 23, O = 16, Ca = 40, C = 12)

Try it first, then show the solutionSolution

(a) $448/22 400 = 0.02$ mol.
(b) $N a O H + H C l \to N a C l + H_{2} O$ : 0.02 mol NaOH $= 0.8$ g.
(c) $C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$ : 0.01 mol CaCO₃ $= 1$ g.

Answer: (a) 0.02 mol (b) 0.8 g (c) 1 g.

Where students lose marks in Fountain Experiment and Making Hydrochloric Acid

The slips examiners see most often, and what to write instead.

Mistake: Inference missing
Students describe the fountain but forget to state what it proves.
Do this: Always add: 'HCl is highly soluble in water and its solution is acidic.'
Usually costs: 1 mark.
Mistake: Wrong litmus colour
Students write 'blue fountain' for HCl by mixing it up with ammonia.
Do this: Blue litmus → red fountain for HCl; red litmus → blue fountain for NH₃.
Usually costs: 1 mark.
Mistake: Funnel dipped deep in water
In diagrams, students draw the funnel well below the surface, which defeats its purpose.
Do this: Draw the funnel rim just touching the water surface.
Usually costs: 1 diagram mark.
Mistake: Explaining back suction without pressure
Answers say 'water goes back' without the cause.
Wrong working
Water goes back into the flask because HCl is soluble.
Right working
HCl dissolves rapidly, the pressure in the tube falls, and atmospheric pressure pushes water back towards the flask.
Do this: Mention the fall in pressure and atmospheric pressure.
Usually costs: 1 mark.

Exam tips

Use the four CISCE headings (apparatus, procedure, observation, inference) for any fountain question.
Draw the funnel arrangement neatly: delivery tube, inverted funnel, rim at the water surface, beaker.
Revise the HCl and NH₃ fountains side by side; questions often swap the gases.
Quote VD 18.25 for HCl and 8.5 for NH₃ when density is asked.

Practice questions on Fountain Experiment and Making Hydrochloric Acid

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The fountain experiment demonstrates that hydrogen chloride is:
(a) lighter than air
(b) highly soluble in water
(c) a reducing agent
(d) insoluble in water
Hint
Why does water rush in?
Answer
(b).
Q2 · 1 mark · Very short answerBoard level
What is back suction?
Hint
Water going the wrong way.
Answer
The sucking of water back through the delivery tube into the gas-generating flask when a very soluble gas dissolves rapidly.
Q3 · 2 marks · Short answerBoard level
Name the indicator used in the trough and the colour of the fountain for (a) HCl, (b) NH₃.
Hint
Acid and alkali.
Answer
(a) Blue litmus; red fountain. (b) Red litmus; blue fountain.
Q4 · 2 marks · Short answerHarder board level
Why does the rim of the funnel only just touch the water and not dip deep into it?
Hint
Air must be able to enter.
Answer
So that a small rise of water into the funnel lowers the outside level below the rim, letting air in and stopping the rise. If the funnel dipped deep, the water could rise high enough to reach the generator.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The flask used in the fountain experiment must be dry.
Reason (R): Moisture would dissolve some of the gas before the experiment starts.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Less gas, smaller pressure drop.
Answer
(a).
Q6 · 1 mark · Very short answerHarder board level
State the role of atmospheric pressure in the fountain experiment.
Hint
What pushes the water?
Answer
Atmospheric pressure acting on the water in the trough pushes it up the tube into the flask, where the pressure has fallen.
Q7 · 2 marks · Short answerHarder board level
Give the reason why hydrogen chloride is not collected over water, and state, with a reason, how it is collected instead.
Hint
Solubility and density.
Answer
It is extremely soluble in water, so it would dissolve; and it is heavier than air, so it can be collected by upward displacement of air in an upright jar.
Q8 · 2 marks · Short answerHOTS / competency
Suppose a student repeats the fountain experiment with carbon dioxide instead of HCl, using lime water in the trough. Predict whether a strong fountain forms, and explain.
Hint
Compare solubilities.
Answer
No strong fountain: carbon dioxide is only slightly soluble in water, so a few drops of water dissolve very little gas and the pressure inside hardly falls. (The lime water may slowly turn milky where it meets the gas.)

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Colour of the HCl fountain with blue litmus?Show answer
Red.
2.Why does the pressure in the flask fall?Show answer
HCl dissolves in the few drops of water.
3.Device used to prevent back suction?Show answer
An inverted funnel just touching the water.
4.VD of HCl?Show answer
18.25.
5.Ion that makes the solution acidic?Show answer
H₃O⁺ (hydronium).

Fountain Experiment and Making Hydrochloric Acid: frequently asked questions

What does the fountain experiment prove?

That hydrogen chloride (or ammonia) is highly soluble in water; the litmus colour also shows whether the solution is acidic or alkaline.

Why is an inverted funnel used to dissolve HCl gas?

It prevents back suction of water into the generating flask and gives a large surface for the gas to dissolve.

How can you show HCl is heavier than air?

Leave one jar of HCl upright and one inverted; after a minute only the upright jar gives white fumes with an ammonia rod.

Why must the flask be dry in the fountain experiment?

Moisture would dissolve some gas beforehand, reducing the pressure drop so the fountain is weak or fails.