GCSE · Chemistry · Edexcel · Spec 1CH0

Chemical test for ammonia

You can't see ammonium ions in a solution. So make them give themselves away: turn them into a gas, then watch a piece of paper turn blue.

The ammonium-ion test, start to finish

Step through it and ask 'why is this step here?' each time. Every condition has a job.

Start with the solutionYou have a solution that might contain ammonium ions. The ions themselves give you nothing to look at, so the plan is to get a gas out of them.
Add sodium hydroxide solutionThis is the chemical that gets the process going. A solution containing ammonium ions releases ammonia gas when it is heated with sodium hydroxide solution.
Heat it gentlyGentle heating, together with the sodium hydroxide solution, is what releases the gas from the solution.
Ammonia gas is given offAmmonia gas is now coming off the solution. From here on you are testing the gas, not the solution.
Hold damp red litmus paper in the gasThe paper must be damp and it must be red litmus. The ammonia gas dissolves in the water on the damp paper.
The paper turns blueAmmonia dissolving in the water on the paper is what turns it blue. Blue shows that an alkali is present.
Positive result: ammonium ions confirmedBlue paper in the gas means ammonia was released, so the solution contained ammonium ions. The test for the ion is really a test for the gas.

Chemistry · Testing for ions

Why does the paper have to be damp?

Your friend says: "The litmus paper is damp in this test. I reckon that's just a detail." Which idea is closest to what you think the water is doing?

Pick the idea you'd go with, then say how sure you are.

The water just helps the paper stay put in the gas, so it doesn't matter chemically.The water turns the paper blue by itself. The gas is just there to carry it.Ammonia gas dissolves in the water on the damp paper, and that turns the paper blue.Any litmus paper will do. The ammonia works whether the paper is damp or dry.

How sure are you?

I'm sureI think soI'm guessing

The water just helps the paper stay put in the gas, so it doesn't matter chemically.

This is a common misconception

Easy to believe, because lots of lab details are only about convenience. But here the water has a real job: the ammonia gas dissolves in the water on the paper, and that is what turns the paper blue.

The water turns the paper blue by itself. The gas is just there to carry it.

This is a common misconception

Not quite. The blue comes from the ammonia gas dissolving in the water on the paper. The water gives the gas somewhere to dissolve, and the gas is what you are testing for.

Ammonia gas dissolves in the water on the damp paper, and that turns the paper blue.

This matches the evidence

Yes. The gas has to dissolve in the water on the paper, and that is what turns the red litmus blue, showing an alkali is present.

Any litmus paper will do. The ammonia works whether the paper is damp or dry.

This is a common misconception

This is the one people drop. The paper must be damp because the ammonia dissolves in the water on it. Take the water away and the gas has nothing on the paper to dissolve in.

Damp isn't a detail. The ammonia gas dissolves in the water on the paper, and that is what turns it blue.

Why bother with a unique test?

Chemists want a test that says 'ammonium ions' and nothing else. Decide why that matters before you look.

Why do chemists need a test that is unique to one ion?

Chemistry · Spot the broken condition

One line of this method is broken

A student writes a method to test a solution for ammonium ions. One line breaks a condition of the test. Which one?

A student's method — which line goes wrong?

Start with a solution that might contain ammonium ions.
Heat the solution gently, with nothing else added.
Hold damp red litmus paper in the gas given off.
If the paper turns blue, the solution contains ammonium ions.

Chemistry · Your turn to write

Write the test, then mark yourself

Describe how to test a solution for ammonium ions, and explain why chemists need a test that is unique to one ion. [5 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Add sodium hydroxide solution to the solution. (1 mark)
Heat it gently. (1 mark)
Hold damp red litmus paper in the gas given off. (1 mark)
The paper turning blue is a positive result, confirming ammonium ions are present. (1 mark)
A unique test prevents confusion from similar reactions, so a different substance is not falsely identified. (1 mark)

Model answer

Add sodium hydroxide solution to the solution and heat it gently. Hold damp red litmus paper in the gas given off. If the paper turns blue, that is a positive result and ammonium ions are present. Chemists need a unique test because it prevents confusion from similar reactions, so a different substance is not falsely identified.

The method is three conditions and one result. The reason a unique test matters is one more point, and it is easy to forget.

What you need to know

Ammonia gas turns damp red litmus paper blue, and that shows an alkali is present.
The paper has to be damp, because the ammonia dissolves in the water on it and that turns it blue.
Have a goA classmate holds dry red litmus paper in ammonia gas and says: "Any minute now..." What have they forgotten?
To make the paper damp.
The ammonia gas has to dissolve in the water on the paper to turn it blue, so a dry paper has nothing to dissolve in.
To test for ammonium ions, heat the solution gently with sodium hydroxide solution. That releases ammonia gas.
Then hold damp red litmus paper in the gas that comes off.
Have a goWhich comes first: holding the damp red litmus paper in place, or heating the solution gently with sodium hydroxide solution?
Heat it gently with the sodium hydroxide solution first.
The paper tests the gas, so the gas has to be released before the paper has anything to meet.
Blue paper is a positive result, and it confirms ammonium ions are present.
So the test for the ion is really a test for the gas it releases.
Have a goFinish the sentence: if damp red litmus paper in the gas turns blue, the gas was ____ and the solution contained ____.
Ammonia; ammonium ions.
The gas is what turns the paper blue, and the gas only appears because the solution held ammonium ions.
Unique tests identify specific ions accurately, because they prevent confusion from similar reactions and falsely identifying a different substance.

The big picture

You can't see ammonium ions, so you turn them into ammonia gas with sodium hydroxide solution and gentle heat. Then you catch the gas on damp red litmus paper, which turns blue.

Key points

1Ammonia gas turns damp red litmus paper blue, which shows an alkali is present.

2The litmus paper must be damp because ammonia dissolves in the water on the paper, and this turns the paper blue.

3Ammonia gas is released when a solution containing ammonium ions is gently heated with sodium hydroxide solution.

4Heat the sample with sodium hydroxide solution and hold damp red litmus paper in the gas given off: the paper turning blue is a positive result that confirms ammonium ions.

5Unique tests are needed to identify specific substances and ions accurately and to avoid falsely identifying a different substance.

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Worked example

Problem

Damp red litmus paper turns blue in the gas given off by a gently heated mixture of a sample solution and sodium hydroxide solution. What can the student say about the sample, and how do they get there?

⚠ Watch out

Thinking any litmus paper will do. It has to be damp red litmus paper, because the ammonia gas dissolves in the water on the paper and that is what turns it blue.

🧠

Memory hook

Blue paper in the gas means ammonium ions in the solution. Test the gas to find the ion.

✓

Check yourself

Say it out loud from memory: what do you add, what do you do next, and what do you hold in the gas? Then say what 'blue' tells you.

Flashcards

(8)

What happens to damp red litmus paper in ammonia gas?

It turns blue, which shows an alkali is present.

Why must the litmus paper be damp?

Ammonia gas dissolves in the water on the damp paper, and this turns the paper blue.

What releases ammonia gas from a solution containing ammonium ions?

Gently heating it with sodium hydroxide solution.

How do you identify ammonium ions in a sample?

Heat the sample with sodium hydroxide solution and hold damp red litmus paper in the gas given off.

What is a positive result in the ammonium-ion test?

The damp red litmus paper turns blue. This confirms ammonium ions are present.

In the ammonium-ion test, what is the paper actually testing?

The ammonia gas released from the solution. The gas tells you about the ion.

Why are unique tests needed?

They identify specific substances and ions accurately, by preventing confusion from similar reactions and falsely identifying a different substance.

Which litmus paper do you use to detect ammonia gas?

Damp red litmus paper.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Why does the paper have to be damp?

One line of this method is broken

Write the test, then mark yourself

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