Last updated: 7 October 2026
Ammonia is one of those substances that seems simple at first: one nitrogen atom joined to three hydrogen atoms. Yet a surprising amount of O-Level Chemistry can be connected through this one molecule.
Its alkaline behaviour links back to acids and bases. Ammonium salts connect ammonia to qualitative analysis. Its solubility and density determine how the gas is dried, tested and collected. On a much larger scale, ammonia introduces the Haber Process, where industry must balance reaction rate, yield, cost and safety.
The best way to study ammonia is therefore not to memorise a collection of unrelated facts. Instead, keep asking one question: which property of ammonia explains what we observe?
Try these before reading the chapter. The aim is not to get everything right, but to uncover the ideas that need the most attention.
- 5 diagnostic questions
- Properties, gas tests and basic ammonia chemistry
- Instant explanation for every answer option
What is ammonia?
Ammonia is a compound of nitrogen and hydrogen with the molecular formula NH₃. Each molecule contains one nitrogen atom covalently bonded to three hydrogen atoms.
Because ammonia consists of individual molecules rather than a giant structure, it is described as a simple molecular covalent substance.
How the bonds in ammonia form
Nitrogen has five outer-shell electrons. Each hydrogen atom has one outer-shell electron. Nitrogen shares one electron with each of three hydrogen atoms, giving three shared pairs of electrons and therefore three N–H covalent bonds.
The remaining two outer-shell electrons on nitrogen form a lone pair. At O-Level, the most important idea is that ammonia exists as separate neutral NH₃ molecules.
Physical properties of ammonia
Several laboratory procedures involving ammonia can be predicted directly from its physical properties.
| Property | What it tells us | Practical consequence |
|---|---|---|
| Colourless gas | Ammonia cannot be identified from its colour. | A chemical test is required to confirm its identity. |
| Pungent smell | Ammonia has a sharp, irritating smell. | Its smell is not used as the official identification test. |
| Less dense than air | Ammonia has a lower density than air. | Ammonia can be collected by upward delivery. |
| Very soluble in water | Large quantities of ammonia dissolve readily in water. | Ammonia should not be collected over water. |
| Forms an alkaline solution in water | Dissolved NH₃ reacts partially with H₂O to produce OH⁻ ions. | Ammonia turns damp red litmus paper blue. |
Why two properties matter when collecting ammonia
Choosing a gas-collection method requires two separate questions.
- Is the gas soluble in water? Ammonia is very soluble, so collection over water is unsuitable.
- Is the gas denser or less dense than air? Ammonia is less dense than air, so it rises into an inverted gas jar during upward delivery.
Why is aqueous ammonia a weak alkali?
Students sometimes expect every alkali to contain OH⁻ in its chemical formula. Ammonia shows why that idea is incomplete.
Ammonia itself has the formula NH₃. However, when NH₃ dissolves in water, some ammonia molecules react with water molecules:
The OH⁻ ions produced make the solution alkaline.
Why is ammonia described as weak?
Only a small proportion of the dissolved ammonia molecules react with water. Most of the dissolved particles remain as NH₃ molecules.
As a result, only a relatively small amount of OH⁻ is produced. This is why aqueous ammonia is described as a weak alkali.
Weak and dilute describe different ideas
Weak describes the extent to which ammonia reacts with water. Only a small proportion of NH₃ molecules react to form NH₄⁺ and OH⁻ ions.
Dilute, on the other hand, describes a solution containing a relatively small amount of solute in a given volume of solution.
A solution can therefore be concentrated but contain a weak alkali. The words weak and dilute do not mean the same thing.
Ammonia and ammonium are different particles
The names ammonia and ammonium sound similar, but they should never be treated as interchangeable.
| Feature | Ammonia | Ammonium |
|---|---|---|
| Formula | NH₃ | NH₄⁺ |
| Type of particle | Neutral molecule | Positive ion |
| Commonly encountered as | Ammonia gas or aqueous ammonia | Part of ammonium salts such as NH₄Cl |
| Important reaction | Accepts H⁺ to form NH₄⁺ | Reacts with OH⁻ on warming to release NH₃ |
The relationship between the two particles can be seen from:
Ammonia gains H⁺ to form an ammonium ion.
How ammonia reacts with acids
Ammonia behaves as a base. When it reacts with an acid, an ammonium salt is formed.
For example, aqueous ammonia reacts with hydrochloric acid:
The ionic equation is:
Here, H⁺ is transferred to NH₃, forming NH₄⁺.
Not every acid–base reaction produces water
When an acid reacts with a hydroxide, the key reaction is:
When an acid reacts directly with ammonia, the key reaction is instead:
There is no OH⁻ ion in this second equation, so water is not a product of the overall reaction.
How ammonium salts release ammonia
Ammonium salts show the reverse connection. When an ammonium salt is warmed with an alkali such as aqueous sodium hydroxide, ammonia gas is released.
For example:
Sodium ions and chloride ions do not change during the reaction. The ionic equation is:
Ammonium salt + alkali
A useful word equation is:
This reaction is important because it provides the chemical basis for identifying NH₄⁺ ions.
Testing for ammonium ions and ammonia gas
It is important to distinguish between the test for NH₄⁺ ions and the test for the ammonia gas produced.
Test for ammonium ions, NH₄⁺
- Add aqueous sodium hydroxide to the sample.
- Warm the mixture carefully.
- Test any gas released using damp red litmus paper.
If NH₄⁺ ions are present, ammonia gas is released:
Test for ammonia gas
Hold damp red litmus paper near the gas. If ammonia is present, the red litmus paper turns blue.
Why must the litmus paper be damp?
The water on the paper allows ammonia to dissolve. Dissolved ammonia then reacts with water to produce OH⁻ ions:
These OH⁻ ions produce the alkaline conditions that turn red litmus blue.
State the complete observation
A strong answer is: “Ammonia gas is evolved, which turns damp red litmus paper blue.”
Simply writing “turns litmus paper blue” leaves out the important word damp.
Preparing ammonia in the laboratory
Ammonia can be prepared in the laboratory by heating an ammonium salt with a base. One suitable preparation uses ammonium chloride and calcium hydroxide.
During heating, the ammonia leaves the reaction vessel together with water vapour. If dry ammonia is required, this water vapour must be removed before the gas is collected.
The hard-glass boiling tube is positioned so that its mouth slopes slightly downwards towards the delivery tube. This prevents condensed water from running back onto the strongly heated part of the tube.
Drying ammonia with calcium oxide
Calcium oxide, CaO, also called quicklime, is suitable for drying ammonia.
It removes water:
More importantly, calcium oxide does not react with ammonia under the conditions used.
| Drying agent | Suitable? | Reason |
|---|---|---|
| Calcium oxide | Yes | It removes water without reacting with ammonia. |
| Concentrated sulfuric acid | No | It is acidic and reacts with basic ammonia. |
| Fused calcium chloride | No | It absorbs ammonia, so ammonia would be removed from the gas mixture. |
A drying agent has two jobs
- It must remove water vapour.
- It must not react with or absorb the gas being dried.
A substance that removes water but also removes ammonia is not a suitable drying agent for ammonia.
Collecting ammonia by upward delivery
Ammonia is collected by upward delivery, also described as downward displacement of air.
Ammonia is less dense than air. It therefore rises into an inverted gas jar while the denser air is displaced downwards.
Collection over water is unsuitable because ammonia is very soluble in water.
Property → consequence → method
- Very soluble in water → ammonia would dissolve → do not collect over water.
- Less dense than air → ammonia rises → collect by upward delivery.
Manufacturing ammonia: the Haber Process
Laboratory preparation is useful when only a small amount of ammonia is needed. Industry, however, requires enormous quantities of ammonia, particularly for the manufacture of nitrogen-containing fertilisers.
The industrial manufacture of ammonia from nitrogen and hydrogen is called the Haber Process.
Where do the raw materials come from?
| Reactant | Source |
|---|---|
| Nitrogen, N₂ | Obtained from air |
| Hydrogen, H₂ | Obtained from the cracking of crude oil |
The gases are mixed according to the balanced equation:
The coefficients show that nitrogen and hydrogen react in a mole ratio of 1 : 3.
Why is the Haber reaction reversible?
Nitrogen and hydrogen can react to form ammonia:
However, ammonia can also decompose back into nitrogen and hydrogen:
The reversible arrow, ⇌, therefore tells us that both forward and reverse reactions can occur.
This has an important industrial consequence: nitrogen and hydrogen are not completely converted into ammonia in a single pass through the reactor.
Conditions used in the Haber Process
| Condition | Typical value | Why it is used |
|---|---|---|
| Temperature | About 450°C | Provides a sufficiently fast rate while still giving a useful ammonia yield. |
| Pressure | About 200 atm | Increases ammonia yield and reaction rate without making the process excessively costly. |
| Catalyst | Finely divided iron | Increases the reaction rate by providing an alternative pathway with lower activation energy. |
The best condition is usually a compromise
A factory does not simply choose the temperature or pressure that gives the highest possible percentage yield. It also has to consider how quickly ammonia is produced, how much energy is required, the cost of equipment and the safety of operating conditions.
The Haber Process therefore uses conditions that give an economical balance between rate, yield, cost and safety.
Why is a temperature of about 450°C used?
The forward reaction forming ammonia is exothermic. Haber Process data show that lower temperatures give a higher final percentage of ammonia.
That sounds as though the factory should simply use a very low temperature. However, there is another factor: reaction rate.
At a lower temperature, particles have less kinetic energy. There are fewer successful collisions per second, so ammonia is produced much more slowly.
Yield and rate move in opposite directions
- Lower temperature: higher final ammonia yield, but slower reaction.
- Higher temperature: faster reaction, but lower final ammonia yield.
- About 450°C: a compromise giving a useful yield at a sufficiently fast rate.
Why is a pressure of about 200 atm used?
Look first at the balanced equation:
There are four moles of gaseous reactants on the left side of the equation but only two moles of gaseous product on the right.
Haber Process data show that increasing pressure increases the percentage yield of ammonia. Higher pressure also brings gas particles closer together, increasing collision frequency and therefore increasing reaction rate.
So why not use an extremely high pressure?
- Compressing gases requires energy.
- Very high-pressure equipment is expensive to build.
- Reaction vessels must be strong enough to withstand the pressure.
- Operating at very high pressure creates greater safety concerns.
About 200 atm therefore provides a useful industrial compromise.
What does the iron catalyst do?
The Haber Process uses a finely divided iron catalyst.
The catalyst provides an alternative reaction pathway with a lower activation energy. A greater fraction of collisions can therefore lead to reaction, so ammonia is produced more quickly.
The catalyst does not increase the final percentage yield of ammonia under the same temperature and pressure.
Rate and yield answer different questions
- Rate: How quickly is ammonia produced?
- Yield: How much ammonia is obtained?
The iron catalyst increases the rate. It does not increase the final percentage yield of ammonia.
How does the Haber Process operate continuously?
Since the Haber reaction is reversible, only some of the nitrogen and hydrogen are converted into ammonia during each pass through the reactor.
Industry avoids wasting the unreacted gases.
- Nitrogen and hydrogen are supplied to the process.
- The gases are mixed in the required 1 : 3 mole ratio.
- The gas mixture is compressed to high pressure.
- The mixture is heated and passed over a finely divided iron catalyst.
- Some nitrogen and hydrogen react to form ammonia.
- The gases leaving the reactor are cooled.
- Ammonia is separated from the unreacted gases.
- The unreacted nitrogen and hydrogen together are recycled.
- The recycled gases rejoin the common feed before entering the reactor again.
The unreacted nitrogen and hydrogen are both reused
Recycling gives unreacted N₂ and H₂ repeated opportunities to react. The recycle stream therefore contains both nitrogen and hydrogen and rejoins the mixed feed going to the Haber reactor.
It does not simply return to the hydrogen source. Recycling reduces waste and increases the overall amount of ammonia obtained from the raw materials.
How to answer Haber Process data questions
In an examination, you may be given a graph, table or set of industrial data rather than being asked simply to recall the operating conditions.
A useful approach is:
- Identify what is being changed — temperature, pressure, catalyst or another condition.
- Describe the trend from the data before trying to explain it.
- Decide whether the question concerns rate or yield.
- Connect the trend to industrial considerations such as reaction speed, energy use, equipment cost and safety.
For example, suppose a graph shows that decreasing temperature increases the final percentage of ammonia but greatly increases the time needed to reach that value. The industrial choice of about 450°C can then be explained as a compromise: the ammonia yield is lower than it would be at a much lower temperature, but ammonia is produced at a much faster and more economical rate.
Common mistakes in ammonia questions
| Common mistake | Why it is wrong | Correct idea |
|---|---|---|
| Saying that ammonia contains OH⁻ ions. | The molecular formula of ammonia is NH₃. | NH₃ reacts partially with water to produce OH⁻ ions. |
| Saying that weak ammonia simply means dilute ammonia. | Weak and dilute describe different ideas. | Ammonia is weak because only a small proportion reacts with water to form ions. |
| Calling ammonia and ammonium the same particle. | NH₃ is neutral, while NH₄⁺ has a positive charge. | Distinguish carefully between ammonia molecules and ammonium ions. |
| Testing ammonia with dry red litmus paper. | Water is needed for ammonia to produce OH⁻ ions. | Use damp red litmus paper. |
| Collecting ammonia over water. | Ammonia is very soluble in water. | Collect ammonia by upward delivery. |
| Drying ammonia with concentrated sulfuric acid. | Concentrated sulfuric acid reacts with ammonia. | Use calcium oxide. |
| Drawing the heated boiling tube with its mouth sloping upwards. | Condensed water could run back towards the strongly heated part. | The boiling tube should slope slightly downwards towards its mouth and delivery tube. |
| Saying the iron catalyst increases ammonia yield. | A catalyst increases reaction rate but does not increase the final percentage yield under the same conditions. | The catalyst makes the reaction proceed more quickly. |
| Saying the highest possible pressure should be used. | Very high pressure increases energy costs, equipment costs and safety risks. | Industry uses a compromise pressure of about 200 atm. |
| Saying the lowest possible temperature should be used. | A very low temperature would make the reaction too slow. | About 450°C provides a compromise between rate and yield. |
| Showing the recycled Haber gases returning only to the hydrogen supply. | The recycle stream contains unreacted N₂ and H₂ together. | Both gases are recycled to the common feed entering the reactor. |
| Writing N₂ + 3H₂ → 2NH₃ with a one-way arrow. | The Haber reaction is reversible. | Write N₂ + 3H₂ ⇌ 2NH₃. |
Ammonia checklist
Properties of ammonia
- Formula: NH₃
- Simple covalent molecule
- Colourless gas
- Pungent smell
- Less dense than air
- Very soluble in water
Acid–base chemistry
- Aqueous ammonia is a weak alkali.
- Only a small proportion of NH₃ reacts with water.
- OH⁻ ions make the solution alkaline.
- Ammonia reacts with acids to form ammonium salts.
- NH₃ and NH₄⁺ are different particles.
Laboratory chemistry
- NH₄⁺ + OH⁻ → NH₃ + H₂O
- Test ammonia with damp red litmus paper.
- Positive result: red litmus turns blue.
- Dry ammonia using calcium oxide.
- Collect ammonia by upward delivery.
- Do not collect ammonia over water.
Haber Process
- Nitrogen comes from air.
- Hydrogen comes from cracking crude oil.
- N₂ + 3H₂ ⇌ 2NH₃
- About 450°C
- About 200 atm
- Finely divided iron catalyst
- Unreacted N₂ and H₂ are recycled together.
These questions go beyond simple recall. Use the properties, equations, practical methods and industrial reasoning from the chapter.
- 10 mastery questions
- Practical and industrial reasoning
- Instant explanation for every answer option
Questions students often ask about ammonia
These questions connect the main ideas and clear up some of the details that commonly cause confusion.
How can ammonia be an alkali when NH₃ does not contain OH⁻?
An alkali does not need to contain OH⁻ in its original formula. What matters is whether it produces OH⁻ ions in aqueous solution.
Ammonia reacts partially with water:
The OH⁻ ions formed make the solution alkaline.
What exactly does weak alkali mean for ammonia?
Only a small proportion of dissolved NH₃ molecules react with water. Therefore, only a relatively small amount of OH⁻ ions is formed.
That partial reaction is why aqueous ammonia is classified as a weak alkali.
Does weak ammonia mean the same thing as dilute ammonia?
No. Weak describes how completely ammonia reacts with water to form ions. Dilute describes how little solute is present in a given volume of solution.
The two words describe different properties.
Is ammonia the same as the ammonium ion?
No. Ammonia is the neutral molecule NH₃. Ammonium is the positively charged ion NH₄⁺.
Why does ammonia reacting with hydrochloric acid not necessarily produce water?
H⁺ from the acid is accepted directly by NH₃:
There is no OH⁻ ion in this ionic equation to combine with H⁺ and form water.
Why must ammonium salts be warmed with sodium hydroxide during the test for NH₄⁺?
Warming helps release ammonia gas from the reaction:
The ammonia can then be identified using damp red litmus paper.
Why does the boiling tube slope downwards when ammonia is prepared?
Water is formed during the reaction and leaves the hot mixture as water vapour. Some water may condense in cooler parts of the apparatus.
Positioning the mouth of the boiling tube slightly lower than the heated end helps prevent condensed water from running back onto the strongly heated glass.
Why can ammonia not be dried using concentrated sulfuric acid?
Concentrated sulfuric acid is acidic, while ammonia is basic. They react, so the drying agent would remove ammonia as well as water.
Why is calcium oxide suitable for drying ammonia?
Calcium oxide removes water without removing the ammonia gas.
Why is ammonia collected by upward delivery?
Ammonia is less dense than air, so it rises into an inverted gas jar and displaces the denser air downwards.
It cannot conveniently be collected over water because it is very soluble in water.
Why not use a much lower temperature in the Haber Process if it gives more ammonia?
A lower temperature gives a higher final percentage of ammonia but also makes the reaction much slower.
Industry needs to produce a large amount of ammonia per unit time, so about 450°C is used as a compromise between yield and rate.
Why not use an extremely high pressure in the Haber Process?
Although increasing pressure can improve ammonia yield and reaction rate, very high pressures require more energy for compression and much stronger, more expensive equipment.
They also create greater safety concerns. About 200 atm is therefore an industrial compromise.
Does the iron catalyst increase the percentage yield of ammonia?
No. The catalyst provides an alternative reaction pathway with lower activation energy, so the reaction proceeds more quickly.
It does not increase the final percentage yield of ammonia when temperature and pressure are unchanged.
Where do the recycled nitrogen and hydrogen go in the Haber Process?
After ammonia has been separated, the remaining unreacted nitrogen and hydrogen are recycled together.
The recycle stream rejoins the common feed going into the Haber reactor. It does not return only to the hydrogen source.
Ammonia becomes much easier to remember once its properties are used to explain its behaviour. Its partial reaction with water explains why it is a weak alkali and why damp red litmus paper is used for its gas test. Its high solubility and low density explain how it is collected, while its reactions connect NH₃ with ammonium salts. On an industrial scale, the Haber Process shows how chemists and engineers balance reaction rate, yield, cost, safety and recycling. Follow these connections and the topic becomes a single chemical story rather than a list of facts to memorise.