Silver Nitrate

Silver nitrate (AgNO3) is the reagent for identifying halide ions in solution. Its three precipitate colours are frequently tested in Paper 6 (practical) and Paper 4.

Key facts

Property	Value
Formula	AgNO3
Mr	108 + 14 + 3(16) = 170
Bonding	Ionic — Ag+ and NO3-
Key property	White solid, very soluble, light-sensitive (kept in brown bottles)
Main use	Test reagent for chloride, bromide and iodide ions

Where silver nitrate appears in 0620

• Tests for ions: the standard halide test.
• Group 7 halogens: distinguishing the halide ions of the group.
• Paper 6 qualitative analysis: reading a table of observations to identify an unknown salt.

The halide ion test

Method

1. Add dilute nitric acid to remove interfering carbonate and sulfite ions.
2. Add aqueous silver nitrate.
3. Observe the precipitate colour (confirm with dilute ammonia if needed).

Results

Halide ion	Precipitate	Colour	Solubility in ammonia
Chloride (Cl-)	AgCl	White	Soluble in dilute ammonia
Bromide (Br-)	AgBr	Cream	Soluble only in concentrated ammonia
Iodide (I-)	AgI	Yellow	Insoluble in ammonia

Ionic equations

Ag+(aq) + Cl-(aq) -> AgCl(s) (white)

Ag+(aq) + Br-(aq) -> AgBr(s) (cream)

Ag+(aq) + I-(aq) -> AgI(s) (yellow)

These are precipitation reactions — two soluble substances forming an insoluble product. The Na+ and NO3- are spectator ions and are left out.

Why dilute nitric acid, not another acid

Carbonate and sulfite ions also form insoluble silver compounds (a false positive), so the acid clears them first. It must be dilute nitric acid: hydrochloric acid would add chloride ions and sulfuric acid would add sulfate ions — the very kind of ion you are testing for.

Two points examiners reward

• The test works for chloride, bromide and iodide only. Fluoride ions give no precipitate because silver fluoride is soluble.
• The silver halides slowly darken in light as they decompose to silver, which is why silver nitrate is stored in brown bottles.

A colourless solution is thought to contain either chloride ions or iodide ions. Describe a test to find out which is present, and give the result for each. (4 marks)

Mark scheme

• Add dilute nitric acid to the solution [1]
• then add aqueous silver nitrate [1]
• Chloride ions give a white precipitate (of silver chloride) [1]
• Iodide ions give a yellow precipitate (of silver iodide) [1]

Examiner note: the two method marks (acidify, then add silver nitrate) come before the observations. Jumping straight to “add silver nitrate” drops the nitric acid mark, and “goes cloudy” without a colour drops an observation mark.

Aqueous silver nitrate is added to sodium bromide solution. Write the ionic equation for the reaction and state what you would see. (3 marks)

Mark scheme

• Ag+(aq) + Br-(aq) -> AgBr(s) [1]
• A cream precipitate forms [1]
• The precipitate dissolves in concentrated (not dilute) ammonia [1]

Examiner note: include the state symbols and leave out the spectator ions Na+ and NO3-. Writing the full equation with spectators, or omitting (s) on the precipitate, can cost the equation mark.

Common exam mistakes

• Forgetting the dilute nitric acid step, so carbonate or sulfite ions give a false positive.
• Muddling the colours. The order matches the group: chloride white, bromide cream, iodide yellow.
• Acidifying with hydrochloric or sulfuric acid, which introduces chloride or sulfate ions.
• Including spectator ions in the ionic equation or leaving off the state symbol on the precipitate.