Flame Tests and the Test for Ammonium Ions

The Elements of Group 1 and 2

What is a flame test used for?

To identify the presence of certain metal cations in a compound from their characteristic flame colours.

Which groups contain many of the metal ions that can be identified using flame tests?

Groups 1 and 2.

Do flame tests work for all Group 1 and Group 2 metal ions?

No, some ions do not produce a visible flame colour.

How do you carry out a flame test?

Add a few drops of concentrated hydrochloric acid to the solid, mix, dip a clean metal wire or silica rod into the mixture, then place it in a Bunsen flame and observe the colour.

Why is concentrated hydrochloric acid added during a flame test?

It converts the metal compound into a chloride, which is more volatile and therefore gives better flame-test results.

What type of wire can be used for a flame test?

A clean platinum or nichrome wire.

What can be used instead of a metal wire in a flame test?

A silica rod.

What flame colour does Li⁺ produce?

Red.

What flame colour does Na⁺ produce?

Yellow-orange.

What flame colour does K⁺ produce?

Lilac.

What flame colour does Rb⁺ produce?

Red-purple.

What flame colour does Cs⁺ produce?

Blue-violet.

What flame colour does Be²⁺ produce?

No colour.

What flame colour does Mg²⁺ produce?

No colour.

What flame colour does Ca²⁺ produce?

Brick red.

What flame colour does Sr²⁺ produce?

Crimson red.

What flame colour does Ba²⁺ produce?

Apple green.

What flame colour do copper compounds produce?

Blue-green.

What are two main problems with flame tests?

Impurities can mask other flame colours, and descriptions of colours can be subjective.

Why can sodium impurities cause problems in flame tests?

Sodium produces an intense yellow-orange colour that can hide other flame colours.

Why can descriptions of flame-test colours be unreliable?

Colour descriptions are subjective, so different people may describe the same colour differently.

What causes the characteristic colours in flame tests?

Electron transitions between energy levels.

What happens to electrons when they absorb energy from the flame?

They move to higher energy levels.

What is the ground state of an electron?

The lowest possible energy level of the electron.

What is an excited state?

A higher-energy state reached when an electron absorbs energy.

What happens after an electron reaches an excited state in a flame test?

It returns to its ground state and releases energy.

How is light produced during a flame test?

Excited electrons return to lower energy levels and release energy as electromagnetic radiation.

Why do different metal ions produce different flame colours?

They have different energy-level spacings, so electron transitions release different amounts of energy and therefore different wavelengths of visible light.

When is a characteristic flame colour observed?

When the energy released by an electron transition corresponds to radiation in the visible-light spectrum.

What wavelength range is visible light approximately?

400–700 nm.

What wavelength is associated with the sodium flame-test colour?

Approximately 590 nm.

Why does sodium produce a yellow-orange flame?

Its electron transition releases radiation at about 590 nm, which lies in the yellow-orange region of the visible spectrum.

Why do magnesium compounds not produce a flame colour?

The relevant electron transition does not correspond to a wavelength in the visible spectrum.

Does burning magnesium metal in a Bunsen flame count as a flame test for Mg²⁺?

No. Burning magnesium involves changing Mg atoms into Mg²⁺ ions, whereas a flame test uses compounds that already contain Mg²⁺ ions.

Which cation in this topic does not give a flame-test colour but has another chemical test?

The ammonium ion, NH₄⁺.

How do you test for ammonium ions?

Add sodium hydroxide solution to the sample and warm the mixture.

What gas is released when ammonium ions are warmed with sodium hydroxide?

Ammonia, NH₃.

What is the ionic equation for the ammonium ion test?

NH₄⁺ + OH⁻ → NH₃ + H₂O

How can ammonia gas be identified using litmus paper?

It turns damp litmus paper blue.

Why does ammonia turn damp litmus paper blue?

Ammonia is an alkaline gas.

What other property can be used to recognise ammonia?

Its characteristic pungent smell, although directly smelling chemicals should be avoided.

How can hydrogen chloride gas be used to confirm ammonia?

Hydrogen chloride reacts with ammonia to produce white smoke of ammonium chloride.

What is the equation for the reaction between ammonia and hydrogen chloride?

NH₃ + HCl → NH₄Cl

What observation occurs when ammonia reacts with hydrogen chloride gas?

White smoke forms.

What substance forms the white smoke when ammonia reacts with hydrogen chloride?

Ammonium chloride, NH₄Cl.

What safety precaution should be used when carrying out flame tests and ammonium-ion tests?

Carry them out in a fume cupboard.