Flame Tests and the Uses of Group 1 Elements and Compounds

25 min

Colours that name the metal

Sprinkle a few grains of salt into a gas flame and it flares yellow-orange. Nearly every sodium and potassium compound is soluble, so these ions cannot be detected by forming a precipitate. Instead, chemists use the colour each metal gives to a flame. It is the same physics as the hydrogen spectrum in Unit 2: heat excites electrons, and as they fall back they give out light of the exact energies of that element. This final lesson of the unit shows how to do a flame test properly, explains the colours, and surveys how the alkali metals and their compounds are used.

How to do a flame test

Clean a nichrome (or platinum) wire loop by dipping it in concentrated hydrochloric acid and holding it in a hot, blue Bunsen flame. Repeat until the wire gives no colour.
Dip the clean, moist loop into the solid (or a concentrated solution) to pick up a little sample.
Hold the loop at the edge of the hot, blue (non-luminous) part of the flame.
Observe the colour, and how long it lasts.
To see a potassium flame in the presence of sodium, look through cobalt blue glass, which absorbs the strong yellow sodium light.

Table 4.1: Flame colours of the Group 1 ions

Ion	Flame colour	Main wavelengths / nm
$\mathrm{Li^{+}}$	crimson red	671
$\mathrm{Na^{+}}$	intense yellow-orange, long-lasting	589
$\mathrm{K^{+}}$	lilac (pale purple)	766 to 770 (red) and 404 (violet)
$\mathrm{Rb^{+}}$	red-violet	780
$\mathrm{Cs^{+}}$	blue-violet	455 to 459

Definition

Flame test: a test that identifies certain metal ions by the colour they give to a hot flame. The colour is the element's emission spectrum seen without a spectroscope: in the heat, electrons are promoted to higher energy levels, and when they fall back they emit light whose wavelengths are fixed by the energy gaps in that element's atoms.

Worked example 1: identifying three salts

Three white solids, P, Q and R, are known to be lithium chloride, sodium chloride and potassium chloride. In flame tests P gives a persistent yellow-orange flame, Q a crimson flame and R a pale lilac flame. Identify each. A fourth solid S gives a yellow-orange flame that looks pale purple through cobalt blue glass. What does S contain?

Solution. P: sodium chloride (yellow-orange). Q: lithium chloride (crimson). R: potassium chloride (lilac). S: the yellow-orange shows sodium; the cobalt glass removes the yellow and reveals lilac, so S contains both sodium and potassium ions. Without the glass, the sodium colour would hide the potassium.

Worked example 2: the energy of the colours

Calculate the energy of one photon of sodium light (589 nm) and of the main potassium line (766 nm). Which electron jump is larger? ( $h = 6.626 \times 10^{-34}\,\mathrm{J\,s}$ , $c = 3.00 \times 10^{8}\,\mathrm{m\,s^{-1}}$ )

Solution.

E_{\mathrm{Na}} = \frac{hc}{\lambda} = \frac{6.626 \times 10^{-34} \times 3.00 \times 10^{8}}{589 \times 10^{-9}} = 3.37 \times 10^{-19}\,\mathrm{J}

E_{\mathrm{K}} = \frac{6.626 \times 10^{-34} \times 3.00 \times 10^{8}}{766 \times 10^{-9}} = 2.59 \times 10^{-19}\,\mathrm{J}

Answer: the sodium jump (3s ← 3p) is larger; potassium's main line is lower in energy, deep red, and mixes with its weaker violet line to look lilac.

Uses of the Group 1 elements and their compounds

Element or compound	Use	Property that makes it suitable
lithium	rechargeable lithium-ion batteries in phones, laptops and electric vehicles	lightest metal; $\mathrm{Li^{+}}$ is small and moves easily between electrodes; large energy stored per kilogram
lithium carbonate	a medicine for bipolar disorder	supplies $\mathrm{Li^{+}}$ ions in controlled doses
sodium	sodium vapour street lamps; liquid sodium as a coolant in some nuclear reactors	intense 589 nm emission; liquid over a wide range (98 to 883 °C) and an excellent conductor of heat
sodium chloride	food and food preservation; raw material for chlorine, sodium hydroxide and hydrogen	cheap, abundant, soluble
sodium hydroxide	making soap, paper and bleach	strong, cheap alkali
sodium carbonate	glass making; washing soda; softening hard water	basic; precipitates $\mathrm{Ca^{2+}}$ as $\mathrm{CaCO_3}$
sodium hydrogencarbonate	baking powder; antacid for indigestion; some fire extinguishers	releases $\mathrm{CO_2}$ when heated or with acid; a mild base
potassium chloride, potassium sulfate, potassium nitrate	fertilisers (the K of NPK)	soluble source of potassium, a nutrient plants need in large amounts
potassium superoxide	breathing apparatus for miners and rescue workers	absorbs exhaled $\mathrm{CO_2}$ and releases $\mathrm{O_2}$
caesium	atomic clocks	the second is defined by a frequency of caesium-133 atoms: 9 192 631 770 Hz

Worked example 3: potassium in a fertiliser

Potassium chloride is the commonest potassium fertiliser. Calculate the percentage by mass of potassium in KCl, and the mass of potassium in a 50 kg bag of pure KCl. (K = 39.1, Cl = 35.5)

Solution. $M_r(\mathrm{KCl}) = 39.1 + 35.5 = 74.6$ .

\%\,\mathrm{K} = \frac{39.1}{74.6} \times 100 = 52.4\,\% \qquad \text{mass of K} = 50\,\mathrm{kg} \times 0.524 = 26.2\,\mathrm{kg}

Answer: 52.4%; 26.2 kg of potassium per 50 kg bag.

Warning

Common mistakes. (1) Using a dirty wire: traces of sodium from earlier tests or from your fingers give a yellow flame that hides everything else. (2) Holding the sample in the yellow, luminous flame: use the hot, blue flame. (3) Calling the potassium flame "purple" or "violet" without qualification: it is a pale lilac. (4) Thinking the colour comes from the chloride: it comes from the metal.

Safety. Concentrated hydrochloric acid is corrosive and gives off choking fumes: use a small amount in a stoppered tube, wear goggles, and work in a well-ventilated room. Keep hair and sleeves away from the flame.

Tip

Learn the three Group 1 flame colours in order down the group: Li crimson, Na yellow-orange, K lilac. If you see yellow-orange, always check for potassium through cobalt blue glass.

ChemJungle: Flame tests - lithium, sodium, potassium and copper(II) ions (3 min). A real laboratory demonstration: compare the colours with Table 4.1 and notice how long the sodium colour lasts.

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Practice

Describe how to carry out a flame test on a solid, including how to clean the wire.
Give the flame colour of lithium, sodium and potassium ions.
Explain, in terms of electrons, why a sodium compound colours a flame yellow-orange.
Why is cobalt blue glass used when testing a mixture for potassium ions?
Why is a flame test, rather than a precipitation test, used to identify $\mathrm{Na^{+}}$ and $\mathrm{K^{+}}$ ?
Give one use each of: lithium, sodium, sodium carbonate, potassium nitrate, caesium. For each, name the property that makes it suitable.
Calculate the percentage by mass of potassium in potassium nitrate, $\mathrm{KNO_3}$ (K = 39.1, N = 14.0, O = 16.0). Compare it with potassium chloride as a source of potassium.

Show answersHide answers

Clean a nichrome wire by dipping it in concentrated HCl and heating until it gives no colour; dip the moist wire in the solid; hold it at the edge of a hot, blue Bunsen flame; note the colour.
Lithium crimson red; sodium yellow-orange; potassium lilac.
Heat promotes the outer electron of sodium from 3s to 3p; when it falls back it emits a photon of energy $3.37 \times 10^{-19}\,\mathrm{J}$ , wavelength 589 nm, which is yellow-orange.
The intense yellow sodium light hides the pale lilac of potassium; cobalt blue glass absorbs the yellow, so any potassium colour can be seen.
Almost all sodium and potassium compounds are soluble, so no common reagent precipitates these ions.
For example: lithium, batteries (light, small mobile ion); sodium, street lamps (bright 589 nm emission) or reactor coolant (liquid over a wide range, conducts heat); sodium carbonate, glass making or water softening (basic, precipitates $\mathrm{Ca^{2+}}$ ); potassium nitrate, fertiliser (soluble source of K and N); caesium, atomic clocks (precise frequency of Cs-133).
$M_r(\mathrm{KNO_3}) = 39.1 + 14.0 + 48.0 = 101.1$ ; $\frac{39.1}{101.1} \times 100 = 38.7\,\%$ . Less potassium per kilogram than KCl (52.4%), but it also supplies nitrogen.

Quick check

What colour do potassium ions give in a flame test?

Key points

Flame test: clean nichrome wire (conc. HCl), sample, hot blue flame. Li crimson, Na yellow-orange, K lilac (seen through cobalt blue glass if Na is present).
The colour is an emission spectrum: electrons excited by heat fall back and emit photons of fixed energy (Na 589 nm).
Uses: Li in batteries; Na in street lamps and reactor coolant; NaCl, NaOH, $\mathrm{Na_2CO_3}$ and $\mathrm{NaHCO_3}$ in industry and the home; K salts in fertilisers; $\mathrm{KO_2}$ in breathing apparatus; Cs in atomic clocks.

1Colours that name the metal

2How to do a flame test

3Uses of the Group 1 elements and their compounds

Colours that name the metal

How to do a flame test

Uses of the Group 1 elements and their compounds