Lab worksheet · Experiment 02 · Atomic structure – electron energy levels

Flame Test

Do the experiment above and fill in this sheet as you go. Click a box to write in it. Check yourself with “Show answers”.

1Aim

To show that every metal ion gives its own flame colour, because its electrons emit light of fixed energies, and to use this to identify unknown samples.

Research question: Do different metal ions give different, characteristic colours in a flame — and can we identify an unknown ion by its colour?

2Theory in briefread before the lab

Electrons in an atom can only have fixed energy levels (Bohr model). When a metal ion is heated in a flame, an electron takes in energy and jumps to a higher level (excited state).

The excited state is not stable. The electron falls back to a lower level and gives out the extra energy as light (a photon). The energy of the photon decides its wavelength (λ) and its colour.

The gaps between energy levels are different in every element. So every metal ion gives its own colour, like a fingerprint. This is used to identify metals and to colour fireworks.

Reference — colour and wavelength of visible light

Colour	Violet	Blue	Green	Yellow	Orange	Red
λ (nm)	380–450	450–495	495–570	570–590	590–620	620–750

ΔE = E(high level) − E(low level) = h·f = h·c ÷ λ

h = 6.63 × 10⁻³⁴ J·s c = 3.00 × 10⁸ m/s 1 nm = 10⁻⁹ m

Key idea: Bigger energy gap → shorter wavelength → colour towards violet. Each element has its own gaps, so its own colour.

3My prediction (hypothesis)write before you start

If we heat different metal ions in a flame, then each ion will give a different, characteristic colour, because each element has its own energy levels, so its electrons emit light of their own wavelengths. The chloride ion will not change the colour, because it is in every salt we test.

4Variables

Variable	Meaning	In this experiment
Independent	What we change	The metal salt (menu “Metal salt”): Li⁺, Na⁺, K⁺, Ca²⁺, Sr²⁺, Ba²⁺, Cu²⁺
Dependent	What we observe	Flame colour, brightest line λ (nm), photon energy
Controlled	What we keep the same	All salts are chlorides; blue flame; wire cleaned in HCl

5Materials and safetytick each item on your bench

Equipment	Qty	✔
Bunsen burner	1	☐
Nichrome wire loop	1	☐
Beaker of hydrochloric acid (cleaning)	1	☐
Watch glasses with salt samples	7	☐
Spectroscope	1	☐
Cobalt-blue glass	1	☐

Chemical	Amount	✔
LiCl, NaCl, KCl, CaCl₂	a few crystals	☐
SrCl₂, BaCl₂, CuCl₂	a few crystals	☐
Hydrochloric acid, HCl	25 mL	☐

Hazard	What we do
Open flame	Tie back long hair. Never leave the burner alone.
Concentrated HCl is corrosive	Goggles on; never touch the loop after dipping.
BaCl₂ and CuCl₂ are toxic	Never taste or touch the salts; wash hands after a real lab.
In the virtual lab	Tick the safety check before the first step — the lab will not start without it.

6Procedurefollow step by step, tick when done

Part A — Seven metal ions

#	What to do	Done
1	Set Metal salt = Lithium chloride and Clean the wire = Yes. Tick the safety check.	☐
2	Step 1 Light the burner: the room gets dark and the flame is blue.	☐
3	Step 2 Clean the wire: watch the flame the first time the loop goes in → Table 2.	☐
4	Steps 3–4 Pick up the salt, Hold in the flame: read the flame colour → Table 1.	☐
5	Step 5 Use the spectroscope: read the brightest line and photon energy → Table 1.	☐
6	Press Reset trial, choose the next salt and repeat for all 7 salts → Table 1.	☐

Part B — Special tests

#	What to do	Done
7	Potassium chloride: at step 4 choose Look through cobalt blue glass → Table 2.	☐
8	Calcium chloride with Clean the wire = No (traces of sodium). Read colour and line → Table 2.	☐
9	Look at the graph Photon energy against wavelength under the lab → section 8.	☐

7Observations and measurementswrite units in every number

Table 1 — Seven metal ions (clean wire)

Copy the values from the readouts or the Results table of the lab.

Ion (salt)	Expected colour (theory)	Observed flame colour	Brightest line λ (nm)	Photon energy (×10⁻¹⁹ J)
Li⁺ (LiCl)	Crimson red	Crimson red	670.8	2.97
Na⁺ (NaCl)	Yellow-orange	Intense yellow-orange	589.0	3.38
K⁺ (KCl)	Lilac	Lilac (pale violet)	766.5	2.59
Ca²⁺ (CaCl₂)	Orange-red	Orange-red (brick red)	622.0	3.20
Sr²⁺ (SrCl₂)	Red	Bright red	606.0	3.28
Ba²⁺ (BaCl₂)	Pale green	Pale (apple) green	524.2	3.79
Cu²⁺ (CuCl₂)	Blue-green	Blue-green	521.8	3.81

Table 2 — Special tests

Test	What I saw	Why?
Cleaning the wire (first time in the flame)	A yellow flash, then a blue flame	Traces of Na⁺ on the wire burn off
KCl through cobalt-blue glass	The lilac colour is seen clearly	The blue glass absorbs the yellow light
CaCl₂ with a dirty wire	Yellow-orange flame; brightest line 589.0 nm	Na⁺ yellow is so bright it hides Ca²⁺

Calculations

Quantity	Formula	My working	Result
Photon energy of Sr²⁺ (λ = 606 nm)	E = h·c ÷ λ	6.63×10⁻³⁴ × 3.00×10⁸ ÷ 606×10⁻⁹	3.28×10⁻¹⁹ J
Frequency of Cu²⁺ light (λ = 521.8 nm)	f = c ÷ λ	3.00×10⁸ ÷ 521.8×10⁻⁹	5.75×10¹⁴ Hz
Highest photon energy in Table 1	shortest λ	Cu²⁺, 521.8 nm	3.81×10⁻¹⁹ J
Lowest photon energy in Table 1	longest λ	K⁺, 766.5 nm	2.59×10⁻¹⁹ J

8Theory vs experiment

What the theory says	What we measured / saw	Difference	Agree? (✓ ≈ ✗)
Each metal ion gives its own colour and its own line spectrum	Seven different colours and line patterns	Li⁺ and Sr²⁺ are both red, but their lines are different	✓
Shorter wavelength → bigger photon energy (E = hc/λ)	On the graph the points go down from left to right	None — every point fits E = hc/λ	✓
Fixed energy levels → separate lines, not a full rainbow	Separate coloured lines in the spectroscope	None	✓
Sodium's yellow can hide other colours	CaCl₂ with a dirty wire gave yellow and a 589.0 nm line	–	✓

How to decide

Result	When
Agrees ✓	The observed colour matches the expected colour, and the line is in the right colour range.
Roughly agrees ≈	The colour is close but weak or mixed (e.g. potassium without the blue glass).
Does not agree ✗	A clearly different colour or line — was the wire cleaned? Is the right salt chosen?

Sources of error

Possible source of error	Effect on the result	How to improve
Example: wire not cleaned (traces of Na⁺)	Yellow flame and a 589 nm line for every salt	Clean the loop in HCl until the flame stays blue
Bright yellow hides a faint colour	K⁺ lilac is hard to see	Look through cobalt-blue glass
Ca, Sr, Ba, Cu flames also contain small molecules (CaOH, SrOH…)	The real spectrum has fuzzy bands as well as lines	Compare with a data book; use a better spectroscope

9Conclusion (report)

	Write in 1–2 sentences
ClaimWhat did we find?	Each metal ion gave its own flame colour and its own line spectrum.
EvidenceWhich data show it?	Li⁺ crimson 670.8 nm, Na⁺ yellow 589.0 nm, K⁺ lilac 766.5 nm … Cu²⁺ blue-green 521.8 nm; energy rises as λ falls on the graph.
ReasoningWhy? Link to the theory.	Electrons are excited by heat and fall back between fixed energy levels, giving photons with ΔE = hc/λ. Every element has different gaps, so different lines and colours. This supports the theory.

Report summary

The aim of this experiment was to show that each metal ion gives its own flame colour. We saw seven different colours and line spectra, so the result agrees with the theory of fixed energy levels. The highest photon energy came from Cu²⁺ (521.8 nm) and the lowest from K⁺ (766.5 nm). A dirty wire gave a yellow flame and a 589 nm line, because traces of sodium hide the real colour.

Check questions

Why do we see separate lines in the spectroscope and not a full rainbow? Electrons can only fall between fixed energy levels, so only certain photon energies (wavelengths) are given out.
Li⁺ and Sr²⁺ both give a red flame. How can the spectroscope tell them apart? Their brightest lines are at different wavelengths (670.8 nm and 606 nm) — each element has its own line pattern.
Why must the wire be cleaned in HCl before every test? Traces of earlier salts, especially sodium, give their own colour and hide the colour of the new ion.

Observation

Variables

Graph

Results table