2.2.5Organic Chemistry
Flame Tests — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 9 of 42
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 4Carrying Out the Test
A flame test identifies metal ions from the colour they give to a hot flame.
1
Clean a nichrome or platinum wire loop by dipping it in concentrated hydrochloric acid and heating it in a blue Bunsen flame.
2
Repeat the cleaning until the wire gives no colour in the flame, which shows that no earlier sample is contaminating it.
3
Moisten the loop with acid, dip it into the solid sample so that some sticks to it, and hold it at the edge of the blue flame.
4
Do not heat the wire until it glows red, because that glow can be mistaken for a flame colour.
5
Record the colour seen and compare it with known results, testing one metal ion at a time because mixed ions blend their colours.
Part 2 of 4Why a Colour Appears
Excitation
- Excitation Heat gives an outer electron enough energy to jump from its ground-state level to a higher energy level.
Instability
- Instability An electron in a higher level is unstable and quickly falls back to a lower energy level.
Emission
- Emission As the electron drops, the energy difference is released as light of one particular frequency.
Colour
- Colour If that frequency lies in the visible region, the flame appears coloured, and each metal has its own set of energy gaps.
Magnesium
- Magnesium Magnesium gives no visible colour because the energy it emits falls outside the visible part of the spectrum.
Part 3 of 4Colours to Learn
Colours to Learn
Lithium
- Lithium Lithium compounds give a crimson red flame, the deepest red of the Group 1 metals.
Sodium
- Sodium Sodium compounds give an intense yellow-orange flame that can hide other colours in a mixture.
Potassium
- Potassium Potassium compounds give a lilac flame, which is often easier to see through blue cobalt glass.
Calcium
- Calcium Calcium compounds give a brick-red or orange-red flame, which can look similar to strontium.
Strontium
- Strontium Strontium compounds give a deep red flame, and barium compounds give a pale apple-green flame.
| Metal ion | Flame colour |
|---|---|
| Li⁺ | Crimson red |
| Na⁺ | Yellow |
| K⁺ | Lilac |
| Ca²⁺ | Brick red |
| Sr²⁺ | Red |
| Ba²⁺ | Apple green |
| Mg²⁺ | No colour |
Part 4 of 4Exam Checklist & Tips
2.2.5 – Key Points for the Exam
- Flame test A test that uses the colour a metal ion gives to a flame to identify the ion.
- Excited state An arrangement in which an electron occupies a higher energy level than the lowest available one.
- Ground state The arrangement in which electrons occupy the lowest available energy levels.
- Emission Release of energy as light when an electron falls from a higher to a lower energy level.
- Contamination Unwanted ions on the wire that give false colours in the flame.
Common Mistakes
- Saying that the electron is removed from the atom: in a flame test it only moves to a higher level and falls back.
- Writing that heat releases light directlythe light comes from electrons dropping down, not from the heating itself.
- Using a dirty wire or sampling from the hot, glowing tip of the loop, which gives a yellow sodium colour or a false red glow.
- Forgetting that sodium impurities can mask weaker colours such as lilac for potassium.
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