Edexcel IGCSE Chemistry · Spec 2.44-2.46
Tests for Gases & Flame Tests
The chemical tests for common gases and the flame tests for metal ions.
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Tests for Gases & Flame Tests
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Explained
Tests for gases, and flame tests for metal ions
These are recall marks, which makes them the cheapest marks in chemistry and the most annoying to lose. Each test needs the reagent or apparatus named exactly, and the observation named exactly. Getting one word wrong loses the whole mark, as the splint tests show.
The gas tests
- Hydrogen: hold a lit splint at the mouth of the tube. It burns with a squeaky pop.
- Oxygen: use a glowing splint. It relights.
- Carbon dioxide: bubble the gas through limewater. It turns milky.
- Ammonia: hold damp red litmus paper in the gas. It turns blue.
- Chlorine: hold damp litmus paper in the gas. It is bleached white.
Hydrogen needs a lit splint. Oxygen needs a glowing one. They are different pieces of apparatus doing different jobs, and swapping them is the single most common error in this topic.
Both litmus tests need the paper damp. A dry paper will not change colour, because the gas has to dissolve in the moisture to act.
Flame tests
- Lithium: red.
- Sodium: yellow.
- Potassium: lilac.
- Calcium: orange red.
- Copper: blue green.
Clean a platinum or nichrome wire by dipping it in concentrated hydrochloric acid and holding it in the flame until no colour shows. Then dip it in the sample and hold it in a hot blue Bunsen flame.
The cleaning step matters. Sodium gives such a strong yellow that a trace left from a previous test will mask everything else, which is why the wire must show no colour before you start.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Chemistry record exactly the splint problem. The majority gained the mark for a lit splint and a squeaky pop, but a few lost it for saying a glowing splint, and others lost it for saying squeaky pop without mentioning that the splint was lit.
Both halves are needed. Name the apparatus and its state, then name the observation.
On flame tests, the reports note that most candidates knew a flame test was required and usually gave a yellow flame for two marks, and that the most common correct response for potassium was a lilac flame.
Writing the answer
Use the form: do this, observe that, therefore it is this. "Hold a lit splint at the mouth of the tube. There is a squeaky pop. The gas is hydrogen." Three short sentences, and every mark point present.
Spec 2.44-2.46
What you need to know
- Recall the tests for common gases
- Recall the flame test colours
- Describe how to carry out a flame test
Active recall
Quick check
Answer each question before opening the answer.
Give the tests for oxygen, carbon dioxide and hydrogen.
Oxygen relights a glowing splint; carbon dioxide turns limewater milky; hydrogen gives a squeaky pop with a lit splint.
Give the flame-test colours for lithium, sodium, potassium and copper.
Lithium — red; sodium — yellow; potassium — lilac; copper — blue-green.
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