How can we identify the ions and gases present in a substance using chemical tests?
Tests for the common gases (hydrogen, oxygen, carbon dioxide and chlorine), flame tests for metal ions, tests for carbonate, halide and sulfate ions, and the use of these tests to identify an unknown compound.
A focused answer to the OCR Gateway GCSE Combined Science A topic C4 on identifying substances, covering tests for hydrogen, oxygen, carbon dioxide and chlorine, flame tests for metal ions, and tests for carbonate, halide and sulfate ions to identify an unknown compound.
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What this topic is asking
OCR wants you to describe the tests for hydrogen, oxygen, carbon dioxide and chlorine, carry out flame tests for metal ions, test for carbonate, halide and sulfate ions, and use these tests to identify an unknown compound.
Tests for gases
Note carefully the difference between the two splint tests: a lighted (burning) splint is used for hydrogen (and gives a pop), while a glowing splint is used for oxygen (and is relit). Confusing them is a common reason for losing a mark. These gas tests are used throughout chemistry, for example to confirm the gas given off when an acid reacts with a metal (hydrogen) or a carbonate (carbon dioxide).
Flame tests for metal ions
A flame test identifies some metal ions by the colour they give to a flame. The method is to dip a clean wire loop (cleaned by dipping in acid and holding it in the flame until there is no colour) into the sample, then hold it in the edge of a roaring blue Bunsen flame and observe the colour.
- Lithium ions give a crimson (red) flame.
- Sodium ions give a yellow flame.
- Potassium ions give a lilac (purple) flame.
- Calcium ions give an orange-red flame.
- Copper ions give a green flame.
The wire must be clean each time, because contamination (especially by sodium, which gives a strong yellow) masks other colours.
Tests for negative ions
OCR expects three tests for negative ions (anions):
- Carbonate ions: add dilute acid; carbonates fizz and release carbon dioxide, confirmed by turning limewater cloudy.
- Halide ions (chloride, bromide, iodide): add dilute nitric acid then silver nitrate solution; a precipitate forms whose colour identifies the halide. Chloride gives a white precipitate, bromide a cream precipitate, and iodide a yellow precipitate.
- Sulfate ions: add dilute hydrochloric acid then barium chloride solution; a white precipitate of barium sulfate forms.
By combining a flame test (for the metal ion) with an anion test, you can identify the full name of an unknown ionic compound, which is a frequent practical exam task.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksDescribe the tests, and the positive results, for hydrogen gas and for carbon dioxide gas.Show worked answer →
A Chemistry Paper 4 recall question on gas tests. Reward: for hydrogen, hold a lighted (burning) splint at the mouth of the test tube; hydrogen burns rapidly with a squeaky pop. For carbon dioxide, bubble the gas through limewater (calcium hydroxide solution); carbon dioxide turns the limewater cloudy (milky). Markers credit the correct apparatus and the correct positive result for each gas. Common slips are confusing the lighted splint test (hydrogen, pop) with the glowing splint test (oxygen, relights), so state which splint is used.
OCR 20214 marksAn unknown white solid is thought to be a metal carbonate. Describe a flame test to identify the metal ion, and a test to confirm the carbonate ion is present.Show worked answer →
A C4 question on identifying ions. Reward: for the flame test, dip a clean wire loop into the solid and hold it in a blue Bunsen flame; the flame colour identifies the metal ion (for example lithium gives crimson or red, sodium gives yellow, potassium gives lilac, calcium gives orange-red, copper gives green). For the carbonate test, add dilute acid to the solid; if a carbonate is present it fizzes and gives off carbon dioxide, which is confirmed by bubbling it through limewater, turning it cloudy. Markers credit the clean wire and flame colour for the metal ion, and the acid producing carbon dioxide (confirmed with limewater) for the carbonate.
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