How do chemists test for substances and monitor the environment?
Chemical analysis: general practical techniques, gas tests and flame tests, paper chromatography, and the use of analysis to monitor the environment.
An SQA National 5 Chemistry answer on chemical analysis, covering general practical techniques, the standard gas tests for hydrogen, oxygen and carbon dioxide, flame tests for metal ions, paper chromatography, and the use of analysis to monitor the environment.
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What this key area is asking
The SQA wants you to use general practical techniques, carry out the standard gas tests and flame tests, describe paper chromatography and read its results, and explain how chemical analysis is used to monitor the environment. It rewards precise practical detail: a test is only worth a mark if its result is stated too.
General practical techniques
The standard gas tests
A test only scores if you give the result as well as the method.
Flame tests
Paper chromatography
A single substance gives one spot; a mixture gives several, so counting the spots tells you how many substances are present.
Worked example: identifying an unknown gas
Monitoring the environment
Chemical analysis is used to monitor the environment: testing river and drinking water for pollutants and dissolved ions, measuring pH, and checking air quality. The data is compared with safe limits and used to detect and control pollution, which links this key area to the eutrophication problem of the fertilisers key area.
Examples in context
These techniques are the everyday tools of analytical chemists. Forensic scientists use chromatography to compare inks or separate the substances in a sample found at a crime scene; food scientists use it to check for permitted colourings. Gas tests are the quick, reliable checks done routinely in any school or industrial lab, and water-testing laboratories use analysis to make sure drinking water is safe and rivers are not polluted.
Try this
Q1. State the test and result for carbon dioxide. [1 mark]
- Cue. Bubble through limewater; it turns cloudy (milky).
Q2. Explain how paper chromatography shows that a dye contains more than one substance. [2 marks]
- Cue. The components travel at different speeds and separate into several spots; more than one spot means more than one substance.
Q3. Name the gas that relights a glowing splint. [1 mark]
- Cue. Oxygen.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 2019 style3 marksDescribe the test, including the result, that you would use to identify each of the following gases: hydrogen, oxygen and carbon dioxide.Show worked answer →
Markers reward each test with its correct positive result.
Hydrogen: hold a lighted splint at the mouth of the test tube. A positive result is a squeaky pop as the hydrogen burns.
Oxygen: place a glowing splint into the test tube. A positive result is the glowing splint relighting, because oxygen supports combustion.
Carbon dioxide: bubble the gas through limewater. A positive result is the limewater turning cloudy (milky). Each test must be paired with its result to earn the mark.
SQA N5 2021 style3 marksA student uses paper chromatography to separate the colours in a food dye. Explain how chromatography separates the substances, and explain how the student could tell whether the dye contained one substance or several.Show worked answer →
A 3 mark answer needs how the separation works and how to read the result.
In paper chromatography a spot of the mixture is placed on paper and a solvent moves up through it. The different substances in the mixture travel at different speeds, because some are more soluble in the solvent and held less strongly by the paper, so they separate into spots at different heights.
If the dye is a single substance it produces one spot, while if it contains several substances it separates into several spots. Counting the spots therefore tells the student how many different coloured substances are present.
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Sources & how we know this
- SQA National 5 Chemistry Course Specification — SQA (2019)