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GCSE / IGCSE Chemistry – Analysis and the Atmosphere

Chromatography, gas and ion tests, the atmosphere, resources.

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12 questions  ·  35 marks  ·  answers included  ·  GCSE / IGCSE Chemistry (Edexcel/AQA/Cambridge style)
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This worksheet covers:

  • Questions on chromatography
  • Tests for gases and ions
  • The composition and evolution of the atmosphere
  • The use of the Earth’s resources

A calculator may be used where numerical answers are required.

Questions

Section A – Chromatography and Chemical Tests

1

In a paper chromatography experiment testing food dyes, a spot travels $6.4\,\text{cm}$ from the baseline while the solvent front travels $8.0\,\text{cm}$.(a) Calculate the $R_f$ value of the dye.(b) Explain how $R_f$ values can be used to identify an unknown substance.

Show answer
(a) $R_f=\dfrac{\text{distance travelled by spot}}{\text{distance travelled by solvent}}=\dfrac{6.4}{8.0}=0.80$.(b) Each substance has a characteristic $R_f$ value in a given solvent; by comparing the $R_f$ value of an unknown spot with the $R_f$ values of known reference substances run in the same solvent, the unknown substance can be identified.
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2

Describe the chemical test, and the positive result, for each of the following gases: (a) hydrogen, (b) oxygen, (c) carbon dioxide.

Show answer
(a) Hydrogen: hold a lit splint at the open end of a test tube of the gas; a squeaky pop confirms hydrogen.(b) Oxygen: insert a glowing splint into the test tube; the splint relights, confirming oxygen.(c) Carbon dioxide: bubble the gas through limewater (aqueous calcium hydroxide); the limewater turns cloudy/milky, confirming carbon dioxide.
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3

Describe a chemical test, including the reagent(s) used and the positive result, for the presence of (a) chloride ions and (b) sulfate ions in an aqueous solution.

Show answer
(a) Chloride ions: add dilute nitric acid followed by silver nitrate solution; a white precipitate (of silver chloride) confirms chloride ions.(b) Sulfate ions: add dilute hydrochloric acid followed by barium chloride solution; a white precipitate (of barium sulfate) confirms sulfate ions.
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4

Describe a flame test used to identify metal ions, and state the flame colour produced by (a) sodium ions and (b) copper(II) ions.

Show answer
A clean nichrome/platinum wire loop is dipped in the sample and held in a blue Bunsen flame; the colour of the flame produced identifies the metal ion present.(a) Sodium ions produce a yellow/orange flame.(b) Copper(II) ions produce a blue-green (green-blue) flame.
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Section B – The Atmosphere

5

State the approximate percentage, by volume, of nitrogen and of oxygen in the Earth’s present-day atmosphere.

Show answer
Nitrogen makes up approximately $78\%$ and oxygen approximately $21\%$ of the atmosphere by volume (with the remainder mostly argon, about $0.9\%$, and a small amount of carbon dioxide, about $0.04\%$).
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6

A sample of air of volume $500\,\text{cm}^3$ is assumed to contain $21\%$ oxygen by volume. Calculate the volume of oxygen present in this sample.

Show answer
$V(\text{O}_2)=500\times\dfrac{21}{100}=105\,\text{cm}^3$.
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7

Describe, in outline, the theory for how the Earth’s early atmosphere is thought to have evolved from being dominated by carbon dioxide (with little or no oxygen) to today’s atmosphere, mentioning the role of algae and plants.

Show answer
The early atmosphere was produced mainly by volcanic activity and was thought to consist mostly of carbon dioxide, with little or no oxygen, together with water vapour, nitrogen and small amounts of other gases. As the Earth cooled, water vapour condensed to form the oceans, and much of the carbon dioxide dissolved into the oceans or was locked up in sedimentary rocks and fossil fuels. Algae and, later, plants evolved and carried out photosynthesis, removing carbon dioxide from the atmosphere and releasing oxygen, gradually increasing the oxygen level to approximately today’s value over billions of years.
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8

Explain how increasing levels of atmospheric carbon dioxide and methane, produced by human activities, contribute to global warming (the enhanced greenhouse effect).

Show answer
Carbon dioxide and methane are greenhouse gases: they allow short-wavelength radiation from the Sun to pass through the atmosphere and warm the Earth’s surface, but absorb the longer-wavelength infrared radiation re-emitted by the surface and re-radiate some of it back down, trapping additional thermal energy. Human activities such as burning fossil fuels and agriculture/deforestation have increased the concentrations of these gases, enhancing this natural greenhouse effect and causing average global temperatures to rise.
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Section C – Using the Earth’s Resources

Exam-style
9

Distinguish between a finite resource and a renewable resource, giving one example of each in the context of energy or materials.

Show answer
A finite resource is formed much more slowly than it is used, so it will eventually run out (e.g. crude oil, coal, natural gas, and metal ores). A renewable resource is replenished naturally at a rate comparable to or faster than its rate of use, so it will not run out (e.g. wood from sustainably managed forests, or wind and solar energy).
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10

Explain, using the terms reduce, reuse and recycle, how the environmental impact of using limited raw materials such as metals can be lessened.

Show answer
Reduce: using less of a raw material in the first place, for example through more efficient design or lighter products, conserves the finite resource. Reuse: using an item again for the same or a different purpose (e.g. reusing glass bottles) avoids the energy and resources needed to manufacture a new one. Recycle: melting down and reprocessing used materials (e.g. scrap metal) to make new products reduces the amount of ore that needs to be mined and the energy used compared with extracting and processing new raw material.
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11

A local water treatment plant makes water safe to drink using sedimentation, filtration and chlorination. Explain the purpose of each of these three stages.

Show answer
Sedimentation: larger insoluble particles are allowed to settle out under gravity, forming a sediment that is removed. Filtration: the water is passed through a filter bed (e.g. sand) to remove any remaining small suspended solid particles. Chlorination: chlorine is added to kill any remaining microorganisms (pathogens), making the water safe (potable) to drink.
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12

Life-cycle assessment (LCA) is used to evaluate the overall environmental impact of a product. Describe two stages of a product’s life cycle that would be considered in an LCA, and explain one limitation of using LCAs to compare products.

Show answer
Any two stages, e.g.: extracting and processing the raw materials; manufacturing and packaging the product; using/operating the product; and disposing of the product at the end of its life (which may include transport at each stage). Limitation: some data used in an LCA, such as the exact energy used or pollution produced at each stage, can be difficult to measure accurately or may be estimated, and assigning a numerical value to environmental damage or to social factors involves judgements that are open to bias, so LCAs comparing similar products can be used selectively to support a particular conclusion.
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