Some plants produce ethene gas.
(a) (i) Draw the displayed formula for a molecule of ethene.
(ii) The incomplete combustion of ethene produces a small amount of carbon dioxide. Name two other products of the incomplete combustion of ethene.
(b) A student extracts a mixture of coloured compounds from a plant. Fig. 4.1 shows the results of chromatography of this mixture.
(i) Complete Fig. 4.1, to show:
● where the mixture of coloured compounds is placed on the chromatography paper at the start of the chromatography
● the level of the solvent at the start of the chromatography.
(ii) State two characteristics of a mixture.
Most-appropriate topic codes (Cambridge IGCSE Chemistry 0620):
• Topic 11.5 — Alkenes (Part (a)(i))
• Topic 10.3 — Air quality and climate / Organic combustion (Part (a)(ii))
• Topic 12.3 — Chromatography (Part (b)(i))
• Topic 2.1 — Elements, compounds and mixtures (Part (b)(ii))
(a)(i)
For the correct displayed formula: The structure should show a carbon-carbon double bond (C=C), with each carbon atom bonded to two hydrogen atoms. All bonds must be clearly drawn as lines.
Ethene (C₂H₄) is an unsaturated hydrocarbon containing a double covalent bond between the two carbon atoms, which is the defining feature of the alkene functional group.
(a)(ii)
carbon monoxide and carbon (or water). Any two of: carbon monoxide / CO, carbon (soot) / C, water / H₂O.
Incomplete combustion occurs when there is a limited supply of oxygen. Instead of forming only carbon dioxide and water, ethene also produces toxic carbon monoxide and particulates of carbon (soot).
(b)(i)
For the correct diagram: 1. A spot or cross (the origin) drawn on the baseline vertically below the separated spots. 2. The solvent level (meniscus) drawn as a horizontal line below this spot, showing the paper dipping into the solvent.
The baseline must be drawn in pencil (insoluble) so the components do not dissolve into the solvent before the run begins. The solvent level must start below the sample spots to ensure the components are eluted up the paper, not washed off.
(b)(ii)
Any two characteristics: (1) does not have a fixed composition / variable composition; (2) the components are not chemically combined; (3) can be separated by physical means; (4) each component retains its own properties.
Unlike compounds, which have fixed formulas and require chemical reactions to separate, mixtures are physical blends of substances. Air, crude oil, and plant extracts are common examples of mixtures.
