CIE iGCSE Co-Ordinated Science C2.1 Elements, compounds and mixtures Exam Style Questions Paper 3
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C11.4 — Alkanes (Part (a)(i), (a)(ii), (a)(iii) & (a)(iv))
• Topic C3.2 — Relative masses of atoms and molecules (Part (b))
• Topic C2.3 — Covalent bonding (Part (c))
• Topic C9.1 — Fossil fuels (Part (d))
• Topic C2.1 — Elements, compounds and mixtures (Part (e))
▶️ Answer/Explanation
(a)(i)
methane + oxygen $\rightarrow$ carbon dioxide + water
Complete combustion requires a plentiful supply of oxygen and converts all the carbon in the hydrocarbon to carbon dioxide and all the hydrogen to water. The balanced symbol equation is $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$, confirming a 1:2 molar ratio of methane to oxygen.
(a)(ii)
carbon monoxide (also accept: carbon / soot)
Incomplete combustion occurs when the oxygen supply is limited, preventing full oxidation of the carbon atoms. Carbon monoxide ($\text{CO}$) is a colourless, odourless, and highly toxic gas produced as a result, making it a serious health hazard in poorly ventilated spaces such as rooms with gas appliances.
(a)(iii)
Methane contains only single covalent bonds between carbon and hydrogen atoms; there are no carbon-carbon double or triple bonds.
A saturated hydrocarbon is one in which every carbon atom forms the maximum number of single bonds with other atoms, leaving no possibility for addition reactions. Because methane has only one carbon atom bonded to four hydrogen atoms by single bonds, it meets the definition of a saturated compound.
(a)(iv)
orange / brown to colourless
Bromine water is orange-brown in colour due to dissolved bromine molecules. When mixed with an unsaturated hydrocarbon, bromine adds across the carbon-carbon double bond in an addition reaction, consuming the bromine and removing its colour. A saturated hydrocarbon would leave the orange colour unchanged since it cannot undergo this reaction.
(b)
$M_r = A_r(\text{C}) + 4 \times A_r(\text{H})$
$M_r = 12 + (4 \times 1)$
$\boxed{M_r = 16}$
Relative molecular mass is found by adding together the relative atomic masses of all atoms present in one molecule. Methane contains one carbon atom ($A_r = 12$) and four hydrogen atoms ($A_r = 1$ each), giving a total of 16.
(c)
The completed diagram shows the carbon atom at the centre with four pairs of shared electrons, one pair bonded to each of the four surrounding hydrogen atoms. Each hydrogen contributes one electron (shown as a cross) and carbon contributes one electron (shown as a dot) to each shared pair. All four bonds are single covalent bonds, giving carbon a full outer shell of 8 electrons and each hydrogen a full shell of 2 electrons.
(d)
coal (also accept: crude oil / petroleum)
Fossil fuels are non-renewable energy sources formed over millions of years from the remains of ancient organisms buried under heat and pressure. Coal, crude oil, and natural gas are the three main fossil fuels; burning them releases carbon dioxide, contributing to the greenhouse effect and global warming.
(e)
Carbon is an element because it contains only one type of atom.
Methane is a compound because it contains two or more different types of atoms that are chemically combined.
An element cannot be broken down into simpler substances by chemical means, whereas a compound can be decomposed into its constituent elements. Methane contains carbon and hydrogen atoms bonded together in a fixed ratio of $1:4$, which is characteristic of a pure compound.
Question
☐ copper forms coloured compounds
☐ copper has a low melting point
☐ copper has a low density
☐ copper is ductile
Topic codes:
• Topic C2.2 — Atomic structure and the Periodic Table (Part (a)(i) & (a)(ii))
• Topic C8.4 — Transition elements (Part (b))
• Topic C3.1 — Formulas (Part (c)(i))
• Topic C2.1 — Elements, compounds and mixtures (Part (c)(ii))
• Topic C9.3 — Alloys and their properties (Part (d)(i), (d)(ii) & (d)(iii))
▶️ Answer/Explanation
(a)(i) Proton number (atomic number)
The number 29 is the proton number (also called atomic number) of copper. This is the number of protons in the nucleus of a copper atom. All atoms of the same element have the same proton number.
(a)(ii) 35 neutrons
Number of neutrons = mass number − proton number = 64 − 29 = 35.
The mass number (64) is the total number of protons and neutrons in the nucleus.
(b) The three correct statements are:
✓ copper acts as a catalyst
✓ copper forms coloured compounds
✓ copper is ductile
Transition elements are metals with high densities and high melting points (not low). They form coloured compounds, often act as catalysts, and are ductile (can be drawn into wires).
(c)(i) 2Cu(s) + O₂(g) → 2CuO(s)
The balanced equation shows 2 copper atoms reacting with 1 oxygen molecule to form 2 copper(II) oxide units. This ensures the number of atoms of each element is the same on both sides: 2 Cu on both sides, and 2 O on both sides.
(c)(ii) Gas
The symbol (g) indicates that oxygen is in the gaseous state at room temperature and pressure.
(d)(i) An alloy is a mixture of a metal with other elements.
Alloys are mixtures of metals with other metals or non-metals to enhance specific properties such as strength, hardness, or corrosion resistance.
(d)(ii) Structure Y
Structure Y best represents an alloy because it shows atoms of different sizes arranged in a metallic lattice. In alloys, atoms of different sizes disrupt the regular arrangement of atoms, making it harder for layers to slide over each other, which increases hardness and strength.
(d)(iii) Stainless steel is used in cutlery because of its hardness and resistance to rusting (corrosion).
Stainless steel is an alloy of iron with chromium, nickel, and carbon. It is hard and does not rust easily, making it ideal for knives, forks, and other cutlery items that need to maintain a sharp edge and withstand frequent washing.