CIE iGCSE Co-Ordinated Science C6.1 Physical and chemical changes Exam Style Questions Paper 3
Question
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C11.3 — Fuels: fractional distillation of petroleum (Part (a))
• Topic C11.3 — Fossil fuels (Part (b)(i))
• Topic C10.2 — Air quality: greenhouse gases (Part (b)(ii))
• Topic C6.1 — Physical and chemical changes (Part (b)(iii))
▶️ Answer/Explanation
(a)(i) Refinery gas
Fraction X has the lowest boiling point and is collected at the top of the fractionating column.
This fraction is refinery gas, used for heating and cooking.
(a)(ii) Used as a chemical feedstock for making other chemicals
Naphtha is not only burned as a fuel.
It is also used as a feedstock to manufacture other chemical products, such as plastics.
(a)(iii) Fractional distillation
Petroleum is a mixture of hydrocarbons with different boiling points.
Fractional distillation separates these hydrocarbons into useful fractions based on their boiling point ranges.
(a)(iv) Mass of diesel oil = 130 kg
Diesel oil makes up 26% of the 500 kg sample.
Mass \(= \dfrac{26}{100} \times 500 = 130\,\text{kg}\).
(a)(v) Carbon and hydrogen
Petroleum fractions are hydrocarbons.
Hydrocarbons are compounds that contain only the elements carbon and hydrogen.
(b)(i) Coal or natural gas
The three fossil fuels are coal, natural gas, and petroleum.
Since petroleum is already given, either coal or natural gas is an acceptable answer.
(b)(ii) Methane
Methane is a common greenhouse gas released from sources such as decomposing vegetation and animal digestion.
It is therefore an acceptable alternative answer to carbon dioxide.
(b)(iii) Chemical change cannot easily be reversed by physical means; chemical change produces new substances
In a chemical change, new substances with different properties are formed, and the change is usually difficult to reverse.
In a physical change, no new substance is formed and the change can usually be reversed easily, for example by cooling or condensing.
Question
Look at the symbol equation for the reaction between hydrogen and oxygen.
This equation is not balanced.
Oxygen is also made.
Balance the symbol equation for this reaction.
Show only the outer-shell electrons.
Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):
• Topic C10.2 — Air quality and climate (Part (a))
• Topic C8.5 — Noble gases (Part (b))
• Topic C6.1 — Physical and chemical changes (Parts (c)(i)–(ii))
• Topic C2.5 — Simple molecules and covalent bonds (Parts (c)(iii)–(iv))
• Topic C10.1 — Water (Part (c)(v))
▶️ Answer/Explanation
(a) oxygen = 21%; nitrogen = 78%
Clean, dry air is mostly nitrogen and oxygen.
These are the standard accepted percentages by volume.
(b) e.g. name: argon; use: filling filament lamps
Noble gases are unreactive (inert), which makes them useful in specific applications.
Argon is used inside filament lamps to prevent the hot filament from reacting with oxygen.
(c)(i) The number of oxygen atoms differs on each side
On the left-hand side there are 2 oxygen atoms (from \(\text{O}_2\)).
On the right-hand side there is only 1 oxygen atom (in \(\text{H}_2\text{O}\)), so the equation is unbalanced.
(c)(ii) \(2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2\)
Placing a coefficient of 2 in front of both \(\text{H}_2\text{O}_2\) and \(\text{H}_2\text{O}\) balances the atoms.
This gives 4 H atoms and 4 O atoms on each side.
(c)(iii) 
Each hydrogen atom shares one electron with oxygen to form a covalent bond.
Oxygen keeps two lone (non-bonding) pairs of outer-shell electrons.
(c)(iv) covalent (bonds)
Covalent bonding involves the sharing of electron pairs between non-metal atoms.
Hydrogen and oxygen are both non-metals, forming covalent bonds in water.
(c)(v) anhydrous copper sulfate turns from white to blue (or cobalt chloride paper turns from blue to pink)
Water reacts with anhydrous copper(II) sulfate to form hydrated copper sulfate.
The colour change from white to blue confirms the presence of water.