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CIE iGCSE Co-Ordinated Science C7.2 Oxides Exam Style Questions Paper 3

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Question

(a) A calcium atom has a proton number of 20 and a nucleon number of 40.
Deduce the number of protons, neutrons and electrons in this atom of calcium.
(b) When calcium atoms react, they change into calcium ions, \(\text{Ca}^{2+}\).
Describe how calcium atoms change into calcium ions.
(c) Lime is produced from limestone, calcium carbonate, by thermal decomposition.
(i) State the chemical name for lime.
(ii) State the name of the gas formed in this thermal decomposition reaction.
(iii) Explain why farmers spread limestone onto soil that is used for growing crops.
(d) The main compound in limestone is calcium carbonate, \(\text{CaCO}_3\).
(i) State the number of different elements in calcium carbonate.
(ii) State the total number of atoms shown in the formula \(\text{CaCO}_3\).

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic C2.2 — Atomic structure and the Periodic Table (Part (a))
• Topic C2.4 — Ions and ionic bonds (Part (b))
• Topic C7.2/C9.6 — Oxides / thermal decomposition (Part (c)(i)–(ii))
• Topic C7.1 — Characteristic properties of acids and bases: neutralisation (Part (c)(iii))
• Topic C2.1 — Elements, compounds and mixtures (Part (d))

▶️ Answer/Explanation

(a) Protons = 20; neutrons = 20; electrons = 20

The proton number (20) gives the number of protons, which equals the number of electrons in a neutral atom.
The number of neutrons is found from \(\text{nucleon number} – \text{proton number} = 40 – 20 = 20\).

(b) Calcium atoms lose two electrons to form \(\text{Ca}^{2+}\) ions

A calcium atom has two electrons in its outer shell.
It loses these two electrons, forming a positively charged calcium ion, \(\text{Ca}^{2+}\).

(c)(i) Calcium oxide

Heating calcium carbonate causes it to break down (thermally decompose).
The solid product formed is calcium oxide, commonly known as lime.

(c)(ii) Carbon dioxide

The thermal decomposition of calcium carbonate is represented by \(\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2\).
The gas released in this reaction is carbon dioxide.

(c)(iii) Limestone neutralises acidic soil, making it more suitable for growing crops

Many soils become too acidic for crops to grow well.
Limestone is a base, so it reacts with and neutralises the excess acid in the soil.

(d)(i) 3

The formula \(\text{CaCO}_3\) contains the elements calcium, carbon, and oxygen.
This gives a total of three different elements.

(d)(ii) 5

The formula \(\text{CaCO}_3\) shows 1 calcium atom, 1 carbon atom, and 3 oxygen atoms.
Adding these together gives a total of \(1 + 1 + 3 = 5\) atoms.

Question

(a) Sodium forms a basic oxide. Carbon forms acidic oxides. State why they are different.
(b) Carbon is a solid and carbon dioxide is a gas.
Describe the differences between a solid and a gas using ideas about particle separation and particle motion.
(c) Diamond is one form of carbon. Fig. 11.1 shows the arrangement of carbon atoms in diamond.
Circle two words from the list to describe the structure and bonding in diamond.
simple giant metallic ionic covalent polymer
(d)(i) Sodium metal reacts with chlorine gas to make sodium chloride.
Balance the symbol equation for this reaction.
……..Na + \(\text{Cl}_2\) → ……..NaCl
(ii) During the reaction sodium atoms form sodium ions, \(\text{Na}^+\), and chlorine atoms form chloride ions, \(\text{Cl}^-\).
Fig. 11.2 shows the electronic structure of a sodium ion and a chloride ion.
Write down the electronic structure of a sodium atom and a chlorine atom.
(iii) Sodium and lithium are both in Group I of the Periodic Table.
Sodium reacts violently with water.
Describe the reaction of lithium with water.
Describe the trend in the reactivity of Group I elements as shown by sodium and lithium.

Most-appropriate topic codes (Cambridge IGCSE Co-ordinated Sciences 0654):

• Topic C7.2 — Oxides (Part (a))
• Topic C1.1 — Solids, liquids and gases (Part (b))
• Topic C2.6 — Giant covalent structures (Part (c))
• Topic C2.4 — Ions and ionic bonds (Part (d)(i), (ii))
• Topic C8.2 — Group I properties (Part (d)(iii))

▶️ Answer/Explanation

(a) non-metals form acidic oxides; metals form basic oxides

Sodium is a metal, and metals typically form basic oxides.
Carbon is a non-metal, and non-metals typically form acidic oxides.

(b) particle separation: solid closer together, gas further apart; particle motion: solid vibrates about a fixed point, gas moves freely and faster

In a solid, particles are packed closely together and only vibrate about fixed positions.
In a gas, particles are far apart and move freely and rapidly in random directions.

(c) giant; covalent

Diamond consists of a giant covalent (macromolecular) structure.
Each carbon atom forms four strong covalent bonds with neighbouring carbon atoms.

(d)(i) \(2\text{Na} + \text{Cl}_2 \rightarrow 2\text{NaCl}\)

The equation must be balanced so the number of atoms of each element is equal on both sides.
Using a coefficient of 2 for both sodium and sodium chloride balances the equation.

(d)(ii) sodium atom: 2,8,1; chlorine atom: 2,8,7

A sodium atom has the electronic structure 2,8,1, with one electron in its outer shell.
A chlorine atom has the electronic structure 2,8,7, with seven electrons in its outer shell.
During the reaction, sodium loses its outer electron to chlorine, forming ions with full outer shells.

(d)(iii) reaction: lithium reacts less rapidly/vigorously than sodium; explanation: reactivity increases down Group I

Lithium reacts with water, but less vigorously and more slowly than sodium.
This shows that as you go down Group I, the reactivity of the metals increases.

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