The diagrams show part of the structures of five substances, $\mathbf{A}, \mathbf{B}, \mathbf{C}, \mathbf{D}$ and $\mathbf{E}$.
(a) Answer the following questions about these structures.
Each structure may be used once, more than once or not at all.
(i) Which two of these structures, $\mathbf{A}, \mathbf{B}, \mathbf{C}, \mathbf{D}$ or $\mathbf{E}$, are covalently bonded? [2]
(ii) Which one of these structures, $\mathbf{A}, \mathbf{B}, \mathbf{C}, \mathbf{D}$ or $\mathbf{E}$, is a diatomic molecule? [1]
(iii) Which one of these structures, $\mathbf{A}, \mathbf{B}, \mathbf{C}, \mathbf{D}$ or $\mathbf{E}$, is a compound? [1]
(iv) Which one of these structures, $\mathbf{A}, \mathbf{B}, \mathbf{C}, \mathbf{D}$ or $\mathbf{E}$, is very soluble in water? [1]
(v) Which one of these structures, $\mathbf{A}, \mathbf{B}, \mathbf{C}, \mathbf{D}$ or $\mathbf{E}$, is used in cutting tools? [1]
(vi) Which one of these structures, $\mathbf{A}, \mathbf{B}, \mathbf{C}, \mathbf{D}$ or $\mathbf{E}$, is used in electrical wiring? [1]
(b) Substance $\mathbf{B}$ is an element.
What is meant by the term element? [1] [Total: 8]
▶️ Answer/Explanation
(a)(i) Ans: B and D
B (diamond) has a giant covalent structure, and D (N₂) has a simple covalent bond.
(a)(ii) Ans: D
D (N₂) is a diatomic molecule consisting of two nitrogen atoms bonded together.
(a)(iii) Ans: C
C (LiCl) is a compound because it consists of lithium and chlorine chemically bonded.
(a)(iv) Ans: C
C (LiCl) is very soluble in water due to its ionic nature, which allows it to dissociate into ions.
(a)(v) Ans: B
B (diamond) is extremely hard and is used in cutting tools.
(a)(vi) Ans: E
E (copper) is a good conductor of electricity and is used in electrical wiring.
(b) Ans: An element is a substance in which all the atoms have the same proton number or a substance containing only one type of atom.
The table shows some properties of four Group I elements.
(a)(i) Complete the table by estimating:
-
- the boiling point of sodium
- the relative hardness of lithium. [2]
(ii) Predict the physical state of lithium at 200°C.
Give a reason for your answer. [2]
(b) Potassium reacts with water.
2K + 2H2O → 2KOH + H2
Describe two observations when potassium reacts with water.[2]
(c) Lithium is extracted by the electrolysis of molten lithium chloride.
(i) Name a non-metal used to make the electrodes.[1]
(ii) Give one property, other than the conduction of electricity, that makes this substance suitable for use as an electrode.[1]
(iii) State the products of the electrolysis of molten lithium chloride at:[2]
the negative electrode (cathode)
the positive electrode (anode)
(d) Lithium chloride conducts electricity when molten and when in aqueous solution.
Give two other physical properties of lithium chloride that show it is an ionic compound.[2]
▶️ Answer/Explanation
(a)(i) Ans: Boiling point of Na ≈ 900°C; Hardness of Li ≈ 1.5 MPa
From the trend in Group I, sodium’s boiling point lies between potassium (774°C) and lithium (1342°C). Lithium’s hardness is intermediate (softer than Na but harder than K).
(a)(ii) Ans: Liquid; 200°C is between Li’s melting (180.5°C) and boiling points (1342°C)
Lithium melts at 180.5°C and boils at 1342°C, so at 200°C, it exists as a liquid.
(b) Ans: (1) Effervescence (H₂ gas bubbles); (2) Lilac flame (from burning H₂)
Potassium reacts violently with water, producing hydrogen gas (bubbles) and enough heat to ignite the gas, showing a characteristic lilac flame.
(c)(i) Ans: Graphite
Graphite is commonly used as an inert electrode due to its conductivity and high melting point.
(c)(ii) Ans: High melting point
Graphite remains solid at the high temperatures required for electrolysis of molten LiCl.
(c)(iii) Ans: Cathode: Lithium metal; Anode: Chlorine gas
During electrolysis, Li⁺ ions gain electrons at the cathode (reduction), while Cl⁻ ions lose electrons at the anode (oxidation).
(d) Ans: (1) High melting point; (2) Soluble in water
Ionic compounds like LiCl have strong electrostatic forces (high melting points) and dissociate in polar solvents like water.