CIE iGCSE Co-Ordinated Science C2.5 Simple molecules and covalent bonds Exam Style Questions Paper 1
Question
Which statement about a carbon dioxide molecule is correct?
A. It is composed of metallic elements, which are covalently bonded.
B. It is composed of metallic elements, which are ionically bonded.
C. It is composed of non-metallic elements, which are covalently bonded.
D. It is composed of non-metallic elements, which are ionically bonded.
▶️Answer/Explanation
Answer: C
Explanation: Carbon dioxide (CO₂) is composed of non-metallic elements (carbon and oxygen) that are covalently bonded, meaning they share electrons.
Question
Which row describes the properties of a covalent compound?
| Melting Point (°C) | Solubility in Water | Undergoes Electrolysis When Molten |
|---|---|---|
| A. 100 | Insoluble | No |
| B. 110 | Soluble | Yes |
| C. 950 | Insoluble | No |
| D. 2200 | Soluble | Yes |
▶️Answer/Explanation
Answer: A. 100, Insoluble, No
Explanation: Covalent compounds generally have low melting points (e.g., 100°C), are often insoluble in water, and do not conduct electricity or undergo electrolysis when molten.
Question
A model of a molecule is shown.
Which row shows the formula of this molecule and describes the type of bonding between the atoms?
A 2BH3 / covalent
B 2BH3 / ionic
C B2H6 / covalent
D B2H6 / ionic
▶Answer/Explanation
Answer: C
Explanation: The molecule shown is B2H6, and the bonding between boron and hydrogen is covalent, as they share electrons.
Question
Which diagram represents molecules of a compound?
▶️Answer/Explanation
Ans : B
Question
The structures of some compounds are shown.
In which rows does the formula match the structure?
A 1 and 2 B 1 and 3 C 2 and 4 D 3 and 4
▶️Answer/Explanation
Ans : B
Question
Which dot-and-cross diagram represents a molecule of ammonia?
▶Answer/Explanation
Ans : C
Question
Which substances exist as a covalent molecules?
1. helium
2. chlorine
3. sodium chloride
4. ethanol
(A) 1 and 2 (B) 1 and 3 (C) 2 and 4 (D) 3 and 4
▶️Answer/Explanation
Ans: C
Question
What is the order of reactivity of metals, from highest to lowest?
(A) aluminium → calcium → copper → iron
(B) calcium → iron → copper → aluminium
(C) calcium → magnesium → iron → copper
(D) aluminium → magnesium → iron → copper
▶️Answer/Explanation
Ans: C
Question
The electronic structures of carbon and of hydrogen are shown.
▶️Answer/Explanation
Ans: D
Question
Which row describes a covalent compound?
| solubility in water | volatility | |
|---|---|---|
| A | high | low |
| B | high | high |
| C | low | low |
| D | low | high |
▶️Answer/Explanation
Answer: D. low, high
Explanation: Covalent compounds generally have low solubility in water and high volatility due to weak intermolecular forces.
Question
Which dot-and-cross diagram represents a molecule of ammonia?
▶️Answer/Explanation
Answer: C) Diagram C
Explanation: Ammonia (NH3) has one nitrogen atom sharing three electrons with three hydrogen atoms, forming three covalent bonds. Diagram C correctly shows this structure.
Question
What is a property of a typical covalent compound?
A. electrical insulator
B. high melting point
C. low volatility
D. soluble in water
▶️Answer/Explanation
Answer: A
Explanation: Covalent compounds are typically electrical insulators because they do not have free electrons or ions to conduct electricity. They often have low melting points, are volatile, and may or may not be soluble in water depending on their polarity.
Question
What is the dot-and-cross diagram for a water molecule?
▶️Answer/Explanation
Answer: A
Explanation: The correct dot-and-cross diagram for a water molecule (H₂O) shows two hydrogen atoms sharing electrons with one oxygen atom. Diagram A correctly represents this arrangement.
Question
In which molecule are all the outer electrons of the atoms used in covalent bonds?
A. CH4
B. HCl
C. H2O
D. NH3
▶️Answer/Explanation
Answer: A. CH4
Explanation: In methane (CH4), all the outer electrons of the carbon atom are used in forming covalent bonds with hydrogen atoms. This results in a stable molecule with no lone pairs of electrons on the carbon atom.