CIE iGCSE Co-Ordinated Science C2.5 Simple molecules and covalent bonds Exam Style Questions Paper 1
▶️ Answer/Explanation
For a compound to conduct electricity when molten, it must contain ions that are free to move. Ionic compounds conduct electricity when molten because the ions are free to move and carry charge. Covalent compounds do not conduct electricity because they have no free ions or electrons.
Looking at each option:
- A. carbon and hydrogen — These are both non-metals, so they form covalent compounds (e.g., methane, \( \text{CH}_4 \)). Covalent compounds do not conduct electricity when molten because they have no free ions.
- B. lithium and bromine — Lithium is a metal (Group I) and bromine is a non-metal (Group VII). They form an ionic compound (lithium bromide, LiBr). Ionic compounds conduct when molten.
- C. magnesium and chlorine — Magnesium is a metal (Group II) and chlorine is a non-metal (Group VII). They form an ionic compound (magnesium chloride, \( \text{MgCl}_2 \)). Ionic compounds conduct when molten.
- D. potassium and iodine — Potassium is a metal (Group I) and iodine is a non-metal (Group VII). They form an ionic compound (potassium iodide, KI). Ionic compounds conduct when molten.
Therefore, only the compound formed by carbon and hydrogen (covalent) does not conduct electricity when molten.
✅ Answer: (A)
▶️ Answer/Explanation
In CH₄ (methane), carbon has four outer electrons and hydrogen has one each. Carbon shares its four electrons with four hydrogen atoms, forming four covalent bonds.
All of carbon’s outer-shell electrons and hydrogen’s outer electrons are used in bonding, leaving no lone pairs.
✅ Answer: (D)
All of carbon’s outer-shell electrons and hydrogen’s outer electrons are used in bonding, leaving no lone pairs.
✅ Answer: (D)
▶️ Answer/Explanation
Simple molecules are formed by covalent bonds between non-metallic elements. They have low melting points and poor electrical conductivity because they have no mobile ions or free electrons. They typically contain only non-metals, not metallic elements.
✅ Answer: (C)
✅ Answer: (C)
▶️ Answer/Explanation
Covalent bonds are formed between non‑metal atoms by sharing pairs of electrons. In the Periodic Table shown, X and Y are non‑metals (e.g. carbon and oxygen or similar), so they form a covalent bond. W is a metal and Z is a noble gas – metals form ionic bonds and noble gases are generally unreactive.
✅ Answer: (C)
✅ Answer: (C)
▶️ Answer/Explanation
Ammonia (\(\text{NH}_3\)) has a nitrogen atom with 5 outer electrons forming 3 single covalent bonds with 3 hydrogen atoms, leaving 1 lone pair on nitrogen.
The correct dot-and-cross diagram shows nitrogen sharing one electron with each of the three hydrogen atoms, with a lone pair remaining on nitrogen — this is option C.
✅ Answer: (C)
The correct dot-and-cross diagram shows nitrogen sharing one electron with each of the three hydrogen atoms, with a lone pair remaining on nitrogen — this is option C.
✅ Answer: (C)