International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which state of matter is likely for substances with a simple molecular structure at room temperature?

1. A solid with a high melting point

2. A liquid or gas

3. A crystalline solid with ionic bonds

4. A gas only

The correct answer is: A liquid or gas

Substances with a simple molecular structure typically consist of small molecules held together by weak intermolecular forces. At room temperature, many of these substances exist as liquids or gases because the energy provided by typical room temperatures is sufficient to overcome these weak forces. For instance, common substances like water (H₂O) and carbon dioxide (CO₂) exhibit simple molecular structures. Water is a liquid at room temperature, while carbon dioxide exists as a gas. The weak forces, such as Van der Waals forces or dipole-dipole interactions, allow these molecules to move freely, giving them the characteristics of liquids or gases. In contrast, a solid with a high melting point typically involves a stronger bonding arrangement, such as covalent or ionic bonds, which are not characteristic of simple molecular structures. Similarly, crystalline solids with ionic bonds are distinct from simple molecular substances, as they involve much stronger attractions between particles, resulting in high melting points and being solid at room temperature. A gas only option is too restrictive, as it overlooks the potential for these substances to exist as liquids as well. Thus, the presence of weak intermolecular forces in substances with simple molecular structures allows them to exhibit liquid or gas states at room temperature, making this the most accurate