Which characteristic is typical of molecular substances and helps distinguish them from ionic compounds?

Introduction / Context:
In basic chemistry, substances are often classified as ionic, molecular, metallic, or network covalent solids. Each type shows characteristic physical properties. Molecular substances, made of discrete molecules held together by weak intermolecular forces, behave very differently from ionic solids. This question focuses on one important property that helps identify molecular substances in comparison to ionic compounds and metals.

Given Data / Assumptions:

• The question is about molecular substances, also called molecular solids when in solid form.
• Options list properties such as thermal and electrical conductivity and melting point values.
• We assume typical room temperature and pressure conditions.
• We compare molecular substances with ionic salts and metallic solids known from school chemistry.

Concept / Approach:
Molecular substances consist of neutral molecules held together in the solid state by weak forces such as van der Waals forces or hydrogen bonds. Because these forces are relatively weak, molecular solids generally have low melting points and often exist as gases or liquids at room temperature. In contrast, ionic solids and metals have high melting points and often conduct heat and electricity well. Therefore, a low melting point is a characteristic property of molecular substances.

Step-by-Step Solution:
Step 1: Recall examples of molecular substances such as ice, solid carbon dioxide, naphthalene, iodine, and many organic compounds like sugar and wax. Step 2: Notice that many of these substances melt or sublime at relatively low temperatures compared to ionic compounds like sodium chloride or metallic solids like iron. Step 3: Good heat conductivity and good electrical conductivity are features typically associated with metals, not molecular solids. Step 4: High melting point and hard crystalline lattices are features of ionic solids and some network covalent solids such as diamond and quartz. Step 5: Molecular substances usually do not conduct electricity, and their melting points are often below a few hundred degrees Celsius. Step 6: Thus, among the given options, low melting point is the correct property associated with molecular substances.

Verification / Alternative check:
If you compare table salt and sugar in the kitchen, you will find that sugar, a molecular compound, caramelises and melts easily on heating, while table salt, an ionic compound, requires much higher temperatures to melt. Similarly, naphthalene balls sublime at room temperature, showing low intermolecular forces. Metals like copper or silver conduct heat and electricity efficiently and have high melting points. This everyday evidence supports the idea that low melting point is typical for molecular substances.

Why Other Options Are Wrong:
Good heat conductivity: This is mainly a property of metals, where free electrons carry thermal energy efficiently.
High melting point: Ionic and network covalent solids usually have high melting points, not ordinary molecular substances.
Good electrical conductivity in solid form: Again typical for metals; molecular substances lack free charges for conduction.
Formation of hard crystalline lattices with strong ionic bonds: Describes ionic crystals such as sodium chloride, not molecular solids held by weak intermolecular forces.

Common Pitfalls:
Students sometimes confuse molecular substances with ionic compounds simply because both can form crystals. The key difference is the type of bonding and the strength of forces between particles. Another mistake is to assume that any solid must have a high melting point. By remembering that molecular substances are made of neutral molecules and have relatively weak intermolecular forces, you can correctly associate them with low melting points and poor conductivity.

Final Answer:
A low melting point is a characteristic property of molecular substances.