Molecular Substances and Particle Attractions

Introduction

Solid carbon dioxide, liquid water and solid iodine contain molecules. Each molecule has atoms joined by covalent bonds, while separate molecules also attract one another. A phase change such as melting or boiling usually separates or rearranges the molecules rather than ripping their atoms apart. Keeping the two energy scales distinct prevents common errors about “weak covalent substances.”

Core explanation

In H₂O, O–H covalent bonds connect the atoms within each water molecule. Water molecules also interact with nearby water molecules through hydrogen bonding and other intermolecular effects. When water boils under ordinary conditions, the molecules move farther apart into a gas; they remain mostly H₂O molecules. The O–H bonds are not generally split into separate H and O atoms by ordinary boiling. Energy is required mainly to overcome attractions among molecules and to change phase.

In CO₂, each molecule contains covalent C=O bonds. Solid dry ice consists of many separate CO₂ molecules held together by intermolecular forces. It sublimes to CO₂ gas at ordinary atmospheric pressure under suitable conditions; the molecular C=O links remain. If a student says “dry ice sublimes because carbon–oxygen bonds are weak,” the explanation targets the wrong interaction. Its molecular units are strong internally, while forces among them are much less able to hold a solid at ordinary conditions.

Iodine, I₂, gives another useful case. The I–I covalent bond connects two atoms in each molecule; attractions among I₂ molecules hold the molecular solid. Heating can produce iodine vapor without turning each I₂ molecule into two I atoms as the main phase-change process. The fact that iodine is a solid at room temperature while smaller halogens can be gases reflects differences in intermolecular interactions as molecular size and electron-cloud polarisability change, not simply a ranking of internal halogen–halogen bond strengths.

Molecular substances can have a range of melting and boiling temperatures. Dispersion forces act even between nonpolar molecules; permanent dipole attractions act between polar molecules; hydrogen bonding can be especially significant where suitable donors and acceptors are present. Molecular size, shape and packing influence the result. Thus “all molecules boil at low temperature” is too broad, even though many small molecular substances change phase more easily than comparable ionic or giant-network solids.

Some molecular substances react when heated, so a practical heating experiment can include both phase change and chemical change. To distinguish them, ask whether the molecular identity remains the same. H₂O(l) → H₂O(g) is a phase change. H₂O → H₂ + O₂ is a chemical decomposition requiring bond rearrangement and suitable conditions. Equations, evidence and temperatures clarify which process is being discussed.

Step-by-step reasoning

1. Identify the discrete molecule and its internal covalent bonds. 2. Identify attractions between separate molecules in the condensed phase. 3. Read the process: melting, boiling or subliming versus chemical reaction. 4. If formula identity is unchanged across the phase transition, explain separation of molecules rather than atom-bond cleavage. 5. Use molecular size, polarity and structure to compare intermolecular effects qualitatively.

Visual explanation

Draw three water molecules with solid O–H lines within each and dotted links between different molecules. Place H₂O(l) → H₂O(g) underneath and show the dotted links spread apart while the solid O–H lines remain. Beside it draw a dry-ice cluster of intact CO₂ units changing into separated CO₂(g) molecules. Label the two line styles clearly.

Real-world analogy

Several people can stand in a group while each carries a firmly attached backpack. Spreading the group apart breaks the group arrangement, not the backpack straps. Molecular phase changes similarly alter between-molecule contacts while often preserving internal bonds. The analogy does not specify actual force strengths or reaction pathways.

Real-world example

Dry ice is used for cooling and theatrical fog effects because solid CO₂ can pass directly into gas under ordinary pressure. Its sublimation does not produce carbon and oxygen atoms in the air. The explanation rests on separate molecular particles and relatively weak between-particle attractions compared with the covalent bonds inside each CO₂ molecule.

Why?

Why does boiling water not ordinarily produce hydrogen and oxygen gases? Boiling supplies energy for molecules to escape liquid-phase attractions. Splitting O–H covalent bonds and forming H₂ and O₂ is a different chemical reaction with different energy and pathway requirements.

Common misconception

“A molecular substance has weak covalent bonds because it boils easily.” Low boiling can reflect relatively weak forces between molecules even while bonds within each molecule are strong. Name the interaction being overcome.

Worked example

Classify the changes I₂(s) → I₂(g) and 2H₂O(l) → 2H₂(g) + O₂(g). In the iodine process, the molecular formula I₂ remains on both sides; the main event is sublimation and reduced attractions between I₂ molecules. The I–I covalent bonds remain in the gas molecules. In the water process, the products have different formulas and O–H bonds must be broken while H–H and O=O bonds form. That is chemical change, not boiling. The equations themselves reveal which bond scale matters.

Quick check

1. Which interactions are mainly overcome when ordinary liquid water boils into water vapor? Answer: Attractions between separate water molecules are overcome; their internal O–H covalent bonds largely remain intact.

Exam focus

Compare formulas and states across the equation. Use “within molecule” for covalent atom bonds and “between molecules” for condensation forces. Do not infer internal bond weakness from a molecular solid's low melting or sublimation temperature.

Advanced insight

The exact boundary between a weak bond and a strong intermolecular interaction can be context-dependent in some systems, but the distinction is robust for introductory examples such as H₂O phase changes and CO₂ sublimation. Thermodynamic phase transitions involve collective interactions, entropy and molecular motion, not merely one bond energy.

Summary

Molecular substances contain discrete molecules with internal covalent bonds and attractions among molecules. Ordinary melting, boiling and sublimation usually rearrange or separate the molecules without changing their formulas. Chemical reactions, in contrast, break and form intramolecular bonds.

Practice questions

1. Are O–H bonds normally broken when water boils? Answer: No. Water molecules separate from one another but remain H₂O. 2. What holds separate CO₂ molecules together in dry ice? Answer: Intermolecular attractions, principally dispersion forces for nonpolar CO₂. 3. Does I₂(s) → I₂(g) require making single iodine atoms as its main product? Answer: No. The vapor consists chiefly of intact I₂ molecules in this phase-change description. 4. What feature of an equation signals chemical decomposition rather than phase change? Answer: The product molecular formulas differ from those of the reactants.