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Small molecules have weak intermolecular forces between them, so they have low melting and boiling points. Free notes with key terms and 11 questions answered.
Physical Properties of Small Molecules is part of How bonding and structure are related to the properties of substances in GCSE AQA Higher Chemistry.
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Small molecules have weak intermolecular forces between them, so they have low melting and boiling points. Free notes with key terms and 11 questions answered.
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Use this lesson to stay inside How bonding and structure are related to the properties of substances while connecting the detail back to the wider Topic 2: Bonding, structure, and the properties of matter topic.
Read the core explanations from Physical Properties of Small Molecules before testing yourself from memory.
Water boils at 100°C, but diamond melts at over 3500°C. Both contain strong covalent bonds, so why the massive difference? The answer lies in what happens between the molecules, not within them.
Small molecules like water ($H_2O$), methane ($CH_4$), and carbon dioxide ($CO_2$) are usually gases or liquids at room temperature. They have relatively low melting and boiling points compared to giant structures like diamond or salt.
Here's the crucial distinction that trips up many students. In a substance like water, there are actually two different types of forces at work:
Covalent bonds hold the atoms together within each molecule. These are strong - it takes a lot of energy to break the O-H bonds in a water molecule.
Intermolecular forces hold the molecules together. These are much weaker - like a gentle attraction between neighbouring water molecules.
The weakness of intermolecular forces explains why small molecules have such different properties from giant structures. When you heat ice, you're not breaking the strong O-H bonds - you're just giving molecules enough energy to overcome the weak attractions between them.
Think of it like this: imagine trying to separate a group of people holding hands (strong covalent bonds) versus a group just standing close together (weak intermolecular forces). The second group is much easier to separate.
Worked example: Comparing Bond Strengths
Explain why methane ($CH_4$) is a gas at room temperature while diamond is solid.
Answer: Methane molecules are held together by weak intermolecular forces that are easily overcome at room temperature, while diamond has continuous strong covalent bonding throughout its structure.
Once you understand the intermolecular force concept, you can predict the properties of molecular substances. The weaker the intermolecular forces, the lower the melting and boiling points.
Most small molecules are gases or liquids at room temperature because the intermolecular forces are weak enough to be overcome by the kinetic energy of particles at normal temperatures.
Students often confuse what happens during melting and boiling. Remember: you're not breaking the molecule apart - you're just separating whole molecules from each other.
Exam precision note: In Physical Properties of Small Molecules - Properties of small molecules - How bonding and structure are related to the properties of substances, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
The important GCSE distinction is between the strong covalent bonds inside a molecule and the weaker intermolecular forces between separate molecules. When a simple molecular substance melts or boils, the covalent bonds inside each molecule stay intact. Only the weak forces between molecules are overcome. Giant covalent structures are different because strong covalent bonds extend throughout the whole structure.
Weak intermolecular forces explain why simple molecular substances usually have low boiling points, but they do not explain everything about a substance on their own. Students should compare simple molecules such as methane with giant covalent structures such as diamond and with giant metallic or ionic structures. The key idea is to identify what is being overcome when a substance changes state. In methane, weak intermolecular forces between molecules are overcome. In diamond, strong covalent bonds extend throughout the giant structure, so much more energy is needed. This comparison helps explain why boiling and melting points depend on structure, not just on whether the word covalent appears in the description.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Intermolecular forces are weak forces of attraction between molecules that hold them together in liquids and solids. They are much weaker than the covalent bonds inside each molecule, which is why simple molecular substances have low melting and boiling points.
Methane is a gas at room temperature because room temperature provides enough energy to overcome the weak intermolecular forces between methane molecules. The strong C-H covalent bonds within each molecule are not broken, so the molecules simply separate.
Small molecules are substances made of covalently bonded atoms that exist as discrete molecules with weak forces between them. Examples include water, methane and carbon dioxide, which are usually gases or liquids at room temperature.
Water (H₂O), methane (CH₄) and carbon dioxide (CO₂) are examples of small molecules. Each is made of covalently bonded atoms that exist as discrete molecules with weak forces between them, so they are usually gases or liquids at room temperature.
Covalent bonds are the strong bonds that hold atoms together within each molecule, while intermolecular forces are the weak forces of attraction between molecules. Intermolecular forces are much weaker than covalent bonds, so they are far easier to overcome.
The strength of intermolecular forces sets the melting and boiling points of a molecular substance: the weaker the intermolecular forces, the lower the melting and boiling points. Most small molecules are gases or liquids at room temperature because their weak forces are overcome by the kinetic energy of particles at normal temperatures.
Small molecules have low melting and boiling points because only the weak intermolecular forces between molecules are overcome when they melt or boil. The strong covalent bonds inside each molecule stay intact, so relatively little energy is needed.
Covalent bonds are not broken when a simple molecular substance melts or boils. The covalent bonds inside each molecule stay intact and only the weak intermolecular forces between separate molecules are overcome, so whole molecules separate from each other.
The strong O-H bonds inside each water molecule stay intact when ice melts. Heating only gives the molecules enough energy to overcome the weak attractions between neighbouring water molecules, so whole molecules separate from each other.
Water boils at 100°C while diamond melts at over 3500°C because different forces are overcome. Boiling water only overcomes weak intermolecular forces between molecules, but melting diamond means breaking strong covalent bonds that extend throughout its giant structure, which needs much more energy.
A simple molecular substance, such as methane, is made of separate molecules held together by weak intermolecular forces, so it has low melting and boiling points. In a giant covalent structure, such as diamond, strong covalent bonds extend throughout the whole structure, so much more energy is needed to melt it.