Structure and bonding · GCSE Chemistry
States of matter
GCSE Chemistry revision on states of matter: particle arrangement in solids, liquids and gases, changes of state, and how bonding type decides melting and boiling points.
Solid: regular, vibrating. Liquid: close, flowing. Gas: far apart, fast, random. Changing state does not break covalent bonds in simple molecules — it overcomes the forces between particles.
The important bits
What you need to know
- 1
In a solid, particles are tightly packed in a regular arrangement and vibrate about fixed positions. Solids keep their shape and have a fixed volume.
- 2
In a liquid, particles are close but irregular and can move past each other. Liquids flow and take the shape of the container, with a fixed volume.
- 3
In a gas, particles are far apart, moving randomly at high speed. Gases fill their container and can be compressed because of the empty space.
- 4
Melting and boiling happen at a fixed temperature for a pure substance. Temperature stays constant during the change while energy goes into separating particles.
- 5
For simple molecules, melting and boiling overcome weak intermolecular forces; the covalent bonds inside the molecules stay intact. H₂O(l) and H₂O(g) are still water.
- 6
For giant ionic, metallic and giant covalent structures, melting must overcome strong bonds throughout the lattice, so melting points are much higher.
- 7
A heating curve shows sloped sections (temperature rising) and flat sections (state changing). The flat at 0 °C for water is melting; the flat at 100 °C is boiling.
- 8
Gas pressure is caused by particles colliding with the walls. Raising temperature at constant volume raises pressure because collisions are more frequent and more forceful.
Quotations worth analysing
Short evidence. Real method.
“H₂O(s) ⇌ H₂O(l) ⇌ H₂O(g)”
Ice, water and steam are the same covalent molecules. Only the arrangement and the intermolecular forces change. Mass is conserved: melting 18 g of ice still gives 18 g of water.
“Melting point of NaCl ≈ 801 °C; boiling point of Cl₂ = −34 °C.”
Giant ionic lattice versus simple molecules. Room temperature sits between those numbers, so salt is solid and chlorine is a gas. That is bonding, not a coincidence.
“Temperature is a measure of the average kinetic energy of the particles.”
Hotter gas particles move faster. During melting the potential energy of the arrangement increases while kinetic energy — and temperature — stays constant until the solid has all melted.
Go deeper
State at room temperature is a bonding question
Do not memorise a list of “chlorine is a gas” without the reason. Chlorine is Cl₂ molecules with weak intermolecular forces, so it has already boiled at −34 °C. Iodine is still I₂, but the molecules are larger, so intermolecular forces are stronger and iodine is a solid that sublimes. Sodium chloride is a giant ionic lattice, solid. Diamond is giant covalent, solid. Copper is metallic, solid. If a question gives a melting point of −218 °C, the substance is simple molecular. If it is 1700 °C, look for a giant structure. Linking state to bonding is the whole point of this page sitting in structure-and-bonding rather than in Physics.
Go deeper
The heating curve is energy going to two different jobs
On the slope, energy increases the kinetic store and temperature rises. On the flat, energy increases the potential store as forces between particles are overcome, and temperature holds. Students mark the flats as “no energy transferred”. The opposite is true: energy is still going in, but it is not raising temperature. For water, use the Chemistry language: hydrogen bonds / intermolecular forces between molecules, not “the bonds in water break” unless you mean decomposition. A pure substance has a sharp melting point; a mixture melts over a range. That is how you tell impure salt from a pure sample in a practical.
Go deeper
Predict the state from the formula and the structure
CO₂(g), CH₄(g), H₂(g) and N₂(g) are small molecules. Br₂(l) is still simple molecular; the forces are a little stronger. Fe(s), Cu(s) and NaCl(s) are giant. SiO₂(s) is giant covalent. When you write equations, the state symbol must match that prediction unless the question specifies otherwise. Hydrogen from a metal-plus-acid reaction is H₂(g) even if the acid is (aq). Water produced in combustion of a hydrocarbon in a bunsen flame is often shown as H₂O(g) because it leaves as steam, then condenses. Read the question: “steam” means (g); “water” usually means (l).
See the idea in action
A substance melts at −114 °C and boils at 78 °C. It does not conduct as a solid or as a liquid. Identify the structure. Low melting and boiling points mean weak forces between particles, so it is simple molecular (ethanol is the usual example). No conduction as a liquid rules out ionic (molten ions would conduct) and metallic (delocalised electrons would conduct). Covalent bonds inside the molecules stay intact when it boils.
Exam technique
Turn knowledge into marks
Name the particles (atoms, ions or molecules), their arrangement, and the force overcome on melting. For simple molecules, stress that covalent bonds are not broken when the substance boils.
Common mistakes
Do not give these marks away
- 01
Saying particles in a solid do not move, or that mass changes when ice melts.
- 02
Claiming covalent bonds break when water boils.
- 03
Predicting that molten ionic compounds do not conduct, or that a low boiling point means a giant structure.
What happens to water molecules when liquid water boils?
AO–H covalent bonds break to make hydrogen and oxygen
BMolecules stay as H₂O and intermolecular forces are overcome
CThe molecules become ions
DMass is lost because atoms are destroyed
Show the answer
Molecules stay as H₂O and intermolecular forces are overcome. Boiling is a physical change of state. Steam is still H₂O(g). Breaking covalent bonds would be a chemical decomposition, not boiling.
Quick questions
If this is the bit you searched
What is the particle model of solids liquids and gases GCSE Chemistry?
Solids: regular, vibrating about fixed positions. Liquids: close, irregular, able to flow. Gases: far apart, moving randomly at high speed, filling the container.
Does boiling water break covalent bonds?
No. Boiling overcomes intermolecular forces between H₂O molecules. The O–H covalent bonds stay intact, so steam is still water.
Why are some covalent substances gases and others solids?
Simple molecules with weak intermolecular forces are gases or low-boiling liquids. Giant covalent structures such as diamond and SiO₂ have covalent bonds throughout, so they are solids with very high melting points.
Why does temperature stay constant while a pure solid melts?
Energy is used to overcome forces between particles rather than to raise their kinetic energy, so temperature holds until melting is complete.