Three states of matter and state symbols
Everything is made of tiny particles. In a solid they are packed tight and only shake. In a liquid they are still close but can slide past each other. In a gas they are far apart and move fast.
To melt or boil a substance you must give energy to pull particles apart. The stronger the forces between particles, the more energy you need, so the higher the melting point (where solid turns to liquid) and boiling point (where liquid turns to gas).
The simple particle model treats particles as hard little balls with no forces shown. That is a limit of the model: real particles are not solid spheres, and there are forces between them.
In equations we write the state in brackets: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water (aqueous). Example: NaCl(s) → NaCl(aq) when salt dissolves.
Ionic compounds
An ionic compound such as sodium chloride is a giant lattice: a huge, regular pattern of positive and negative ions. Opposite charges pull hard in every direction.
- High melting and boiling points: lots of energy is needed to break so many strong pulls.
- Do not conduct as a solid: ions are locked in place.
- Conduct when melted or dissolved: now the ions can move and carry charge.
Small molecules and polymers
Water, chlorine, oxygen and methane are made of small molecules. Inside a molecule the covalent bonds are strong. But between molecules the forces (intermolecular forces) are weak. When you boil water you do not break O–H bonds; you only pull molecules away from each other.
- Low melting and boiling points; many are gases or liquids at room temperature.
- Bigger molecules have stronger intermolecular forces, so they boil higher.
- They do not conduct electricity: molecules have no overall charge.
Polymers such as poly(ethene) are very long chain molecules. The chains are long, so the forces between them add up. That is why most polymers are solid at room temperature.
Giant covalent structures
In diamond, graphite and silicon dioxide (sand) every atom is joined to its neighbours by covalent bonds, making one giant molecule. To melt them you must break many strong bonds, so their melting points are very high (diamond does not melt until above 3500 °C). Most do not conduct electricity, because there are no free charges. Graphite is the exception: each carbon has one free electron.
Metals and alloys
A metal is a giant structure of positive ions with delocalised (free) electrons moving between them. The pull between the ions and the electron sea is strong, so most metals melt high.
- Good conductors of electricity and heat: the free electrons move and carry charge and energy.
- Bendy (malleable): layers of ions can slide over each other.
An alloy is a metal mixed with other elements. The different-sized atoms upset the neat layers, so the layers cannot slide easily. Alloys are harder than pure metals. Examples: steel (iron + carbon), brass (copper + zinc), and 22-carat gold used in jewellery.
Try it: melt race in the kitchen
With an adult, put a pinch of salt, a pinch of sugar and a small piece of candle wax in three separate old steel spoons. Warm them gently over a low flame. Wax melts first (small molecules), then sugar (bigger molecules, it also browns). Salt does not melt at all (giant ionic lattice). Then try the slider in the 3D to check your prediction.
Key formulas and definitions
- Giant ionic lattice → high melting point; conducts only when molten or dissolved
- Small molecules → low melting point; weak forces between molecules; no conduction
- Giant covalent → very high melting point; usually no conduction (graphite conducts)
- Metal → free electrons → conducts heat and electricity; malleable
- Alloy = metal + other element → harder, because layers cannot slide
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water
Worked examples
1. Substance X melts at 1600 °C and conducts only when molten. What is its structure?
High melting point means a giant structure. Conducting only when molten means ions that must be free to move. So X is a giant ionic lattice.
2. Substance Y melts at −95 °C and never conducts. What is its structure?
Very low melting point means weak forces between particles, so Y is made of small (simple) molecules.
3. Why is steel used for bridges instead of pure iron?
Pure iron's layers slide easily, so it bends. In steel, carbon atoms of a different size jam the layers, so steel is harder and stronger.
4. Write the equation for solid magnesium reacting with hydrochloric acid with state symbols.
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
Common mistakes
- Saying covalent bonds break when water boils. Only the weak forces between molecules are overcome.
- Saying solid salt conducts. Its ions are fixed; it conducts only when molten or in solution.
- Thinking all covalent substances melt low. Giant covalent ones like diamond melt extremely high.
- Saying metals conduct because ions move. It is the free (delocalised) electrons that move.