ansferred to the solid, so its particles vibrate more strongly about their fixed positions. 2. At the melting point, the particles have enough energy to overcome some of the forces holding them in the regular solid arrangement. 3. The particles are still close together, but they can now move past each other, so the substance has become a liquid. When a substance is heated, do not say “the particles expand”. The **particles stay the same size**. Usually, the spaces between particles increase and the particles move more. ## Predicting the state from temperature data You may be given a melting point, a boiling point and a temperature, then asked to predict the state of the substance. Use this rule: - below the melting point: **solid** - between the melting point and boiling point: **liquid** - above the boiling point: **gas** This number line is a useful way to visualise the decision. ![Temperature number line showing solid below melting point, liquid between melting and boiling point, and gas above boiling point](https://assets.yesgenie.com/notes/51a35283-b71d-4272-b1f2-49729989198b.png) At the exact melting point, solid and liquid can both be present while melting or freezing happens. At the exact boiling point, liquid and gas can both be present while boiling or condensing happens. A substance has a melting point of -114 °C and a boiling point of 78 °C. Predict its state at -120 °C, 25 °C and 100 °C. 1. Compare -120 °C with the melting point. Since -120 °C is below -114 °C, the substance is a **solid**. 2. Compare 25 °C with both points. Since 25 °C is above the melting point but below the boiling point, the substance is a **liquid**. 3. Compare 100 °C with the boiling point. Since 100 °C is above 78 °C, the substance is a **gas**. Draw a small temperature line: melting point first, boiling point second. Then place the given temperature on the line and read off solid, liquid or gas. ## Why different substances change state at different temperatures Different substances have different melting points and boiling points because their particles are held together by different strengths of forces. A **force between particles** is an attraction that holds particles near each other. The stronger these forces are, the more energy is needed to separate the particles enough for a change of state. The stronger the forces between particles, the higher the melting point and boiling point of the substance. This links directly to bonding and structure: - In some substances, the particles are **molecules** with relatively weak forces between molecules. - In ionic substances, there are many oppositely charged ions held in a giant lattice by strong electrostatic attractions. - In giant covalent substances, many atoms are joined by strong covalent bonds throughout a huge structure. The “particles” in the simple model can therefore represent different things depending on the substance: atoms, molecules or ions. Sodium chloride has a much higher melting point than chlorine. 1. Sodium chloride has a giant ionic structure, so there are strong attractions between oppositely charged ions throughout the lattice. 2. Chlorine is made of small molecules, so when chlorine melts, only the weak forces between molecules need to be overcome. 3. Because sodium chloride has much stronger forces between its particles, much more energy is needed to melt it, so it has a much higher melting point. For simple molecular substances, melting and boiling overcome forces **between molecules**. They do not usually break the covalent bonds inside the molecules. ## Bulk properties