A sample arrives with no name on the bottle. You are told only that it is a white solid, it melts at a high temperature, it does not conduct electricity as a solid, but it does conduct when molten and when dissolved in water. A Pearson-style answer is not "it is probably salt" and then a memory list. The answer has to show why the evidence points to a particle model.

Use this order whenever the substance is unfamiliar:

The important habit is to avoid jumping from one clue to one label. High melting temperature alone does not prove "ionic"; metals and giant covalent substances can also have high melting temperatures. Conductivity alone does not prove "metallic"; graphite conducts because it contains delocalised electrons, and molten ionic compounds conduct because ions are mobile.

Most deductions in this part of Topic 2 use four structure families. The word "bonding" means different things in each family, so name the particles before naming the force.

Notice the phrase "where relevant" for intermolecular forces. Intermolecular forces are relevant for simple molecular substances because there are molecules with attractions between them. They are not the explanation for the high melting temperature of sodium chloride, magnesium oxide, diamond or copper.

Feynman diagnostic: the molecule trap

Explain to a younger student why "covalent" is not enough information to predict melting temperature. Your explanation must compare iodine with diamond. The diagnostic trap is saying "covalent bonds are strong, so all covalent substances have high melting temperatures."

Good explanation

Iodine is covalent and simple molecular. During melting, the covalent bond inside each iodine molecule is not broken; only London forces between iodine molecules are overcome, so the melting temperature is relatively low. Diamond is also covalent, but each carbon atom is covalently bonded in a giant network, so melting requires disruption of many strong covalent bonds through the structure.

When data are supplied, treat each value as evidence. A useful deduction normally combines at least two of melting or boiling temperature, conductivity, solubility and composition.

Worked example: deducing an unknown

An unknown crystalline solid has these properties.

Deduce the type of structure and bonding present.

Route

1. The high melting temperature suggests a giant structure, not a simple molecular substance.
2. The solid does not conduct, so there are no mobile charged particles in the solid. That rules against a metal and against graphite-like carbon conductivity.
3. The molten substance conducts, so charged particles become mobile after melting. That is typical of ions in an ionic lattice.
4. The solution conducts, which is consistent with ions moving in water.

Conclusion

The substance is likely to have a giant ionic lattice. The particles are positive and negative ions, held by strong electrostatic attractions in all directions. The solid does not conduct because the ions are fixed in position; the molten liquid and aqueous solution conduct because the ions are mobile.

Guided practice

An unknown solid melts above 1600 deg C, does not conduct electricity as a solid or when molten, and is insoluble in water. The formula contains only non-metal atoms.

Complete the deduction:

• The very high melting temperature suggests ...
• The lack of conductivity suggests ...
• The non-metal composition supports ...
• The most likely structure is ...

Answer

The very high melting temperature suggests a giant structure. The lack of conductivity suggests there are no mobile ions or delocalised electrons. The non-metal composition supports covalent bonding rather than metallic bonding. The most likely structure is giant covalent.

For row 2B.27, prediction means giving the expected property and supporting it with the model. The support must mention the relevant particles, structure and bonding or intermolecular forces.

Melting and boiling temperature

For a simple molecular substance, melting and boiling overcome intermolecular forces between molecules. The covalent bonds inside the molecules usually remain intact. Stronger intermolecular forces give higher boiling temperatures, but this is still a different scale of explanation from giant lattices.

For a giant ionic lattice, melting requires enough energy to overcome many strong electrostatic attractions between oppositely charged ions. Higher ionic charge and smaller ionic radius can make ionic attractions stronger, but in this lesson the usual deduction is simply "giant ionic lattice, strong electrostatic attractions, high melting temperature."

For a metal, melting disrupts strong electrostatic attractions between positive metal ions and delocalised electrons. For a giant covalent substance, melting or subliming requires disruption of many strong covalent bonds in the network, so the melting temperature is very high.

Electrical conductivity

Conductivity requires mobile charged particles. Metals conduct because delocalised electrons move through the structure. Graphite and graphene conduct because delocalised electrons can move along the carbon layers or sheet. Ionic solids do not conduct because their ions are fixed in the lattice, but molten ionic compounds and aqueous ionic solutions conduct because ions can move. Simple molecular substances usually do not conduct because their molecules are neutral.

Solubility in water

Water is polar and can form strong attractions with ions and with some polar molecules. Many ionic compounds dissolve when attractions between water molecules and separated ions compensate for the attractions in the ionic lattice. Some simple molecular substances dissolve if they can form hydrogen bonds with water, such as small alcohols; many non-polar molecular substances do not. Metals and giant covalent lattices are usually insoluble because water cannot pull the extended structure apart under ordinary conditions.

Worked example: predicting properties from a structure

Predict three physical properties of magnesium oxide, MgO, from its structure.

Route

MgO contains Mg2+ and O2- ions arranged in a giant ionic lattice. The ions are held by strong electrostatic attractions in all directions.

Predictions

• Melting temperature: high, because many strong electrostatic attractions between oppositely charged ions must be overcome.
• Electrical conductivity: solid MgO does not conduct because ions are fixed in position; molten MgO conducts because ions can move.
• Solubility in water: expected to be low compared with many simple Group 1 salts because the Mg2+ and O2- ions give very strong lattice attractions; if ions are present in solution, the solution conducts.

Common-error contrast

Do not write "MgO has strong intermolecular forces." MgO is not made of molecules. The mark-earning words are ions, giant ionic lattice, strong electrostatic attractions and mobile ions when molten or aqueous.

Real substances do not always fit a one-clue shortcut. Edexcel can give enough evidence for a justified prediction, so use all of it.

Small molecular substances can have surprisingly high boiling temperatures

Water, ammonia, hydrogen fluoride and small alcohols can hydrogen bond. Their boiling temperatures are higher than similar-sized molecules with only London forces, but they are still simple molecular substances. During boiling, intermolecular forces are overcome; covalent bonds within the molecules are not broken.

Some covalent structures conduct

Diamond does not conduct because all four outer electrons on each carbon atom are used in covalent bonds. Graphite and graphene conduct because each carbon atom has one electron that becomes delocalised. Therefore, "contains carbon" and "covalent" do not automatically predict the same electrical behaviour.

Water solubility needs a reason

Ionic substances may dissolve if hydration of the ions is favourable, and the resulting solution conducts. Small polar molecules may dissolve through hydrogen bonding or permanent dipole interactions with water, but if they remain as neutral molecules the solution does not conduct. Non-polar molecular substances, metals and giant covalent lattices are usually poorly soluble in water.

Feynman diagnostic: the conductivity trap

Explain why sodium chloride does not conduct electricity as a solid but does conduct when molten, without using the word "electrons" as the charge carrier.

Diagnostic answer

Solid sodium chloride contains ions, but they are locked in fixed positions in the giant ionic lattice, so there are no mobile charged particles. When sodium chloride melts, the lattice breaks down enough for the ions to move through the liquid, and these mobile ions carry charge.

A strong 9CH0 deduction is a short chain of evidence, not a long list of memorised properties. The command word predict needs an expected result supported by the relevant model; deduce needs the conclusion and the evidence used.

Use this sentence structure:

1. "The substance is likely to be ..."
2. "The particles present are ..."
3. "The structure is ..."
4. "This explains property X because ..."
5. "This also explains property Y because ..."

Worked example: comparing three unknowns

Deduce the most likely structure family for each.

Model answer

X is likely to be metallic. It conducts as a solid and when molten, so it has mobile delocalised electrons in both states. The relatively low melting temperature compared with many giant lattices does not rule out metallic bonding because metallic melting temperatures vary between metals.

Y is likely to be simple molecular. Its melting temperature is low compared with giant lattices, and it does not conduct in either state because it has neutral molecules rather than mobile ions or delocalised electrons. Its insolubility in water suggests the molecules do not form sufficiently favourable attractions with water.

Z is likely to be giant covalent. Its melting temperature is very high, it does not conduct in either state, and it is insoluble in water. The evidence points to atoms joined by strong covalent bonds through a network, with no mobile charged particles.

Pearson-aware recap

Rows 2B.26 and 2B.27 are about moving from evidence to model to property. The best answers name the particles, state the structure, identify the bonding or intermolecular forces, and then use mobile charged particles or attractions between particles to explain the property. Avoid one-word structure labels unless the question asks only for identification; most marks come from the reasoning that connects the label to the data.