States of matter

• Gas: Pa, m³, K — convert before you substitute. Solid: what is overcome decides the melting point; what can move decides the conductivity.

• 4.1.1 I can explain — explain the origin of pressure in a gas in terms of collisions between gas molecules and the wall of the container
• 4.1.2 I can understand — understand that ideal gases have zero particle volume and no intermolecular forces of attraction
• 4.1.3 I can state — state and use the ideal gas equation pV = nRT in calculations, including in the determination of Mr
• 4.2.1 I can describe — describe, in simple terms, the lattice structure of a crystalline solid which is: (a) giant ionic, including sodium chloride and magnesium oxide (b) simple molecular, including iodine, buckminsterfullerene C60 and ice (c) giant molecular, including silicon(IV) oxide, graphite and diamond (d) giant metallic, including copper
• 4.2.2 I can describe — describe, interpret and predict the effect of different types of structure and bonding on the physical properties of substances, including melting point, boiling point, electrical conductivity and solubility
• 4.2.3 I can deduce — deduce the type of structure and bonding present in a substance from given information

New at AS, and not to be assumed: the two ideal-gas assumptions in the syllabus’s words; pV = nRT with its SI units; Mr from gas data; silicon(IV) oxide; and the three simple molecular lattices, iodine, C₆₀ and ice. None of the O Level ideas is re-taught below at O Level depth.

• “n = pV/RT = (98.5 × 250)/(8.31 × 25)” Correct Pa, m³, K: convert before you substitute. R = 8.31 J K⁻¹ mol⁻¹ only works with p in Pa, V in m³ and T in K. 98.5 kPa = 9.85 × 10⁴ Pa, 250 cm³ = 2.50 × 10⁻⁴ m³, 25 °C = 298 K. The unconverted line above gives 118 mol instead of 9.94 × 10⁻³ mol — out by a factor of about 12 000. (4.1.3)
• “1 m³ = 1000 cm³.” Correct 1 m³ = 10⁶ cm³ = 10³ dm³. A cube 1 m on a side is 100 cm on a side, and 100³ = 10⁶. (4.1.3)
• “Gas pressure is caused by the molecules colliding with each other.” Correct Pressure is caused by molecules colliding with the walls of the container: each collision exerts a force on the wall, and pressure is force per unit area. (4.1.1)
• “An ideal gas has very small particles and weak forces between them.” Correct Zero particle volume and no intermolecular forces of attraction. “Small” and “weak” describe a real gas, which is why a real gas only approaches ideal behaviour. (4.1.2)
• “C₆₀ is a giant covalent structure, like diamond and graphite.” Correct C₆₀ is simple molecular: each molecule has exactly sixty atoms, and the molecules are held to one another only by id–id forces. (4.2.1)
• “Melting iodine breaks the I–I bonds.” Correct Melting or subliming a simple molecular solid overcomes only the forces between molecules. The covalent bond inside each I₂ molecule is untouched. (4.2.2)
• “It has a high melting point, so it is ionic.” Correct Giant molecular solids melt higher still. The test that separates ionic from giant molecular is conductivity when molten or in aqueous solution. (4.2.3)
• “Gas pressure is caused by the molecules colliding with each other.” Repair Pressure is caused by molecules colliding with the walls of the container. Each collision exerts a force on the wall; pressure is the total force per unit area.
• “An ideal gas has small molecules and weak forces between them.” Repair The two assumptions are zero particle volume and no intermolecular forces of attraction. Real gases have small volumes and weak forces, which is why they only approach ideal behaviour.
• “Gases behave most ideally at low temperature.” Repair At high temperature and low pressure. At low temperature the molecules move slowly, the attractions between them take effect, and the gas eventually liquefies.
• “n = pV/RT = (98.5 × 250) / (8.31 × 298)” Repair Convert first: 250 cm³ = 2.50 × 10⁻⁴ m³ and 98.5 kPa = 9.85 × 10⁴ Pa. R is in J K⁻¹ mol⁻¹, so p must be in Pa and V in m³.
• “T = 25 in pV = nRT, because the question said 25 °C.” Repair 298 K. Every temperature in the gas equation is a thermodynamic temperature, in kelvin: add 273 to a Celsius value.
• “1 m³ = 1000 cm³.” Repair 1 m³ = 10⁶ cm³ = 10³ dm³. It is 1 dm³ that equals 1000 cm³.
• “Mr = 46.0 g mol⁻¹.” Repair M = 46.0 g mol⁻¹ has the unit; Mr is the same number with no unit: Mr = 46.0.
• “One mole of gas at 298 K and 101 kPa occupies 24.0 dm³.” Repair The equation gives 24.5 dm³ at 298 K. The Data-section 24.0 dm³ mol⁻¹ is for room conditions, about 293 K. Use pV = nRT whenever a temperature and pressure are stated.
• “Sodium chloride is made of NaCl molecules.” Repair It is a giant ionic lattice. NaCl is the ratio of ions in it; each ion is attracted equally to six neighbours, and no molecule exists.
• “Ionic compounds conduct electricity because their electrons move.” Repair They conduct because their ions move, which the ions can do only when the compound is molten or in aqueous solution.
• “Graphite conducts because its layers slide.” Repair Graphite conducts because each carbon has one delocalised electron free to move along the layer. Sliding layers explain its softness, a different property.
• “C₆₀ is a giant molecular structure like graphite.” Repair C₆₀ is a simple molecular solid: discrete molecules of sixty atoms held to one another by id–id forces. It is soft and a poor conductor.
• “Iodine has a low melting point because the I–I bond is weak.” Repair Melting iodine overcomes only the id–id forces between I₂ molecules. The I–I covalent bond is not broken.
• “Ice floats because its molecules are further apart as they vibrate more.” Repair Ice is colder than the water it floats on. It floats because its hydrogen bonds hold the molecules in an open hexagonal lattice; on melting the lattice collapses and the molecules pack more closely.
• “A high melting point proves a substance is ionic.” Repair Giant molecular solids melt higher still. Conductivity when molten is the deciding test.
• “Silicon dioxide is a gas like carbon dioxide because they are in the same group.” Repair Silicon is too large to form strong π bonds, so it forms four Si–O single bonds in a giant covalent network; SiO₂ is a solid that melts well above 1500 °C.

• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.
• Interleave with the chapters that use this one. Topic 5 assumes that melting a molecular solid overcomes forces, not bonds: re-answer “what is overcome when iodine sublimes?”. Topic 7 uses gas pressures in equilibria: re-answer “what are the SI units in pV = nRT?”. Topic 8 explains reaction rates through molecular motion and collisions. Topic 9 applies the four structure types across Period 3: re-answer “how do you tell giant ionic from giant molecular?”. Topic 23 treats the strength of ionic lattices quantitatively. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Chemistry 9701 has five components. Topic 4 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E.
• A Paper 1 item on this topic is a discrimination: which conditions make a gas most nearly ideal, which substance is simple molecular, which statement explains a property of graphite, which unit conversion is right. A Paper 2 structured question asks you to explain gas pressure through collisions with the walls, state the two ideal-gas assumptions, calculate with pV = nRT, describe one of the ten named lattices, explain a property from the particles and the forces, or deduce a structure from a table of properties. A full explanation names the particle, the force, what is overcome and what can move.
• Every calculation is pV = nRT, rearranged for n, V, p, T or, through n = m/M, for Mr. R = 8.31 J K⁻¹ mol⁻¹ and the molar gas volumes (22.4 dm³ mol⁻¹ at s.t.p., 24.0 at room conditions) are printed in the Data section and supplied with the paper, as are the Ar values. The unit conversions are not: cm³, dm³, kPa and °C to m³, Pa and K are yours to do. Melting points are not in the Data section; a question that needs one supplies it.
• This topic is the theory behind gas collection (a volume converted to moles at a stated temperature and pressure) and behind the determination of Mr by vaporising a liquid in a gas syringe, a planning and evaluation context for Paper 5. At the Paper 3 bench it supplies observation exercises: heating, conductivity and solubility tests on unknown solids. The Practical skills section sets out both.
• Read the command word before you decide how much to write. This syllabus has twenty-two of them: analyse, calculate, compare, consider, contrast, deduce, define, demonstrate, describe, determine, discuss, evaluate, examine, explain, give, identify, justify, predict, show (that), sketch, state and suggest. Comment, estimate, name and outline are not among them: where a question wants something named it says identify, which the syllabus glosses as “name/select/recognise”. State and give want a fact and nothing more. Describe wants the points or the features. Explain wants the reasons and the relationships — a describe-level answer to an explain question is incomplete however well written it is. Deduce and determine want a conclusion reached from the information given, with the reasoning visible.