CIE A Level Chemistry States of Matter Exam Questions

About CIE A Level Chemistry States of Matter

States of matter in Cambridge International 9701 links the behaviour of particles to the properties you can measure. The first part deals with gases: the assumptions of the kinetic theory, the ideal gas equation pV = nRT, and why real gases deviate from ideal behaviour at high pressure and low temperature. The second part covers solids, including giant ionic, giant covalent, giant metallic and simple molecular lattices, with examples such as sodium chloride, diamond, graphite, silicon(IV) oxide, copper and iodine. This AS topic builds on chemical bonding and supports periodicity. Examiners frequently set pV = nRT calculations and ask you to explain melting points and conductivity from structure.

What you need to know

• Ideal gas assumptions Gas molecules have negligible volume compared with the container and there are no intermolecular forces between them. Collisions are perfectly elastic and molecules move randomly.
• Ideal gas equation pV = nRT with p in Pa, V in m3, T in K and R = 8.31 J K-1 mol-1. Rearranging with n = m/M lets you find the Mr of a volatile liquid or gas.
• Real gases Gases deviate most from ideal behaviour at high pressure, where molecular volume is significant, and low temperature, where intermolecular forces become important. Small, non-polar molecules such as helium behave most ideally.
• Giant lattices Giant ionic (NaCl, MgO), giant covalent (diamond, graphite, SiO2) and giant metallic (Cu) structures have high melting points because strong bonds must be broken throughout the lattice.
• Simple molecular lattices Iodine and ice are examples. They have low melting points because only weak intermolecular forces are overcome. Ice is less dense than water because hydrogen bonds hold molecules in an open lattice.
• Properties from structure Ionic compounds conduct when molten or aqueous as ions are mobile. Graphite conducts along its layers because of delocalised electrons, while diamond does not conduct.

Common mistakes in States of Matter

• Using V in cm3 or dm3 and p in kPa in pV = nRT; convert to m3 (divide cm3 by 10^6) and Pa.
• Using temperature in degrees Celsius; always add 273 to get kelvin.
• Saying real gases deviate at high temperature; it is low temperature and high pressure.
• Explaining the high melting point of SiO2 using intermolecular forces; it is a giant covalent lattice with strong covalent bonds.
• Saying solid ionic compounds do not conduct because there are no electrons; the reason is that the ions are fixed in the lattice.

Try three States of Matter exam-style questions