NCEA Level 1 Chemistry 92023 Physical properties of materials: past papers, practice and marking
- NCEA Level 1
- Chemistry
- AS92023
- 4 credits
- External exam
About 92023
AS92023, Demonstrate understanding of how the physical properties of materials inform their use. Molecular, metallic, ionic, polymer and covalent network materials: what their particles are, how strongly they hold together, and why that makes each one right or wrong for a job. The 4-credit Level 1 external, in the shapes it keeps asking.
What 92023 covers
- Types of materialClassifying a substance as molecular, metallic, ionic, polymer or covalent network from data or a particle diagram, naming the particles (atoms, ions, molecules, electrons) and the attractive forces between them
- Covalent networksDiamond, graphite and silicon dioxide: every atom covalently bonded into a network, very high melting points, graphite's layers and delocalised electrons, diamond's 3D network and hardness, linked to a use
- Molecular substancesWater, iodine and carbon dioxide: strong covalent bonds inside each molecule, weak forces between molecules, low melting and boiling points, no charged particles free to move, which bonds break when it melts or boils
- MetalsPositive metal ions in a sea of delocalised electrons, electrical and thermal conduction, malleability and ductility, density, alloys of different-sized atoms, choosing a metal or alloy from data
- Ionic materialsA lattice of positive and negative ions, high melting point, brittleness when like charges line up, conducting only when molten or dissolved, how water molecules pull ions out of the lattice, nitrate fertilisers
- PolymersLong chains of repeating units, covalent bonds along each chain and weak forces between chains, natural and synthetic polymers, branched and unbranched chains, density, flexibility, stiffness and insolubility linked to a use
- Choosing a materialReading a table of physical properties, comparing the numbers with units, density and floating, identifying unknown solids from tests, and justifying the best material for a job from its structure and bonding
92023 past papers
Open a 92023 past paper beside your own Ako page, then get it marked, part by part.
Work through a past paper All NCEA Level 1 Chemistry past papers
Practice questions
Question 1: Types of material
A potter needs a material to line a kiln that runs at 1200 °C. Solid X (a covalent network) melts at 1710 °C. Solid Z (a molecular substance) melts at 80 °C. Neither conducts electricity.
- Using the data, compare X and Z to explain why X could line the kiln but Z could not.(up to Excellence)
- Explain why neither X nor Z conducts electricity.(up to Merit)
Check your answer
Part a: what gets the marks
- X's melting point of 1710 °C is above the kiln's 1200 °C, but Z's melting point of 80 °C is far below it.
- Z melts at only 80 °C because melting overcomes only the weak attractions between its molecules, which needs little energy; the covalent bonds inside the molecules do not break.
- X melts only at 1710 °C because every atom is held by strong covalent bonds in a 3D network, and many of these bonds must be broken to melt it, which needs a lot of energy.
- At 1200 °C the kiln supplies far more energy than Z's weak attractions between molecules can withstand, so Z would melt and run out of the lining, but not enough to break X's covalent bonds, so X stays solid and keeps its shape.
A full answer: Z is a molecular substance. Melting it only overcomes the weak attractions between its molecules (the covalent bonds inside the molecules do not break), so little energy is needed and it melts at 80 °C. X is a covalent network: every atom is joined to its neighbours by strong covalent bonds, and many covalent bonds must be broken to melt it, so it melts only at 1710 °C. In a kiln at 1200 °C, Z would melt and run out of the lining almost at once, while X is still 510 °C below its melting point, so it stays solid and keeps lining the kiln.
Part b: what gets the marks
- Neither X nor Z contains ions or delocalised electrons.
- In X every outer electron is held in a covalent bond between atoms, and Z is made of neutral molecules with their electrons held in covalent bonds, so in both there are no charged particles free to move and carry a current.
A full answer: A material conducts only if it has charged particles that are free to move. In X, every atom uses all its outer electrons in covalent bonds, so there are no delocalised electrons and no ions. Z is made of neutral molecules whose electrons are held in covalent bonds. In both, nothing is free to move, so neither conducts.
What gets each grade
Question 2: Types of material
Solid W (a metallic solid) melts at 1538 °C and conducts electricity both as a solid and when molten. Solid Y (an ionic material) melts at 772 °C and conducts only when molten.
- Explain why W conducts electricity as a solid, but Y conducts only once it has melted. Compare the particles in each.(up to Excellence)
- Explain why both W and Y have high melting points, naming the attraction that has to be overcome in each.(up to Merit)
Check your answer
Part a: what gets the marks
- W contains delocalised electrons and Y is made of positive and negative ions.
- In W the positive metal ions sit in a sea of delocalised electrons that are free to move through the lattice even in the solid, so they carry the charge.
- In solid Y the ions are held in fixed positions in the lattice by strong ionic bonds, so they cannot move; once Y melts at 772 °C the ions are free to move and carry the charge.
- Both need charged particles that are free to move: W's delocalised electrons can move in the solid and the liquid, but Y's ions are locked in place until the ionic bonds are overcome on melting, so Y conducts only above 772 °C.
A full answer: Both substances need charged particles that are free to move. W is metallic: its positive ions sit in a sea of delocalised electrons, which can move through the lattice even in the solid, so W conducts as a solid and as a liquid. Y is ionic: its positive and negative ions are held in fixed positions in the lattice by strong ionic bonds, so in the solid they cannot move and Y does not conduct. When Y melts at 772 °C the lattice breaks down and the ions are free to move, so molten Y conducts. The difference is that W's charge carriers are already free in the solid, while Y's are only freed when it melts.
Part b: what gets the marks
- W is held together by the attraction between its positive metal ions and the delocalised electrons, and Y by the attraction between its oppositely charged ions (ionic bonds).
- Both attractions are strong and act throughout the whole lattice, so a lot of energy is needed to overcome them, which is why W melts at 1538 °C and Y at 772 °C.
A full answer: W is held by the strong attraction between its positive ions and the delocalised electrons (metallic bonding), and Y by strong ionic bonds between oppositely charged ions. In both, these attractions act through the whole lattice, so a lot of energy is needed to overcome them and the solids melt only at high temperatures (1538 °C and 772 °C).