Structures, Properties and Nanoparticles - Worksheets, Questions and Revision

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GCSE · Chemistry

C2b Structures, Properties and Nanoparticles

AQA 8464 · Calculator allowed · about 95 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

A Manchester Discovery: Isolating Graphene

Original text written for Revision Library.

Graphite has been used in pencils for centuries, but it took until 2004 for scientists to isolate a single layer of it. At the University of Manchester, Andre Geim and Konstantin Novoselov repeatedly pressed strips of sticky tape onto a small piece of graphite, then onto each other, splitting the layers apart each time. Eventually they were left with flakes of carbon just one atom thick, a material now called graphene. Because each carbon atom in graphene is joined to three others by strong covalent bonds in a flat hexagonal sheet, the material is remarkably strong for its very low mass, and the one electron per carbon atom that is not used in bonding is free to move through the sheet, so graphene also conducts electricity extremely well. Geim and Novoselov were awarded the Nobel Prize in Physics in 2010 for this work, and graphene is now studied worldwide for use in flexible electronics, composite materials and water filters.

Water can exist as a solid, a liquid or a gas, depending on its temperature.

(a)State the name of the change of state when a liquid turns into a gas.(1)

(b)Describe how the arrangement, movement and energy of the water particles change as the liquid evaporates to form a gas.(3)

(c)A pure substance has one fixed melting point. Explain why melting point data can be used to help show whether a sample of a substance is pure.(1)

(Total for Question 1 is 5 marks)

Magnesium oxide (MgO) is a giant ionic lattice made up of Mg2+ ions and O2- ions.

(a)Describe the structure of a giant ionic lattice such as magnesium oxide.(2)

(b)Magnesium oxide has a much higher melting point than sodium chloride. Use the charges on the ions in each compound to explain why.(2)

(c)Explain why solid magnesium oxide does not conduct electricity, but molten magnesium oxide does.(2)

(Total for Question 2 is 6 marks)

Iodine exists as simple molecules with the formula I2.

(a)State the type of bonding that joins the two iodine atoms together within an I2 molecule.(1)

(b)Explain, in terms of the forces between particles, why iodine has a low melting point even though the covalent bond within each molecule is strong.(2)

(c)Explain why iodine does not conduct electricity in any state (solid, liquid or gas).(2)

(Total for Question 3 is 5 marks)

Diamond and graphite are both giant covalent structures made only of carbon atoms, yet they have very different physical properties.

Compare the structures of diamond and graphite, and use these structures to explain the differences in their hardness and electrical conductivity.

(Total for Question 4 is 6 marks)

Buckminsterfullerene and carbon nanotubes both belong to the fullerene family of carbon structures.

(a)Give the molecular formula of buckminsterfullerene, the first fullerene to be discovered.(1)

(b)State one use of carbon nanotubes, and explain how a property of nanotubes makes them suitable for this use.(2)

(c)Give one other use of fullerene molecules (other than the use given in part (b)).(1)

(Total for Question 5 is 4 marks)

Poly(ethene) is an example of a thermosoftening polymer. Some other polymers are thermosetting.

(a)Describe, in terms of bonding between polymer chains, the structure of a thermosoftening polymer.(2)

(b)Explain why thermosoftening polymers can be melted and reshaped repeatedly, while thermosetting polymers cannot.(2)

(c)State one factor, other than the presence of cross-links, that affects the properties (such as strength or melting point) of a polymer.(1)

(Total for Question 6 is 5 marks)

Bronze is an alloy made from copper and tin.

(a)Describe the type of bonding present in a pure metal such as copper, including reference to delocalised electrons.(2)

(b)Explain why pure copper is malleable (can be hammered or bent into shape).(2)

(c)Bronze contains tin atoms mixed in with the copper atoms. Explain, in terms of particle arrangement, why bronze is harder than pure copper.(2)

(Total for Question 7 is 6 marks)

Nanoparticles are structures with dimensions much smaller than those we can see, but larger than individual atoms.

(a)State the approximate size range, in nanometres, of a nanoparticle.(1)

(b)Place the following in order of increasing size (smallest first): fine particle, atom, nanoparticle.(1)

(c)Give one everyday use of nanoparticles, and one possible risk associated with their use.(2)

(Total for Question 8 is 4 marks)

A student compares two cube-shaped samples of the same material to investigate how particle size affects surface area to volume ratio.

(a)Calculate the surface area to volume ratio of a cube with a side length of 1 cm. Give your answer in the form n : 1.(2)

(b)Calculate the surface area to volume ratio of a cube of the same material with a side length of 0.1 cm.(1)

(c)State how the surface area to volume ratio of a cube changes as its side length gets smaller.(1)

(Total for Question 9 is 4 marks)

A manufacturer compares a cube-shaped silica nanoparticle with a side length of 20 nm to a bulk cube of the same silica with a side length of 2 cm.

(a)Calculate the surface area to volume ratio of the nanoparticle cube, in nm^-1.(2)

(b)Calculate the surface area to volume ratio of the bulk cube, in cm^-1.(2)

(c)Convert your answer to part (a) into cm^-1, and hence show that the nanoparticle's surface area to volume ratio is approximately 1 x 10⁶ (one million) times greater than the bulk cube's ratio found in part (b).(3)

(d)Explain why this large difference in surface area to volume ratio makes a nanoparticle catalyst more effective, per gram, than the same bulk material used as a catalyst.(2)

(Total for Question 10 is 9 marks)

A cosmetic company uses titanium dioxide nanoparticles with a diameter of 50 nm in a sun cream.

(a)Convert 50 nm into metres. Give your answer in standard form.(1)

(b)A single human hair has a diameter of 0.05 mm. Calculate how many of these nanoparticles, placed side by side in a straight line, would span the width of one human hair. Give your answer in standard form.(2)

(Total for Question 11 is 3 marks)

Priya carries out a practical investigation into four unlabelled solid substances, A, B, C and D, in order to classify the type of structure and bonding in each. For each substance she tests its melting point, and whether it conducts electricity as a solid and (separately) once it has been melted.
Table 1

Substance	Melting point	Conducts as a solid?	Conducts when molten?
A	High	No	Yes
B	High	No	No
C	Low	No	No
D	High	Yes	Yes

(a)Describe how Priya could safely test whether a solid substance conducts electricity.(2)

(b)Priya then needs to melt each solid to test whether it conducts electricity as a liquid. Suggest one safety precaution she should take when heating a solid until it melts.(1)

(c)State one variable that Priya should control to make her comparison of melting points a fair test.(1)

(d)Using the results in Table 1, identify the most likely type of structure and bonding for each of substances A, B, C and D.(4)

(e)Explain why substance A only conducts electricity when molten, and not when solid.(2)

(f)Evaluate one limitation of using melting point alone to identify the type of bonding in an unknown solid, and suggest one improvement to Priya's method.(2)

(g)Explain why it would be dangerous for Priya to test the electrical conductivity of a substance while it is still molten by holding bare wires against it by hand.(2)

(h)State the independent variable in Priya's investigation.(1)

(Total for Question 12 is 15 marks)

Graphene, a single layer of graphite, was first isolated at the University of Manchester in 2004.

(a)State what is meant by the term 'graphene'.(1)

(b)Explain, in terms of its structure, why graphene is a good conductor of electricity.(2)

(c)Suggest one use of graphene in electronics, and state the property that makes it suitable for this use.(1)

(Total for Question 13 is 4 marks)

Ionic compounds share several typical physical properties.

(a)State one physical property, other than having a high melting point, that is typical of ionic compounds.(1)

(b)Explain why ionic compounds are often brittle, shattering rather than bending when a force is applied.(2)

(Total for Question 14 is 3 marks)

Pure metals are often mixed with other elements to form alloys for use in everyday products.

(a)Give one reason why an alloy such as steel is often used in construction rather than pure iron.(1)

(b)Name one everyday alloy, other than steel or bronze, and state one of its uses.(2)

(Total for Question 15 is 3 marks)

A materials scientist compares three samples of the same polymer that differ only in their average chain length.
Sample 1 (short chains): melting point 85 degC
Sample 2 (medium chains): melting point 120 degC
Sample 3 (long chains): melting point 160 degC

(a)Describe the trend shown by the data.(1)

(b)Explain, in terms of intermolecular forces, why increasing chain length increases the melting point of a polymer.(3)

(Total for Question 16 is 4 marks)

An atom has a diameter of approximately 0.1 nm. A nanoparticle at the top of the typical size range has a diameter of 100 nm.

(a)Calculate how many atoms, placed side by side in a straight line, would span the diameter of this nanoparticle. Give your answer in standard form.(2)

(b)State the order of magnitude difference, as a power of 10, between the diameter of this nanoparticle and the diameter of an atom.(1)

(Total for Question 17 is 3 marks)

Silver nanoparticles are increasingly used as an antibacterial coating in wound dressings, replacing the traditional use of bulk silver metal.

(a)Explain, in terms of surface area to volume ratio, why silver nanoparticles are more effective antibacterial agents than the same mass of bulk silver.(3)

(b)Suggest one economic advantage to a manufacturer of using nanoparticle silver rather than bulk silver in a wound dressing, if the total mass of silver used stays the same.(1)

(c)Evaluate the risks that should be considered before using silver nanoparticles in medical products, compared with using bulk silver.(3)

(Total for Question 18 is 7 marks)

Mark scheme · C2b Structures, Properties and Nanoparticles

Question 1

(a) B1 evaporation (accept boiling if it occurs throughout the liquid at its boiling point) oe
(a) Answer: Evaporation
(b) B1 the particles change from being close together in a random arrangement to being far apart in a random arrangement (oe)
(b) B1 the particles move faster and move freely/randomly in all directions, rather than moving around each other while staying close together (oe)
(b) B1 the particles gain energy (the change is endothermic), which is used to overcome the forces of attraction between them (oe)
(b) Answer: The particles move from close together (random arrangement) to far apart (random arrangement); they move much faster and more freely in all directions; they gain energy, which is used to overcome the forces of attraction between them
(c) B1 an impure sample melts over a range of temperatures and/or at a lower temperature than the pure substance, so if the sample melts sharply at the known melting point it is likely to be pure (oe)
(c) Answer: A pure substance melts sharply at one fixed temperature; an impure sample melts over a range of temperatures (often starting lower), so comparing the melting behaviour to the known value indicates purity

Question 2

(a) B1 a regular, repeating (three-dimensional) arrangement of oppositely charged ions (oe)
(a) B1 held together by strong electrostatic forces of attraction between the oppositely charged ions, acting in all directions (ionic bonds) (oe)
(a) Answer: A regular, repeating 3D arrangement of oppositely charged ions, held together by strong electrostatic forces of attraction (ionic bonds) acting in all directions
(b) M1 magnesium oxide contains Mg2+ and O2- ions (2+ and 2- charges), whereas sodium chloride contains Na+ and Cl- ions (1+ and 1- charges) (oe)
(b) A1 the higher ionic charges in magnesium oxide give stronger electrostatic forces of attraction between the ions, so more energy is needed to overcome them and separate the lattice, giving a much higher melting point (oe)
(b) Answer: Mg2+ and O2- have higher charges than Na+ and Cl-, so the electrostatic forces of attraction between the ions in magnesium oxide are stronger; more energy is needed to overcome these stronger forces, giving magnesium oxide a much higher melting point (MgO approximately 2852 degC compared with NaCl approximately 801 degC)
(c) M1 in the solid, the ions are held in fixed positions in the lattice and cannot move, so there are no charge carriers free to move (oe)
(c) A1 when molten, the ions are free to move (throughout the liquid) and can carry electrical charge, so the liquid conducts (oe)
(c) Answer: In the solid, ions are fixed in place in the lattice and cannot move to carry charge; when molten, the ions are free to move and can carry electrical charge, so molten magnesium oxide conducts

Question 3

(a) B1 covalent bonding oe
(a) Answer: Covalent bonding
(b) M1 melting iodine only requires overcoming the weak intermolecular forces (forces of attraction between molecules), not breaking the strong covalent bonds within the molecules (oe)
(b) A1 these intermolecular forces are weak, so only a small amount of energy is needed to overcome them, giving a low melting point (oe)
(b) Answer: Melting iodine only overcomes the weak forces of attraction between I2 molecules, not the strong covalent bonds within them; since these intermolecular forces are weak, little energy is needed, giving a low melting point
(c) M1 iodine molecules have no overall electrical charge (oe)
(c) A1 there are no ions and no free/delocalised electrons able to move and carry charge through the substance (oe)
(c) Answer: Iodine molecules are uncharged and there are no ions or delocalised electrons present to move and carry electrical charge, so iodine cannot conduct in any state

Question 4

Level 3 (5-6): A clear, detailed and accurate comparison of both structures, correctly linked to both hardness and electrical conductivity. Reference is made to the number of covalent bonds per carbon atom, the three-dimensional rigid lattice of diamond versus the layered structure of graphite, the presence or absence of delocalised electrons, and the weak forces between graphite's layers.
Level 2 (3-4): Structural detail is given for at least one of diamond or graphite, and this is used to explain at least one property (hardness or conductivity). The explanation may be one-sided or only partly linked to the structure.
Level 1 (1-2): Simple, largely unlinked statements about diamond and/or graphite are given, such as 'diamond is hard' or 'graphite conducts electricity', with little or no reference to structure or bonding.
Level 0 (0): No relevant content.
Indicative content:
- In diamond, each carbon atom forms four covalent bonds to four other carbon atoms, arranged in a rigid, three-dimensional tetrahedral lattice.
- This rigid lattice of strong covalent bonds, extending throughout the structure in all directions, means a large number of strong bonds must be broken to break or scratch diamond, so diamond is extremely hard.
- All four outer electrons of each carbon atom in diamond are used in covalent bonding, so there are no free or delocalised electrons and no ions; diamond does not conduct electricity.
- In graphite, each carbon atom forms three covalent bonds to three other carbon atoms, arranged in flat hexagonal layers (sheets).
- Only three of each carbon atom's four outer electrons are used in covalent bonding, so one electron per carbon atom is delocalised and free to move within a layer.
- These delocalised electrons can move and carry charge, so graphite conducts electricity (unlike diamond).
- The layers in graphite are held together only by weak intermolecular forces between layers, with no covalent bonds between them.
- Because these forces between layers are weak, the layers can slide over each other easily, so graphite is soft and slippery, and is used as a lubricant and in pencils.
- Diamond's hardness makes it useful in cutting tools and drill tips, whereas graphite's softness and electrical conductivity make it useful for very different applications, such as electrodes.

Question 5

(a) B1 C60 cao
(a) Answer: C60
(b) B1 a valid use, e.g. reinforcing materials/composites (such as tennis racket frames or bicycle frames), or as tiny electrical wires/components in nanotechnology, or as industrial catalysts (any one) oe
(b) B1 a linked property, e.g. high tensile strength for their very low mass/density makes them good for reinforcing composites without adding much weight; or their ability to conduct electricity makes them useful in tiny electrical circuits; or their very large surface area to volume ratio makes them effective catalysts (must match the use given) oe
(b) Answer: For example: reinforcing composite materials (e.g. sports equipment), because nanotubes have a very high tensile strength for their low mass, making the material strong without adding much weight
(c) B1 any valid use, e.g. delivering drugs into the body (drug delivery), acting as lubricants, or acting as catalysts (any one, must differ from the use given in (b)) oe
(c) Answer: For example: delivering drug molecules to specific cells in the body (drug delivery)

Question 6

(a) B1 thermosoftening polymers consist of individual, tangled polymer chains (oe)
(a) B1 only weak intermolecular forces act between the separate chains, with no covalent bonds (cross-links) joining one chain to another (oe)
(a) Answer: Thermosoftening polymers are made of individual, tangled chains held together only by weak intermolecular forces between chains, with no covalent cross-links between them
(b) M1 on heating a thermosoftening polymer, the weak intermolecular forces between chains are overcome, allowing the chains to move/slide past each other so the polymer softens and can be remoulded, then this can be repeated (oe)
(b) A1 thermosetting polymers have covalent cross-links (strong covalent bonds) between chains, forming a rigid giant covalent network; these strong cross-links do not break on heating, so the polymer does not melt/soften and cannot be reshaped (oe)
(b) Answer: Heating overcomes the weak forces between the separate chains of a thermosoftening polymer, letting the chains slide and the polymer be reshaped repeatedly; thermosetting polymers have strong covalent cross-links between chains that do not break on heating, so they cannot be melted or reshaped
(c) B1 the length of the polymer chains (or the degree of branching of the chains) oe
(c) Answer: The average length of the polymer chains (or the amount of branching in the chains)

Question 7

(a) B1 a giant structure of positive metal ions arranged in a regular lattice (oe)
(a) B1 surrounded by a 'sea' of delocalised (free) electrons, with strong electrostatic forces of attraction between the positive ions and the delocalised electrons (metallic bonding) (oe)
(a) Answer: A giant lattice of positive metal ions surrounded by a sea of delocalised electrons, held together by strong electrostatic forces of attraction between the ions and the delocalised electrons (metallic bonding)
(b) M1 the metal ions are arranged in layers that can slide over each other when a force is applied (oe)
(b) A1 the delocalised electrons continue to hold the ions together as the layers move, so the metal bends/changes shape without shattering (oe)
(b) Answer: The layers of metal ions can slide over each other when a force is applied, while the delocalised electrons continue to hold the structure together, so the metal bends into a new shape rather than shattering
(c) M1 tin atoms are a different size to copper atoms, so they distort the regular layers of the copper lattice (oe)
(c) A1 this makes it harder for the layers of atoms to slide over each other, so more force is needed to bend/deform the alloy, making bronze harder than pure copper (oe)
(c) Answer: The different-sized tin atoms distort the regular layers of copper atoms, making it harder for the layers to slide over one another, so bronze is harder than pure copper

Question 8

(a) B1 1 nm to 100 nm (accept values within this range) cao
(a) Answer: 1 nm to 100 nm
(b) B1 atom, nanoparticle, fine particle cao
(b) Answer: Atom, then nanoparticle, then fine particle
(c) B1 a valid use, e.g. titanium dioxide/zinc oxide nanoparticles in sun cream, silver nanoparticles in antibacterial deodorants/dressings/food packaging, nanoparticle catalysts, or nanoparticles in electronics/sensors/sports equipment (any one) oe
(c) B1 a valid risk, e.g. the effects of nanoparticles on human health and the environment are not yet fully known/understood, or nanoparticles may be able to pass through cell membranes/skin and enter the bloodstream more easily than larger particles (any one) oe
(c) Answer: For example: use, titanium dioxide nanoparticles in sun cream; risk, the long-term effects of nanoparticles on human health and the environment are not yet fully understood

Question 9

(a) M1 correct surface area and volume found: surface area = 6 x 1² = 6 cm², volume = 1³ = 1 cm³ (oe)
(a) A1 ratio = 6 : 1 (accept 6 cm^-1) cao
(a) Answer: 6 : 1 (6 cm^-1)
(b) B1 60 : 1 (60 cm^-1) cao
(b) Answer: 60 : 1 (60 cm^-1)
(c) B1 the surface area to volume ratio increases as the side length (particle size) decreases, ft consistent with (a) and (b) oe
(c) Answer: The surface area to volume ratio increases as the cube gets smaller

Question 10

(a) M1 surface area = 6 x 20² = 2400 nm², volume = 20³ = 8000 nm³ (oe)
(a) A1 0.3 nm^-1 (accept 3 : 10 or 0.3 : 1) cao
(a) Answer: 0.3 nm^-1
(b) M1 surface area = 6 x 2² = 24 cm², volume = 2³ = 8 cm³ (oe)
(b) A1 3 cm^-1 (accept 3 : 1) cao
(b) Answer: 3 cm^-1
(c) M1 correct conversion factor used: 1 nm = 1 x 10^-7 cm, so 1 nm^-1 = 1 x 10⁷ cm^-1 (oe), ft from (a)
(c) A1 0.3 nm^-1 = 0.3 x (1 x 10⁷) = 3 x 10⁶ cm^-1, ft from (a)
(c) A1 comparison to (b): 3 x 10⁶ / 3 = 1 x 10⁶, so the nanoparticle's ratio is approximately one million (10⁶) times greater than the bulk cube's ratio, ft from (a) and (b)
(c) Answer: 3 x 10⁶ cm^-1, which is approximately 1 x 10⁶ (one million) times greater than the bulk cube's ratio of 3 cm^-1
(d) M1 a much greater proportion of the atoms/particles in a nanoparticle sample are on the surface, exposed and available to react with reactant particles, for a given mass (oe)
(d) A1 so reactions can take place faster, or the same catalytic effect can be achieved using a much smaller mass of nanoparticle catalyst than bulk catalyst (oe)
(d) Answer: A much greater proportion of the particles in a nanoparticle sample are on the exposed surface for a given mass, so more collisions with reactant particles can happen; this means reactions are catalysed faster, or the same effect is achieved using far less material

Question 11

(a) B1 5 x 10^-8 m cao
(a) Answer: 5 x 10^-8 m
(b) M1 0.05 mm converted to nm: 0.05 x 1 x 10⁶ nm = 5 x 10⁴ nm, then 5 x 10⁴ / 50 (oe method, ft from consistent unit conversion)
(b) A1 1 x 10³ (1000) cao
(b) Answer: 1 x 10³ (1000 nanoparticles)

Question 12

(a) M1 connect the solid sample into a circuit with a low-voltage power supply and a bulb or ammeter, using electrodes/crocodile clips that touch the sample (oe)
(a) A1 observe whether the bulb lights (or the ammeter shows a current), and only handle the circuit using the insulated wires/clips, not the electrodes directly (oe)
(a) Answer: Connect the solid into a circuit with a low-voltage power supply and a bulb or ammeter, using electrodes that touch the sample; observe whether the bulb lights (or a current is shown), handling the circuit only by its insulated parts
(b) B1 any valid precaution, e.g. wear eye protection/heat-resistant gloves, use tongs/a heatproof stand rather than holding the container, heat gently and do not overheat, or work in a fume cupboard if the substance may give off fumes (any one) oe
(b) Answer: For example: wear eye protection and use tongs or a heatproof mat/stand rather than holding the heated container directly
(c) B1 any valid controlled variable, e.g. the mass/amount of each sample used, the rate of heating, or the type/purity of each sample (any one) oe
(c) Answer: For example: the mass of each sample used should be the same
(d) B1 A: giant ionic (lattice) - high melting point, and only conducts once the ions are free to move (i.e. when molten) oe
(d) B1 B: giant covalent (e.g. diamond or silicon dioxide) - high melting point, but no ions or delocalised electrons free to move, so it never conducts oe
(d) B1 C: simple molecular - low melting point (only weak intermolecular forces to overcome), and no charged particles free to move, so it never conducts oe
(d) B1 D: metallic - high melting point, and conducts as both a solid and a liquid because delocalised electrons are free to move in both states oe
(d) Answer: A = giant ionic lattice; B = giant covalent structure; C = simple molecular substance; D = metal (metallic structure)
(e) M1 in the solid, the ions in A are held in fixed positions in the lattice and cannot move to carry charge, ft from (d) oe
(e) A1 once molten, the ions are free to move throughout the liquid and can carry electrical charge, so it conducts, ft from (d) oe
(e) Answer: As a solid, A's ions are fixed in the lattice and cannot move; once molten, the ions are free to move and carry charge, so molten A conducts electricity
(f) B1 limitation, e.g. both giant ionic and giant covalent substances can have similarly high melting points, so melting point alone cannot always distinguish between them (oe)
(f) B1 improvement, e.g. also test electrical conductivity in the solid and molten states (as Priya did), or test solubility in water, to help distinguish between structures with similar melting points (oe)
(f) Answer: Limitation: giant ionic and giant covalent substances can both have very high melting points, so melting point alone cannot reliably tell them apart. Improvement: also test electrical conductivity as a solid and when molten, since this can distinguish ionic (conducts only when molten) from covalent (never conducts) structures
(g) M1 the molten substance is likely to be extremely hot, so touching it or bare wires near it risks severe burns (oe)
(g) A1 if the molten substance conducts electricity, holding bare wires by hand also risks an electric shock; suitable insulated apparatus (e.g. electrodes held in a stand, not by hand) should be used instead (oe)
(g) Answer: The molten substance will be very hot, so holding bare wires near it risks serious burns; if it conducts electricity, there is also a risk of electric shock, so insulated electrodes held in a stand, not by hand, should be used instead
(h) B1 the substance/sample being tested (A, B, C or D) oe
(h) Answer: The identity of the substance being tested (A, B, C or D)

Question 13

(a) B1 a single layer of graphite, one carbon atom thick, with carbon atoms arranged in hexagons (oe)
(a) Answer: A single layer of graphite, one carbon atom thick, with the carbon atoms arranged in a hexagonal pattern
(b) M1 each carbon atom in graphene forms three covalent bonds, so one outer electron per carbon atom is not used in bonding (oe)
(b) A1 these electrons are delocalised and free to move across the sheet, allowing them to carry electrical charge, so graphene conducts electricity well (oe)
(b) Answer: Each carbon atom forms only three covalent bonds, leaving one electron per atom delocalised and free to move across the sheet; these delocalised electrons carry charge, making graphene an excellent conductor
(c) B1 a valid use with a matching property, e.g. flexible touchscreens because graphene is thin, strong and conducts electricity, or as a very thin, lightweight electrical conductor in circuits (any one, use and property must match) oe
(c) Answer: For example: flexible touchscreens, because graphene is extremely thin, strong and conducts electricity well

Question 14

(a) B1 they are often soluble in water, and/or they are hard but brittle (any one) oe
(a) Answer: Many ionic compounds are soluble in water (they are also typically hard but brittle)
(b) M1 when a force is applied, the layers of ions can shift slightly out of place relative to each other (oe)
(b) A1 this brings ions of the same charge next to each other; the like charges repel strongly, forcing the layers apart and shattering the lattice (oe)
(b) Answer: A force can shift the layers of ions so that ions of the same charge line up next to each other; these like charges repel strongly, forcing the layers apart, so the lattice shatters rather than bending

Question 15

(a) B1 steel (an alloy) is harder/stronger than pure iron (oe)
(a) Answer: Steel is harder and stronger than pure iron
(b) B1 a valid named alloy, e.g. brass, or a named alloy of aluminium, or solder (any one) oe
(b) B1 a matching, valid use, e.g. brass used for door fittings/musical instruments, aluminium alloys used in aircraft bodies, solder used to join electrical components (must match the alloy named) oe
(b) Answer: For example: brass, used to make door fittings and musical instruments

Question 16

(a) B1 as the average chain length increases, the melting point increases (oe)
(a) Answer: As average chain length increases, the melting point of the polymer increases
(b) M1 longer polymer chains have more points of contact with neighbouring chains (oe)
(b) M1 so there are more intermolecular forces acting between the chains, and/or the total (combined) strength of these forces between neighbouring chains is greater (oe)
(b) A1 more energy is needed to overcome these stronger/more numerous intermolecular forces and separate the chains, so the melting point is higher (oe)
(b) Answer: Longer chains have more points of contact with neighbouring chains, so there are more (and a greater combined strength of) intermolecular forces between them; more energy is needed to overcome these forces and separate the chains, so the melting point increases

Question 17

(a) M1 correct method: 100 / 0.1 (oe)
(a) A1 1 x 10³ (1000) cao
(a) Answer: 1 x 10³ (1000 atoms)
(b) B1 10³ (three orders of magnitude), ft from (a) oe
(b) Answer: 10³ (the nanoparticle's diameter is three orders of magnitude, i.e. about a thousand times, greater than the atom's diameter)