9.35C–9.36CSizes of particles and their properties
Chemistry only — not on the Combined Science specification
Nanoscience refers to structures that are 1–100 nm in size, of the order of a few hundred atoms.
Nanoparticles are smaller than fine particles (PM2.5), which have diameters between 100 and 2500 nm (1 × 10⁻⁷ m and 2.5 × 10⁻⁶ m). Coarse particles (PM10) have diameters between 1 × 10⁻⁵ m and 2.5 × 10⁻⁶ m. Coarse particles are often referred to as dust.
You should be able to compare 'nano' dimensions to typical dimensions of atoms and molecules, and this is where standard form, ratios and order of magnitude calculations are being examined.
An atom has a radius of about 0.1 nm, so a diameter of about 0.2 nm. A nanoparticle 100 nm across is therefore roughly 500 atoms wide — which is what the specification means by "of the order of a few hundred atoms".
Worked example
A nanoparticle has a diameter of 20 nm. An atom has a diameter of about 0.2 nm. How many times larger is the nanoparticle, and what order of magnitude is that?
- Write both in the same unit: 20 nm and 0.2 nm.
- Divide: 20 ÷ 0.2 = 100.
- So the nanoparticle is 100 times wider than an atom.
- 100 is 1 × 10², so this is two orders of magnitude.
Answer: 100 times larger in diameter — two orders of magnitude. (In volume it would be 100³ = 10⁶ times larger.)
As the side of a cube decreases by a factor of 10, the surface area to volume ratio increases by a factor of 10.
This is the key quantitative idea of the sub-topic, and it is worth proving to yourself rather than memorising, because you may be asked to calculate the surface areas and volumes of cubes.
Worked example
Calculate the surface area to volume ratio of a cube of side 10 nm, and of a cube of side 1 nm.
- A cube has 6 faces, so surface area = 6 × side².
- Volume = side³.
- For the 10 nm cube: surface area = 6 × 10² = 600 nm²; volume = 10³ = 1000 nm³; ratio = 600 ÷ 1000 = 0.6.
- For the 1 nm cube: surface area = 6 × 1² = 6 nm²; volume = 1³ = 1 nm³; ratio = 6 ÷ 1 = 6.
- Compare the two ratios: 6 ÷ 0.6 = 10.
Answer: 0.6 for the 10 nm cube and 6 for the 1 nm cube — a tenfold decrease in side length gives a tenfold increase in the surface area to volume ratio, as the specification states.
Nanoparticles may have properties different from those for the same materials in bulk because of their high surface area to volume ratio. It may also mean that smaller quantities are needed to be effective than for materials with normal particle sizes.
That second sentence is the practical consequence, and it appears again and again in the uses below: if almost every atom in a nanoparticle is at the surface, then almost every atom is available to react, absorb light or kill bacteria — so far less material does the same job.
Common mistake
Surface area to volume ratio increases as particles get smaller, not larger. Check by putting numbers in: a 1 nm cube has a ratio of 6, a 10 nm cube 0.6. If your answer says a bigger particle has the higher ratio, the division has been done the wrong way round.
- Nanoscience
- the study of structures 1–100 nm in size, of the order of a few hundred atoms
- Nanoparticle
- a particle 1–100 nm in size, smaller than fine particles, with a very high surface area to volume ratio
- Fine particles
- PM2.5: particles with diameters between 100 and 2500 nm
- Coarse particles
- PM10: particles with diameters between 2.5 × 10⁻⁶ m and 1 × 10⁻⁵ m, often referred to as dust
9.36C–9.37CUses of nanoparticles
Chemistry only — not on the Combined Science specification
Nanoparticles have many applications in medicine, in electronics, in cosmetics and sun creams, as deodorants, and as catalysts. New applications for nanoparticulate materials are an important area of research.
You should consider the advantages and disadvantages of the applications of these nanoparticulate materials, but you do not need to know specific examples or properties other than those specified.
You should be able to, given appropriate information, evaluate the use of nanoparticles for a specified purpose, and explain that there are possible risks associated with the use of nanoparticles.
An evaluation is not a list. Use the information you are given, weigh at least one advantage against at least one disadvantage, and finish with a judgement that answers the actual question asked — is this a good use of nanoparticles here, and why.
The general risk argument, which applies to nearly any use, is worth having ready. Nanoparticles are small enough to be breathed in and to pass into cells; their effects on health and on the environment have not been studied for long, because the materials are new; and the same high surface area that makes them so effective also makes them highly reactive. Because they are so new, there has not yet been time for long-term testing.
Worked example
A sun cream containing titanium dioxide nanoparticles blocks more ultraviolet light than the same mass of ordinary titanium dioxide powder, and is transparent rather than white. Evaluate its use.
- State the advantage from the information given: it blocks more ultraviolet light for the same mass, so it gives better protection against skin cancer, and being transparent it is more likely to be worn.
- Explain why, using the chemistry: the nanoparticles have a much higher surface area to volume ratio, so smaller quantities are needed to be effective.
- State the disadvantage: the particles are small enough that they may be absorbed through the skin, and the long-term effects are not known because the material is new.
- Reach a judgement, and say what would change it.
Answer: The protection against ultraviolet light is a clear, immediate benefit and needs less material, while the risk is uncertain rather than demonstrated — so on the information given the use is justified, but long-term testing of absorption through the skin would be needed to settle it.
9.38C–9.39CCeramics, polymers and composites
Chemistry only — not on the Combined Science specification
Most of the glass we use is soda-lime glass, made by heating a mixture of sand, sodium carbonate and limestone. Borosilicate glass, made from sand and boron trioxide, melts at higher temperatures than soda-lime glass — which is why laboratory glassware, heated over a flame, is made from it.
Clay ceramics, including pottery and bricks, are made by shaping wet clay and then heating in a furnace.
The properties of polymers depend on what monomers they are made from and the conditions under which they are made. For example, low density (LD) and high density (HD) poly(ethene) are produced from ethene.
You should be able to explain how low density and high density poly(ethene) are both produced from ethene. They have the same monomer, so the difference is entirely in the conditions: LD poly(ethene) is made at a high temperature and pressure with a small amount of oxygen as a catalyst, giving branched chains that cannot pack closely, so the polymer is less dense and more flexible. HD poly(ethene) is made at a lower temperature and pressure with a different catalyst, giving straight chains that pack closely, so it is denser, stronger and more rigid.
Thermosoftening polymers melt when they are heated. Thermosetting polymers do not melt when they are heated.
You should be able to explain the difference between them in terms of their structures. A thermosoftening polymer consists of individual polymer chains held together only by weak intermolecular forces. Heating gives the chains enough energy to overcome those forces, so the polymer melts — and it can be melted and reshaped again and again.
A thermosetting polymer has cross-links, strong covalent bonds between the chains, holding them in a fixed position. Heating does not break those bonds, so the polymer does not melt; heated far enough, it chars instead.
Most composites are made of two materials: a matrix or binder surrounding and binding together fibres or fragments of the other material, which is called the reinforcement.
You should be able to recall some examples of composites.
Given appropriate information, you should be able to compare quantitatively the physical properties of glass and clay ceramics, polymers, composites and metals, and explain how the properties of materials are related to their uses and select appropriate materials.
Comparing quantitatively means using the numbers you are given — density, strength, melting point — rather than describing materials in words. When selecting a material, name the property that decides it and quote the figure from the table.
Common mistake
Thermosoftening and thermosetting differ in their structure, not in the strength of the polymer chains themselves. Both have strong covalent bonds within the chains; only the thermosetting polymer has covalent cross-links between them.
- Soda-lime glass
- the glass in everyday use, made by heating a mixture of sand, sodium carbonate and limestone
- Borosilicate glass
- glass made from sand and boron trioxide, which melts at higher temperatures than soda-lime glass, so it is used for laboratory glassware
- Composite
- a material made of two materials with different properties, one acting as a matrix or binder surrounding fibres or fragments of a reinforcement
Before you practise
- Nanoparticles are 1–100 nm across — larger than atoms and simple molecules, far smaller than fine particles.
- As a cube's side falls by a factor of ten, its surface area to volume ratio rises by a factor of ten.
- That huge surface area is why a very small quantity is so effective, and why nanoparticles make good catalysts.
- Uses: sun creams, catalysts, deodorants, electronics and medical delivery — pair each with the property behind it.
- The risk argument is that their effects on health have not been fully tested and they can pass into cells.
- Glass and clay ceramics are hard and brittle; polymers are flexible and easily moulded; composites combine a reinforcement with a matrix.
- Choose a material by matching properties to the job, and say which property matters most for that use.