What you'll learn
Nanoparticles are incredibly small particles, only a few hundred atoms across, that behave differently from the same material in bulk. For AQA GCSE Chemistry you need to know the size ranges of nanoparticles compared with larger particles, understand why their very large surface area to volume ratio makes them so useful, and be able to describe some of their uses and the possible risks. This guide explains the scale of nanoparticles, the surface area to volume ratio and why it matters, the main applications such as catalysts, sunscreens and medicines, and the reasons some people are cautious about their use. By the end you should be able to calculate and compare surface area to volume ratios and explain why nanoparticles are used in a given product.
Key terms and definitions
Nanoparticle — A particle containing a few hundred atoms, with dimensions between roughly 1 and 100 nanometres.
Nanometre (nm) — A unit of length equal to one billionth of a metre (1 × 10⁻⁹ m).
Coarse particles (dust) — Larger particles with diameters between 2500 and 10 000 nm (2.5–10 micrometres).
Fine particles — Particles with diameters between 100 and 2500 nm.
Surface area to volume ratio — The total surface area of a particle divided by its volume; this ratio increases sharply as particles get smaller.
Nanoscience — The study of very small particles (nanoparticles) and their uses.
Catalyst — A substance that speeds up a reaction without being used up; nanoparticles make effective catalysts because of their large surface area.
Core concepts
The scale of nanoparticles
Particles are grouped by size. Nanoparticles are the smallest, with diameters from about 1 to 100 nm — just a few hundred atoms across. Fine particles are larger, from 100 to 2500 nm, and coarse particles (often called dust) are larger still, from 2500 to 10 000 nm. Because nanoparticles are so small, a nanoparticle can be around one hundred times smaller than a fine particle. This tiny size is what gives them their special properties.
Surface area to volume ratio
The key property of nanoparticles is their very high surface area to volume ratio. As a particle gets smaller, its volume falls faster than its surface area, so the ratio of surface area to volume rises. For a cube, if each side is halved, the surface area to volume ratio doubles. Nanoparticles are so small that a huge proportion of their atoms are on the surface, where they can take part in reactions. A high surface area to volume ratio means nanoparticles are much more reactive and effective per gram than the same material in larger lumps.
Why this matters for catalysts
Because so many of their atoms are exposed on the surface, nanoparticles make excellent catalysts. A reaction happens on the surface of a catalyst, so a large surface area lets more reactant molecules react at once. Using nanoparticles means you need only a very small mass of catalyst to have a large active surface, which can make processes cheaper and more efficient.
Uses of nanoparticles
Nanoparticles are used in many products:
- Catalysts — their large surface area speeds up industrial reactions using very little material.
- Sun creams — nanoparticles of titanium dioxide or zinc oxide give better protection against ultraviolet light and rub in without leaving a white layer.
- Medicine and drug delivery — nanoparticles can carry drugs into the body and release them where they are needed.
- Electronics — nanoparticles are used in some computer chips and sensors.
- Cosmetics and deodorants — some contain nanoparticles of silver, which has antibacterial properties.
How the surface area changes with size
It is worth seeing just how quickly the surface area to volume ratio grows as particles shrink. Imagine a cube 1 cm on each side being cut into smaller and smaller cubes. Cutting each side into ten gives 1000 tiny cubes, and the total surface area increases tenfold even though the total volume is unchanged. Keep dividing down to the nanoscale and the surface area becomes enormous compared with the volume. This is why a nanoparticle has such a high proportion of its atoms on the surface: for a particle only a few atoms across, almost every atom is a surface atom, ready to take part in a reaction. This simple geometric fact is the reason nanoparticles behave so differently from the bulk material.
Possible risks
Because nanoparticles are so new and so small, their long-term effects on health and the environment are not fully known. They may be able to pass into cells or the bloodstream in ways larger particles cannot, so some people argue that products containing them should be tested and labelled carefully. There are also questions about what happens to nanoparticles when they are washed off skin and enter rivers and the sea, where their effects on living things are not yet fully understood. This is an example of where science provides the technology, but society must weigh the benefits against the possible risks, and where more research is needed before we can be sure a product is safe.
Worked examples
Example 1: Comparing surface area to volume ratios
A cube has sides of 2 cm. Calculate its surface area to volume ratio. Surface area = 6 × (2 × 2) = 24 cm². Volume = 2 × 2 × 2 = 8 cm³. Ratio = 24 ÷ 8 = 3, written as 3:1. Now compare with a 1 cm cube: surface area = 6 × 1 = 6 cm², volume = 1 cm³, ratio = 6:1. The smaller cube has the higher ratio, showing that smaller particles have a larger surface area to volume ratio.
Example 2: Explaining why less catalyst is needed
Explain why nanoparticles can be used in smaller amounts than ordinary catalysts. Nanoparticles have a very large surface area to volume ratio, so a small mass provides a very large surface area for the reaction. Because the reaction happens on the surface, a small amount of nanoparticle catalyst gives the same effect as a much larger mass of ordinary catalyst.
Example 3: Choosing nanoparticles for sun cream
Why are nanoparticles of titanium dioxide used in modern sun creams? They provide effective protection against ultraviolet light while being small enough that they do not leave a visible white layer on the skin, so the cream is more pleasant to use and still blocks harmful UV.
Example 4: Weighing risks and benefits
A deodorant contains silver nanoparticles that kill bacteria. Suggest one benefit and one concern. A benefit is that the silver nanoparticles reduce body odour by killing bacteria very effectively because of their large surface area. A concern is that the long-term effects of the nanoparticles on the body or the environment are not fully known, so caution and testing are needed.
Common mistakes and how to avoid them
A common mistake is thinking that nanoparticles are useful simply because they are small. The real reason is their very high surface area to volume ratio — always explain the property, not just the size.
Students often get the surface area to volume ratio the wrong way round. As particles get smaller, the ratio gets larger, not smaller. Practise the cube calculation until this is automatic.
When calculating for a cube, remember there are six faces, so the surface area is 6 × (side × side). Forgetting the six is a frequent slip.
Another error is confusing the size categories. Nanoparticles are 1–100 nm, fine particles 100–2500 nm, and coarse particles (dust) 2500–10 000 nm. Learn these ranges.
Finally, do not present nanoparticles as risk-free. The examiners expect you to know that their long-term effects are uncertain, so a balanced answer mentions both the benefits and the concerns.
Exam technique for "Nanoparticles and their uses"
Many questions give you data on particle sizes and ask you to classify them, so learn the three size ranges precisely. Calculation questions on surface area to volume ratio are common and worth several marks — always show the surface area, the volume, and then the ratio, and remember to use six faces for a cube.
For "explain the use" questions, link the property to the use: a large surface area to volume ratio makes nanoparticles effective catalysts, and their small size lets sun creams work without a white layer. When asked about risks, give a balanced answer covering both a benefit and a concern, and mention that long-term effects are not yet fully understood.
Practise the cube ratio calculation for two different sizes and compare them, because "compare the ratios and comment" is a favourite exam task.
Quick revision summary
- Nanoparticles are 1–100 nm across (a few hundred atoms); fine particles are 100–2500 nm; coarse particles (dust) are 2500–10 000 nm.
- Their key property is a very high surface area to volume ratio, which rises as particles get smaller.
- For a cube, surface area = 6 × side², volume = side³; halving the side doubles the ratio.
- A large surface area makes nanoparticles effective catalysts, so less material is needed.
- Uses include catalysts, sun creams (titanium dioxide/zinc oxide), drug delivery, electronics and antibacterial products.
- Their long-term health and environmental effects are uncertain, so benefits must be weighed against risks.