Paper Together: sit a paper in a room with your friends.See how it works
The sizes of coarse, fine and nanoparticles, why surface area to volume ratio matters, and the uses and risks of nanoscience.
Nanoparticles are incredibly tiny pieces of material. They are so small that you cannot see them, even with a school microscope. Yet they are already in some sun creams, sports equipment and medicines.
This matters because a material can behave differently when it is made of nanoparticles. Scientists use this to design useful new products. They also need to ask whether the new products are safe.
By the end you will be able to put particles into size groups. You will calculate a surface area to volume ratio for a cube. You will explain why nanoparticles are special, and weigh up their uses and risks.
Particle sizes are measured in nanometres, written nm. 1 nm is 1 × 10⁻⁹ m, which is a billionth of a metre. Coarse particles, or dust, are 2500 nm to 10 000 nm across.
Fine particles are 100 nm to 2500 nm across. Nanoparticles are 1 nm to 100 nm across. A nanoparticle contains only a few hundred atoms.
To change metres into nanometres, multiply by 10⁹. So 5 × 10⁻⁸ m is 50 nm.
Surface area is the total area of the outside of an object. Volume is the space inside it. Think of a gift box. The wrapping paper is the surface area. What fits inside is the volume.
For a cube with side length L, the surface area is 6 × L². A cube has six faces, and each face is L × L. The volume is L³.
The surface area to volume ratio is the surface area divided by the volume. For a cube this simplifies to 6 ÷ L.
A cube with side 2 cm has a surface area of 6 × 4 = 24 cm². Its volume is 8 cm³. So the ratio is 24 ÷ 8 = 3.
A cube with side 1 cm has a surface area of 6 cm² and a volume of 1 cm³. So the ratio is 6. Halving the side doubles the ratio.
The pattern is simple. A smaller particle has a larger ratio. Each time the diameter is divided by 10, the ratio goes up by a factor of 10.
Nanoparticles have a very large surface area compared with their volume. So a larger share of their atoms are on the surface. Surface atoms can take part in reactions and interact with other substances.
Think of a sugar cube and a spoonful of powdered sugar. The powder dissolves faster, because more of the sugar touches the water.
This is why nanoparticles can have different properties from the same material in bulk. A smaller mass can do the same job.
Nanoparticles of titanium dioxide and zinc oxide are used in sun creams. They block harmful ultraviolet light and are invisible on the skin. Silver nanoparticles are used in some wound dressings and deodorants because they kill bacteria.
A catalyst speeds up a reaction without being used up. Nanoparticle catalysts need less material, because the surface area is so large. Other uses include delivering drugs to particular cells and stronger, lighter materials for sports equipment such as tennis rackets.
The long-term effects of nanoparticles on health and the environment are not fully known. Because they are so small, they may be able to enter the body and cells and could be harmful. They could also build up in the environment.
A new medicine is tested for years before people trust it. Nanoparticles need the same careful testing.
When you evaluate nanoparticles, give a benefit, such as a better sun cream. Then give a risk, such as unknown health effects. Say that more research is needed.
Nanoparticles are 1 nm to 100 nm across. Fine particles are 100 nm to 2500 nm. Coarse particles are 2500 nm to 10 000 nm.
For a cube, surface area to volume ratio = 6 ÷ side length. Smaller particles have a larger ratio.
A large ratio means many atoms on the surface. That is why nanoparticles have different properties and make efficient catalysts.
Uses are useful, but the long-term risks are not fully known.
Calculate the surface area to volume ratio of a cube with sides of 3 cm.
Answer: 2
Explain why a nanoparticle catalyst is more efficient than the same mass of larger particles.
Answer: They have a larger surface area to volume ratio, so more atoms are exposed to the reactants.
A cube of side 20 nm is divided into cubes of side 10 nm. Compare the surface area to volume ratio before and after.
Answer: The ratio goes from 0.3 to 0.6 per nm, so it doubles.
Nanoparticles practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
What unit is used for the size of nanoparticles?
Answer: nm
Which is larger, a fine particle or a nanoparticle?
Answer: fine particle
Name one use of silver nanoparticles.
Answer: in wound dressings
Does a smaller cube have a larger or smaller surface area to volume ratio?
Answer: larger
How many faces does a cube have?
Answer: 6
What is the size range of nanoparticles? Give your answer in nm.
Answer: 1 nm to 100 nm
Calculate the surface area to volume ratio of a cube with sides of 5 cm.
Answer: 1.2
A cube has a surface area to volume ratio of 3. Its side is halved. What is the new ratio?
Answer: 6
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
A cube of a material has a side length of 4 cm. (a) Calculate the surface area to volume ratio of the cube. [3 marks] (b) The cube is cut into much smaller cubes. State what happens to the ratio. [1 mark]
Answer: (a) 1.5 (b) It increases.
Some sun creams contain nanoparticles of titanium dioxide. Evaluate the use of nanoparticles in sun creams. [6 marks]
Answer: Nanoparticles have a very large surface area to volume ratio, so a small amount blocks ultraviolet light well. They are transparent on the skin so the cream looks better and people may use it more, which protects against skin damage. The risk is that nanoparticles are so small that they may be absorbed into the body or enter cells, and the long-term effects on health and the environment are not fully known. More research is needed, so the benefit of protecting against sunburn has to be weighed against this uncertainty.
Saying smaller particles have less surface area for their size
They have more surface area compared with their volume.
Mixing up nm and µm
1 µm is 1000 nm. Nanoparticles are 1 to 100 nm.
Calculating the ratio as volume ÷ surface area
The ratio is surface area ÷ volume.
Giving only benefits when asked to evaluate
Give at least one benefit and one risk, and say more research is needed.
Nanoparticles is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 6.
Saying smaller particles have less surface area for their size. They have more surface area compared with their volume.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
See how the papers are set: AQA Combined Science papers and Edexcel Combined Science papers.
Yumio is not affiliated with, endorsed by or connected to AQA, Pearson Edexcel or any other exam board.