A company makes two sunscreens. Both contain zinc oxide particles. Sunscreen P contains larger particles and sunscreen Q contains nanoparticles.
The table shows the results of tests on the two sunscreens.
| Sunscreen | Diameter of zinc oxide particles / nm | Ultraviolet radiation blocked / % | Appearance on the skin | Zinc oxide found inside the skin in a laboratory test |
|---|---|---|---|---|
| P | 500 | 90 | white layer | none detected |
| Q | 25 | 97 | transparent | very small amounts in the outer layer of dead skin cells only |
A separate study found that zinc oxide nanoparticles were toxic to fish at lower concentrations than larger zinc oxide particles.
Calculate the surface area to volume ratio of a zinc oxide nanoparticle in sunscreen Q. Assume the particle is a cube with sides of 25 nm. Give the unit with your answer.
Determine how many times larger the surface area to volume ratio of the particles in Q is than that of the particles in P. Assume the particles in P are cubes with sides of 500 nm.
Explain why zinc oxide nanoparticles might behave differently from the larger zinc oxide particles.
Evaluate the use of zinc oxide nanoparticles in sunscreen Q compared with the larger particles in sunscreen P. Use the information given and your knowledge.
36 exam-style questions on Edexcel GCSE Chemistry 10.5 Bulk and surface properties of matter including nanoparticles, covering 10.5.1 Nanoparticles: size, uses and risks and 10.5.2 Comparing and selecting materials. Each one has a worked solution and a mark scheme showing where the marks go.