SC26 · Bulk and surface properties of matter including nanoparticlesTopic 9 — Separate chemistry 2
Choosing ceramics, polymers, composites and metals; nanoparticle uses and risks
Revise the key ideas
Comparing bulk materials
Bulk properties describe a material on an everyday scale, such as density, stiffness, strength, hardness, electrical conductivity and thermal conductivity. Read the property and its units carefully; hardness is resistance to scratching, not the same as toughness.
Glass ceramics are often hard, heat-resistant and chemically resistant, but brittle. Transparent glass suits windows; changing composition and treatment changes its properties.
Clay ceramics are made by shaping clay and heating it strongly. Their hardness, heat resistance and durability suit bricks, tiles and pottery, but brittleness limits uses where impacts or bending are common.
Polymers are often low-density and good electrical insulators; they can be flexible or rigid. Heat resistance and mechanical behaviour vary widely between polymers.
Metals generally conduct heat and electricity and can often be shaped. Their density, strength, melting point and corrosion resistance vary; alloys can alter their suitability.
A composite combines different materials to use their properties together. In fibreglass, glass fibres strengthen a surrounding polymer (the matrix). Carbon-fibre composites can be strong while having a low mass.The surrounding matrix transfers loads between fibres; fibre direction affects properties.
Concrete contains stones and sand (aggregate) held together by a cement-based material (matrix). It is strong when squeezed (in compression), but weaker when pulled (in tension). Steel reinforcement helps it resist pulling forces. The composite’s properties depend on how its parts work together.
Choose materials for the intended load, temperature, electrical requirements, environment, lifetime, manufacturing method, cost and disposal. Data should support the choice; no material category is best for every product.
Size, surface area and nanoparticles
Nanoparticles have dimensions on the scale of about 1–100 nanometres. One nanometre is 10⁻⁹ m; typical atoms are smaller, while many molecules have sizes that overlap the lower nanoscale.
A nanoparticle contains many atoms and is not just a single atom. Compare actual size data rather than treating all atoms, molecules and nanoparticles as having one fixed size.
As particles become smaller, surface area per unit volume increases. For a cube of side length L, surface area = 6L², volume = L³ and surface-area-to-volume ratio = 6/L, with inverse-length units.Halving cube size doubles its surface-area-to-volume ratio; use consistent length units.
Splitting a fixed volume into smaller particles increases its total exposed surface without changing the total material volume, if the particles remain separate and their surfaces are accessible.
Nanoparticles have a larger proportion of their atoms at or near the surface. This can make a substance behave differently from a large sample of the same material (the bulk material), for example in its reactions or how it interacts with light.
Nanoparticle catalysts have a large surface area for their mass, so less material may be needed for the same catalytic effect. If particles clump together (aggregate), less of their surface is exposed and the benefit may be reduced.
Applications, risks and evidence
Nanoparticles of titanium dioxide or zinc oxide can be used in sunscreens. They help protect against ultraviolet radiation while looking less white on skin than some formulations containing larger particles.
Other examples include catalytic surfaces and antimicrobial coatings. Effectiveness depends on the material, size, coating and exposure conditions; a nanoscale label alone does not guarantee a benefit.
Some nanoparticles may enter the body by inhalation, ingestion or skin exposure and may interact with cells. Risk depends on exposure, material and form; do not assume that every nanoparticle is either harmless or equally dangerous.
Small particles can reach sites larger particles may not, and their large surface area can increase interactions. Potential effects on organisms and environmental persistence need evidence for each material.
Evaluate benefits against possible health and environmental risks, using measured exposure and toxicity data where available. Lack of evidence of harm is not the same as proof of safety; uncertain data should limit conclusions.
For a material choice, compare both bulk performance and surface effects, and consider manufacturing, use and disposal. Safe handling may require avoiding dust and following the product or laboratory controls.
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