| Date | November 2017 | Marks available | 1 | Reference code | 17N.3.sl.TZ0.4 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | Outline | Question number | 4 | Adapted from | N/A |
Question
It is wise to fill dental cavities before irreversible tooth decay sets in. An amalgam (alloy of mercury, silver, and other metals) is often used although many prefer a white composite material.
Outline the composition of an alloy and a composite.
Outline why an alloy is usually harder than its components by referring to its structure.
At present, composite fillings are more expensive than amalgam fillings.
Suggest why a patient might choose a composite filling.
Explain how Inductively Coupled Plasma (ICP) Spectroscopy could be used to determine the concentration of mercury in a sample of dental filling.
Markscheme
Alloy:
mixture of metal with other metals/non-metals
OR
mixture of elements that retains the properties of a metal
Composite:
reinforcing phase embedded in matrix phase
Award [1 max] for implying “composites only have heterogeneous/nonhomogeneous compositions”.
[Max 2 Marks]
difference in ionic/atomic radius prevents layers sliding over each other
Accept “difference in diameter/packing of cations prevents layers sliding over each other”.
concern about Hg poisoning
OR
«composite» is white «so looks more like tooth»
OR
galvanic response potential exists
OR
local allergic potential
OR
less damage/destruction of healthy tooth tissue
OR
long term corrosion requires replacement
OR
gradual darkening of tooth
Accept other correct responses.
Any three of:
sample injected into argon «plasma»
atoms «of sample» are excited/ionised
OR
electrons are promoted
electrons drop back/recombine with ions AND emit photons of characteristic energies/wavelengths/frequencies
total number of photons is proportional to concentration of element
actual concentration found from calibration/standard curve
Accept "graph/plot" for “curve”.
[Max 3 Marks]
