Materials questions reward knowing why a material behaves as it does, not just its name. Mixing ratios, working time, storage conditions and the consequences of getting them wrong all appear — often framed as 'the mix looks like this, what happened?'.
What materials covers in the Knowledge Test
- Amalgam, composite, glass ionomer and when each is indicated
- Impression materials: alginate, silicones and compound
- Cements, liners and bases
- Mixing technique, working time and setting reactions
- Storage, shelf life and handling requirements
- Equipment used to prepare and deliver materials
Where candidates lose marks
Blaming the material when the fault is proportioning or technique.
Forgetting that temperature changes setting time — a chilled slab is a deliberate choice.
Confusing which materials are light-cured with those that are chemically cured.
Knowing why, not just what
Materials questions are usually diagnostic: you are shown a result — a mix that is too runny, a restoration that stays tacky, an impression with show-through — and asked what went wrong. The answer is almost always technique or proportioning rather than the material itself.
| What you see | Usual cause |
|---|---|
| Mix too thin and runny | Too much liquid — incorrect powder-to-liquid ratio |
| Mix sets too quickly | Warm slab or environment; incorrect ratio |
| Composite still tacky after curing | Inadequate light curing, or the oxygen-inhibited surface layer |
| Impression shows tray through the material | Tray underloaded, or wrong tray size |
| Alginate distorted | Delay in casting, or incorrect storage |
Choosing the material
| Material | Typical use | Note |
|---|---|---|
| Amalgam | Posterior load-bearing restorations | Mercury-containing — hazardous waste |
| Composite | Anterior and posterior, aesthetic | Light cured; shade chosen before isolation |
| Glass ionomer | Linings, temporary restorations, high caries risk | Releases fluoride; chemically bonds |
| Alginate | Study models, opposing arches | Dimensionally unstable — cast promptly |
| Silicone | Precision impressions | More stable than alginate |
Shade before isolation
Shade is selected before the tooth dehydrates under isolation, because a dehydrated tooth appears lighter and produces a mismatch once it rehydrates. This ordering point is examined more often than the shade guide itself.
Storage and handling
Materials have shelf lives and storage conditions, and both affect how they set. Chilling a glass slab deliberately slows a reaction to extend working time — that is a technique, not an error, and questions sometimes offer it as though it were a mistake.
Worked examples
Real questions from our bank, with the reasoning. No sign-in needed.
A nurse loads too little material into a stock tray and the dentist notes show-through areas. What has happened?
- APolymerisation shrinkage
- BInsufficient tray loadingCorrect
- CShade mismatch from dehydration
- DOver-curing
Why: Show-through means the tray is visible through the impression material — there was not enough material to cover it properly.
Both free mocks draw from this topic in the same proportion as the real paper — 65 questions in 90 minutes, with an explanation for every answer and a per-topic breakdown at the end.
Start a free mock examMaterials questions, answered
Why is a glass slab sometimes chilled before mixing?
To slow the setting reaction and extend working time. It is a deliberate technique for materials such as zinc oxide eugenol, not a handling error.
Why choose the shade before placing rubber dam?
Isolation dehydrates the tooth and makes it look lighter. A shade chosen at that point will look wrong once the tooth rehydrates.
Why does a composite feel tacky after curing?
Usually inadequate light curing, or the oxygen-inhibited layer at the surface. It is worth checking the curing light's output as well as the technique.
Why must alginate impressions be cast promptly?
Alginate is dimensionally unstable — it loses or gains water and distorts. Delay changes the fit of whatever is made from it.
Which waste stream does amalgam go in?
Amalgam waste, separately from clinical and domestic waste, because it contains mercury and is classed as hazardous.
The other nine topics
Independent study material, not affiliated with or endorsed by NEBDN. Always check the official NEBDN website for the current syllabus. NEBDN does not publish a pass mark for the Knowledge Test.