Dental materials and radiology basics: dental stone versus plaster: practice question

A Dentistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Dentistry · Dental materials and radiology basics: dental stone versus plaster

Sample item

A laboratory needs a cast stronger and less porous than one made from model plaster. The stone used for this purpose is made of which form of calcium sulphate?

ACalcium sulphate dihydrate that has already been fully hydratedBBeta-hemihydrate with porous, sponge-like irregular particlesCAlpha-hemihydrate with dense particles needing less waterDAnhydrous calcium sulphate with no water of crystallisation

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Alpha-hemihydrate with dense particles needing less water

Dental stone is alpha-hemihydrate, made by heating gypsum under steam pressure, which gives dense prismatic crystals. These need less water for mixing, so the set cast is stronger and harder. Model plaster is beta-hemihydrate, made by open heating, with porous particles that need more water.

Option by option

A. Calcium sulphate dihydrate that has already been fully hydrated
The dihydrate is the set product; it cannot be mixed to form a new setting mass.
B. Beta-hemihydrate with porous, sponge-like irregular particles
Beta-hemihydrate is the plaster form, whose porous particles demand more water and give a weaker cast.
C. Alpha-hemihydrate with dense particles needing less water (correct)
Correct. Alpha-hemihydrate crystals are dense and need less water, giving a stronger cast.
D. Anhydrous calcium sulphate with no water of crystallisation
Anhydrous forms hydrate very slowly and are not used as ordinary cast materials.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Dentistry question

What is the answer to this Dentistry question?

The best answer is C, Alpha-hemihydrate with dense particles needing less water. Dental stone is alpha-hemihydrate, made by heating gypsum under steam pressure, which gives dense prismatic crystals.

Why is "Calcium sulphate dihydrate that has already been fully hydrated" not the answer?

The dihydrate is the set product; it cannot be mixed to form a new setting mass.

Why is "Beta-hemihydrate with porous, sponge-like irregular particles" not the answer?

Beta-hemihydrate is the plaster form, whose porous particles demand more water and give a weaker cast.

Why is "Anhydrous calcium sulphate with no water of crystallisation" not the answer?

Anhydrous forms hydrate very slowly and are not used as ordinary cast materials.

What concept does this question test?

It tests Dental materials and radiology basics: dental stone versus plaster in Dentistry. These need less water for mixing, so the set cast is stronger and harder. Model plaster is beta-hemihydrate, made by open heating, with porous particles that need more water.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Dental materials and radiology basics: dental stone versus plaster?

Learn the dental formula, eruption ages and tooth structure once, as tables. Then use the flashcards and the other Dentistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.