Dental materials and radiology basics: condensation silicone shrinkage: 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: condensation silicone shrinkage

Sample item

A condensation silicone impression is left for a day before pouring and the die is found to be smaller than the prepared tooth. What best explains this?

ARelease of hydrogen gas from the platinum catalystBThermal contraction after leaving the mouth onlyCEvaporation of alcohol formed during cross-linkingDWater uptake that swells the elastomer after removal

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

The answer and the reasoning

C. Evaporation of alcohol formed during cross-linking

Condensation-cured silicones cross-link with release of a volatile by-product, ethyl alcohol, which evaporates and causes continuing shrinkage. This is why they should be poured early. Addition silicones give no by-product and remain the most dimensionally stable.

Option by option

A. Release of hydrogen gas from the platinum catalyst
Hydrogen release is a feature of some addition silicones and causes bubbles in the stone, not shrinkage.
B. Thermal contraction after leaving the mouth only
Thermal contraction from mouth to room temperature is small and does not explain a day of change.
C. Evaporation of alcohol formed during cross-linking (correct)
Correct. Loss of the alcohol by-product keeps shrinking the material after setting.
D. Water uptake that swells the elastomer after removal
Water uptake would enlarge the impression and is a feature of hydrocolloids.

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, Evaporation of alcohol formed during cross-linking. Condensation-cured silicones cross-link with release of a volatile by-product, ethyl alcohol, which evaporates and causes continuing shrinkage.

Why is "Release of hydrogen gas from the platinum catalyst" not the answer?

Hydrogen release is a feature of some addition silicones and causes bubbles in the stone, not shrinkage.

Why is "Thermal contraction after leaving the mouth only" not the answer?

Thermal contraction from mouth to room temperature is small and does not explain a day of change.

Why is "Water uptake that swells the elastomer after removal" not the answer?

Water uptake would enlarge the impression and is a feature of hydrocolloids.

What concept does this question test?

It tests Dental materials and radiology basics: condensation silicone shrinkage in Dentistry. This is why they should be poured early. Addition silicones give no by-product and remain the most dimensionally stable.

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: condensation silicone shrinkage?

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.