EFDA Chapter 46 Impression Materials
Exam Questions and Answers
Section 1: Impression Tray Selection, Hydrocolloids &
Elastomeric Materials (Questions 1–25)
Question 1
Which type of impression material is classified as an irreversible hydrocolloid and is most
commonly used for preliminary impressions, study models, and orthodontic retainers?
A. Polyether
B. Polysulfide
C. Alginate
D. Addition silicone (VPS)
Correct Answer: C
Rationale: Alginate is an irreversible hydrocolloid that undergoes a chemical reaction from a sol
to a gel state. It is widely used for diagnostic study casts and preliminary impressions due to its
ease of use and low cost.
Question 2
When mixing alginate hydrocolloid impression material, using warmer water than recommended
by the manufacturer will have what direct effect on the material?
A. Increase the dimensional expansion
B. Shorten the setting time
C. Lengthen the working time
D. Prevent the sol-to-gel transformation
Correct Answer: B
Rationale: Water temperature directly controls the setting reaction rate of alginate. Warmer
water speeds up the chemical reaction, thereby shortening both working time and setting time.
Question 3
What condition occurs when an alginate or hydrocolloid impression absorbs additional water,
causing the impression to swell and distort?
A. Imbibition
B. Syneresis
C. Hysteresis
D. Polymerization contraction
Correct Answer: A
Rationale: Imbibition is the absorption of fluid by a colloid gel, resulting in swelling and
dimensional distortion. This is why alginate impressions should never be stored submerged in
water.
Question 4
, When an alginate impression is left exposed to open air, moisture exudates onto the surface
and the impression shrinks. What is this process called?
A. Imbibition
B. Viscosity shifting
C. Syneresis
D. Gelation phase shift
Correct Answer: C
Rationale: Syneresis refers to the loss of water by evaporation and liquid exudation from a gel,
leading to shrinkage and distortion of the impression.
Question 5
Which impression material category exhibits the highest elastic recovery and best dimensional
stability over time, allowing models to be poured days later?
A. Reversible hydrocolloid (agar)
B. Polyvinyl siloxane (addition silicone)
C. Irreversible hydrocolloid (alginate)
D. Condensation silicone
Correct Answer: B
Rationale: Polyvinyl siloxane (VPS) produces no volatile by-products during setting, offering
exceptional dimensional stability and high elastic recovery without immediate distortion.
Question 6
Why must latex gloves NOT be worn when mixing putties or handling polyvinyl siloxane (VPS)
impression materials?
A. Sulfur compounds in latex inhibit the platinum catalyst and prevent setting
B. Latex accelerates setting time instantly causing early hardening
C. Latex dissolves the vinyl polymers in the tray adhesive
D. Latex causes severe syneresis in the final impression
Correct Answer: A
Rationale: Sulfur residues found on latex gloves react with and poison the platinum salt catalyst
in addition silicones (VPS), leaving the material tacky and uncured at the surface.
Question 7
What property describes reversible hydrocolloid (agar) melting at a significantly higher
temperature than the temperature at which it solidifies into a gel?
A. Syneresis
B. Hysteresis
C. Viscoelastic elasticity
D. Imbibition delta
Correct Answer: B
Rationale: Hysteresis is the phenomenon where a material melts at one temperature (approx.
85–100°C for agar) and gels at a lower temperature (approx. 37–45°C).
Question 8
Which tray type is used to record the prepared tooth, opposing teeth, and bite registration
simultaneously in a single step?
Exam Questions and Answers
Section 1: Impression Tray Selection, Hydrocolloids &
Elastomeric Materials (Questions 1–25)
Question 1
Which type of impression material is classified as an irreversible hydrocolloid and is most
commonly used for preliminary impressions, study models, and orthodontic retainers?
A. Polyether
B. Polysulfide
C. Alginate
D. Addition silicone (VPS)
Correct Answer: C
Rationale: Alginate is an irreversible hydrocolloid that undergoes a chemical reaction from a sol
to a gel state. It is widely used for diagnostic study casts and preliminary impressions due to its
ease of use and low cost.
Question 2
When mixing alginate hydrocolloid impression material, using warmer water than recommended
by the manufacturer will have what direct effect on the material?
A. Increase the dimensional expansion
B. Shorten the setting time
C. Lengthen the working time
D. Prevent the sol-to-gel transformation
Correct Answer: B
Rationale: Water temperature directly controls the setting reaction rate of alginate. Warmer
water speeds up the chemical reaction, thereby shortening both working time and setting time.
Question 3
What condition occurs when an alginate or hydrocolloid impression absorbs additional water,
causing the impression to swell and distort?
A. Imbibition
B. Syneresis
C. Hysteresis
D. Polymerization contraction
Correct Answer: A
Rationale: Imbibition is the absorption of fluid by a colloid gel, resulting in swelling and
dimensional distortion. This is why alginate impressions should never be stored submerged in
water.
Question 4
, When an alginate impression is left exposed to open air, moisture exudates onto the surface
and the impression shrinks. What is this process called?
A. Imbibition
B. Viscosity shifting
C. Syneresis
D. Gelation phase shift
Correct Answer: C
Rationale: Syneresis refers to the loss of water by evaporation and liquid exudation from a gel,
leading to shrinkage and distortion of the impression.
Question 5
Which impression material category exhibits the highest elastic recovery and best dimensional
stability over time, allowing models to be poured days later?
A. Reversible hydrocolloid (agar)
B. Polyvinyl siloxane (addition silicone)
C. Irreversible hydrocolloid (alginate)
D. Condensation silicone
Correct Answer: B
Rationale: Polyvinyl siloxane (VPS) produces no volatile by-products during setting, offering
exceptional dimensional stability and high elastic recovery without immediate distortion.
Question 6
Why must latex gloves NOT be worn when mixing putties or handling polyvinyl siloxane (VPS)
impression materials?
A. Sulfur compounds in latex inhibit the platinum catalyst and prevent setting
B. Latex accelerates setting time instantly causing early hardening
C. Latex dissolves the vinyl polymers in the tray adhesive
D. Latex causes severe syneresis in the final impression
Correct Answer: A
Rationale: Sulfur residues found on latex gloves react with and poison the platinum salt catalyst
in addition silicones (VPS), leaving the material tacky and uncured at the surface.
Question 7
What property describes reversible hydrocolloid (agar) melting at a significantly higher
temperature than the temperature at which it solidifies into a gel?
A. Syneresis
B. Hysteresis
C. Viscoelastic elasticity
D. Imbibition delta
Correct Answer: B
Rationale: Hysteresis is the phenomenon where a material melts at one temperature (approx.
85–100°C for agar) and gels at a lower temperature (approx. 37–45°C).
Question 8
Which tray type is used to record the prepared tooth, opposing teeth, and bite registration
simultaneously in a single step?
