Gypsum Properties and Applications Quiz
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Questions and Answers

What is the chemical formula for Calcium Sulphate Dihydrate?

  • CaSO4⋅3H2O
  • CaSO4⋅1/2 H2O
  • CaSO4
  • CaSO4⋅2H2O (correct)

    • Closed calcination occurs at temperatures between 230°F to 250°F.

    False

    What is the primary product of open calcination of gypsum?

    Plaster of Paris

    The process of heating gypsum to produce a usable form is known as __________.

    <p>calcination</p> Signup and view all the answers

    Match the following types of calcination with their characteristics:

    <p>Open Calcination = Heating in an open kettle Closed Calcination = Heating in a closed environment Calcination temperature = 110-120°C Product of open calcination = Plaster of Paris</p> Signup and view all the answers

    What is one characteristic of Plaster of Paris?

    <p>Forms very dense crystals</p> Signup and view all the answers

    What is produced when gypsum is heated at temperatures of 110-120°C?

    <p>Calcium Sulphate Hemihydrate</p> Signup and view all the answers

    Dental plaster is formed from β-hemihydrate.

    <p>True</p> Signup and view all the answers

    The chemical equation for the manufacturing of gypsum is CaSO4⋅2H2O -------- __________ + 11/2 H2O.

    <p>CaSO4⋅1/2 H2O</p> Signup and view all the answers

    What type of gypsum is known as Dental Stone with high strength?

    <p>Type V Gypsum</p> Signup and view all the answers

    Type I Gypsum is used for making study casts.

    <p>False</p> Signup and view all the answers

    What is the primary characteristic of the crystals formed in α-hemihydrate?

    <p>Smaller, regularly shaped crystalline particles in the form of rods or prisms</p> Signup and view all the answers

    Type V gypsum is known for its high strength and ________ expansion.

    <p>high</p> Signup and view all the answers

    Match the following types of gypsum with their uses:

    <p>Type I = Used to take impression Type II = Used to make study cast Type IV = Used as investing medium Type V = High strength, lower expansion</p> Signup and view all the answers

    What is the correct pressure and temperature used during closed calcination?

    <p>16 PSI and 120°C</p> Signup and view all the answers

    Crystals formed during β-hemihydrate are more regular in shape than those of α-hemihydrate.

    <p>False</p> Signup and view all the answers

    What is the ADA classification number for gypsum products?

    <p>25</p> Signup and view all the answers

    The vicat needle is used to measure ________ time in dental materials.

    <p>setting</p> Signup and view all the answers

    Which type of gypsum is stronger than plaster of Paris?

    <p>Type III Gypsum</p> Signup and view all the answers

    What happens to the dry strength of a cast as the W/P ratio increases?

    <p>Decreases</p> Signup and view all the answers

    A higher W/P ratio directly correlates to stronger dry strength in casts.

    <p>False</p> Signup and view all the answers

    What is the effect of overmixing during spatulation on the strength of a cast?

    <p>It decreases the strength of the cast.</p> Signup and view all the answers

    The ideal drying process requires that the specimen is free of excess ______.

    <p>water</p> Signup and view all the answers

    Match the properties of a good cast with their descriptions:

    <p>A = Must have high strength – reduces the chance of fracture B = Should be hard – so that surface will have no scratches during carving C = Accurately measured and stable – setting expansion should be controlled D = Compatible with impression material – no reaction</p> Signup and view all the answers

    What is the recommended disinfecting solution for casts according to the manufacturer's instructions?

    <p>1:10 dilution of sodium hypochlorite solution</p> Signup and view all the answers

    What is the purpose of the Gillmore needle?

    <p>To measure the final setting time of gypsum products</p> Signup and view all the answers

    The maximum temperature during the exothermic setting reaction can reach 30°C.

    <p>True</p> Signup and view all the answers

    What is the initial setting time when measuring gypsum?

    <p>Measured from the start of mixing until it reaches a semi-hard stage.</p> Signup and view all the answers

    The working time is generally around ___ minutes for maintaining a uniform consistency.

    <p>3</p> Signup and view all the answers

    Match the terms with their definitions:

    <p>Mixing Time (MT) = The time taken to combine water and gypsum until uniform Setting Time (ST) = Measured until the material reaches a hard stage Working Time (WT) = Time available to use a workable mix Final Setting Time (FST) = Measured from mixing until the second decrease of temperature</p> Signup and view all the answers

    Which term describes the time available to use a workable mix of gypsum?

    <p>Working Time (WT)</p> Signup and view all the answers

    Final Setting Time (FST) is measured from the start of mixing until the first decrease of temperature.

    <p>False</p> Signup and view all the answers

    What is the significance of finding the Initial Setting Time (IST) in plaster work?

    <p>It indicates when to trim off extra plaster or stone that overflowed.</p> Signup and view all the answers

    Gypsum should be soaked in water for ___ seconds before mixing.

    <p>20</p> Signup and view all the answers

    How long does mechanical mixing of gypsum typically take?

    <p>20 to 30 seconds</p> Signup and view all the answers

    What effect does fineness have on setting time?

    <p>Decreases setting time</p> Signup and view all the answers

    Higher water-to-powder ratios result in shorter setting times.

    <p>False</p> Signup and view all the answers

    What is the chemical reaction that occurs during the setting of gypsum?

    <p>H2O + CaSO4·1/2 H2O → CaSO4·2H2O + HEAT</p> Signup and view all the answers

    Accelerators help to _______ the setting time of gypsum.

    <p>shorten</p> Signup and view all the answers

    Match the type of gypsum with its setting time percentage:

    <p>Type I = 0.13% Type II = 0.3% Type III = 0.15-0.2% Type IV = 0.08% Type V = 0.3%</p> Signup and view all the answers

    What happens to setting reaction above 50°C?

    <p>It acts as a retarder</p> Signup and view all the answers

    A mixing time of longer than 2 minutes will necessarily speed up setting time.

    <p>False</p> Signup and view all the answers

    What is a consequence of using too much water in gypsum mixing?

    <p>Air bubbles or longer setting time</p> Signup and view all the answers

    Setting expansion can occur under two conditions: normal setting and _______.

    <p>air</p> Signup and view all the answers

    What is the typical effect of impurities in gypsum during the setting process?

    <p>Slower setting time</p> Signup and view all the answers

    Study Notes

    Gypsum Properties and Applications

    • Gypsum is primarily Calcium Sulfate Dihydrate (CaSO4⋅2H2O).
    • Used extensively in dental applications, due to its near-pure chemical composition.
    • Ground gypsum is produced in powdered form, which varies in strength based on processing.

    Calcination Process

    • Involves heating gypsum to produce different forms, namely β-hemihydrate (Plaster of Paris) and α-hemihydrate (Dental Stone).
    • Open Calcination occurs at 110-120°C, producing β-hemihydrate with irregularly shaped particles, characterized by large crystal size and capillary pores.
    • Closed Calcination utilizes an autoclave at 16 PSI and 120-130°C to produce α-hemihydrate, featuring smaller, prismatic crystals.

    ADA Classification of Gypsum Products

    • Type I: Impression/Soluble Plaster for taking impressions.
    • Type II: Plaster Model/Plaster of Paris for study casts.
    • Type III: Dental Stone (Class 1 Dental Stone) offers high strength for stable casts.
    • Type IV: Die Stone has high strength with lower expansion characteristics.
    • Type V: High Strength, High Expansion Dental Stone suitable for investment models.

    Setting Reaction and Properties

    • An exothermic setting reaction occurs during gypsum hardening, with temperature reaching up to 30°C in bulk.
    • Initial Setting Time (IST) measures time until the material reaches a semi-hard state, typically 7-13 minutes.
    • Final Setting Time (FST) measures until the temperature declines, usually 20 minutes.
    • Properties of good casts include high strength, hardness to prevent scratches, and accurate measurement for expansion control.

    Factors Affecting Strength and Quality

    • Water-to-Powder (W/P) Ratio: Higher ratios lead to greater porosity and weaker casts; lower ratios result in strong, denser casts.
    • Mixing Time: Extended mixing within a minute can improve strength; overmixing can result in weakness.
    • Chemical Modifiers: Use of accelerators or retarders can lower both wet and dry strength of gypsum products.

    Disinfection and Safety

    • Disinfecting solutions, such as a 1:10 dilution of sodium hypochlorite, are recommended for casts, following manufacturer guidelines.
    • Iodophore solutions may also be used for disinfection in accordance with safety protocols.

    Additional Considerations

    • Mixing Techniques: Mechanical mixing is faster than hand spatulation, which can take over a minute to achieve smooth consistency.
    • Surface characteristics: Casts should exhibit good color contrast, be free of air bubbles, and have a smooth surface for further processing in dental applications.

    Terminologies

    • Mixing Time (MT), Working Time (WT), and Setting Time (ST) are essential parameters in assessing gypsum product usability.
    • Excessive impurities can decrease setting time and impair performance.

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    Description

    Test your knowledge on the properties and applications of gypsum, including its chemical makeup and various forms produced during the calcination process. This quiz covers dental materials and the ADA classification of gypsum products, ensuring you understand their uses and characteristics thoroughly.

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