Discussion
What is the major difference bewteen ordinary and dental plaster ?
Answer: A
The key's answer is the expansion factor.
Both ordinary plaster of Paris and dental plaster (dental stone) are calcium sulphate hemihydrate that sets by the same reaction, taking up water to form gypsum. Dental stone is made from denser, more regular crystals, so it needs less water, sets harder and stronger, and shows a smaller and more controlled setting expansion. That controlled expansion is why it gives more accurate casts of footwear impressions.
The chemical reaction is the same for both, so that option is wrong. Drying time and cost differ only incidentally and are not the defining difference.
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