Plaster of Paris is obtained by heating which of the following?
- A.Limestone
- B.Gypsum
- C.Quick lime
- D.Washing soda
Correct answer
Explanation
The correct answer is B, gypsum. Plaster of Paris is calcium sulphate hemihydrate, made by heating gypsum, which is calcium sulphate dihydrate, to about 373 kelvin so that it loses three fourths of its water of crystallisation. When water is added again it sets into a hard mass of gypsum, which is why it is used for plaster casts, moulds, statues and false ceilings, and why it must be stored in a moisture proof container. Option A is wrong because limestone is calcium carbonate and heating it gives quick lime and carbon dioxide. Option C is wrong because quick lime is calcium oxide, the product of that reaction, and adding water to it gives slaked lime. Option D is wrong because washing soda is sodium carbonate decahydrate and heating it simply drives off water of crystallisation. Note also that heating gypsum too strongly destroys the setting property.
Read the full article: Acids, Bases and Salts: pH Scale and Common Salts
Practice Questions
View allWhich of the following contains citric acid?
- A.Tomato
- B.Orange
- C.Tamarind
- D.Sour milk
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Citrus fruits such as orange, lemon, lime and grapefruit owe their sharp sour taste to citric acid, which is the acid textbooks name for this family and the one examiners expect here.
Read the full explanation →The pH of a solution is 3. When its pH changes to 6, then the hydrogen ion concentration
- A.Increases 2 times
- B.Increases three times
- C.Decreases 100 times
- D.Decreases 1000 times
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The pH is the negative logarithm of the hydrogen ion concentration, so each rise of one unit means the concentration has fallen to one tenth.
Read the full explanation →When litmus is added to a solution of borax, it turns _______.
- A.Pink
- B.Orange
- C.Red
- D.Blue
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Borax, sodium tetraborate, is the salt of a strong base, sodium hydroxide, and a weak acid, boric acid, so its solution in water is basic.
Read the full explanation →What is the chemical formula of baking soda?
- A.Na2CO3
- B.NaHCO3
- C.NaOH
- D.CaOCl2
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Baking soda is sodium hydrogen carbonate, a mild, non corrosive base used as an antacid, as an ingredient of baking powder along with a mild edible acid such as tartaric acid, and in soda acid fire extinguishers, where it releases carbon dioxide.
Read the full explanation →The acid present in the sting of an ant is
- A.Formic acid
- B.Citric acid
- C.Lactic acid
- D.Tartaric acid
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The sting of an ant and the hairs of the nettle plant inject formic acid, also called methanoic acid, which is why the skin burns; rubbing on a mild base such as baking soda or the leaf of the dock plant relieves it by neutralising the acid.
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