UPSC Prelims 2003 Q145: Science & Technology – Chemistry
Which one of the following statements is NOT correct?
- (a) The presence of NaCl increases the rate of setting of Plaster of Paris
- (b) Gypsum is added to the cement to slow down its rate of setting
- (c) All alkaline earth metals form hydrated salts
- (d) Barium and Strontium are found free in nature
Answer: (d) Barium and Strontium are found free in nature
Explanation
Statements about plaster of Paris, cement and metals (NOT correct)
| Statement | Verdict |
|---|---|
| a. NaCl increases the rate of setting of plaster of Paris | Correct |
| b. Gypsum is added to cement to slow setting | Correct |
| c. All alkaline earth metals form hydrated salts | Correct |
| d. Barium and strontium are found free in nature | Not correct — they are reactive and occur only as compounds |
The question asks which statement is NOT correct: it is (d), since barium and strontium are very reactive and never occur free in nature. (The stored stem had dropped NOT; it is restored.)
Answer: (d) Barium and Strontium are found free in nature.
Why each option is right or wrong
- (a) Incorrect. Sodium chloride accelerates the setting of plaster.
- (b) Incorrect. Gypsum slows the setting of cement.
- (c) Incorrect. Alkaline earth metal salts are commonly hydrated.
- (d) CORRECT. Barium and strontium are not found free, so this statement is wrong.
Key point
Reactive metals occur combined: Ba as barytes, Sr as celestite.
Remember
Barium and strontium occur only in combined form.
Common mistakes
- Reading the stem as asking for a correct statement.
- Thinking reactive metals occur in the free state.
- Forgetting gypsum's role in cement.
Exam tip
Free state: gold, platinum, silver; combined: all reactive metals.
Sources
- NCERT Class XI Chemistry — s-block elements
- J. D. Lee — Concise Inorganic Chemistry (Group 2: the alkaline earth metals)