An atom has mass number 39 and 20 neutrons. Its atomic number is:
A. 59
B. 20
C. 39
D. 19
Answer: Option D
Solution (By JKExamLibrary)
Atomic number (Z) = Mass number (A) – Number of neutrons (N) = 39 – 20 = 19. This is potassium (K). Number of protons equals atomic number. In a neutral atom, electrons also equal Z.
Explanation:
Temporary hardness is due to dissolved bicarbonates of Ca and Mg, which can be removed by boiling: Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂. Permanent hardness is caused by chlorides and sulfates of Ca and Mg, not removed by boiling. Sodium bicarbonate does not cause hardness. Methods to remove permanent hardness include treatment with washing soda (Na₂CO₃), ion exchange, or calgon.
Explanation:
A solution turning red litmus blue is basic, so pH > 7. Among options, pH 9 is basic. pH 2 and 5 are acidic (blue litmus turns red). pH 7 is neutral. Litmus is a natural indicator extracted from lichens. Red litmus in base remains blue, blue litmus in acid turns red. Practical: Phenolphthalein gives pink in basic solution (pH > 8.3).
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