The biological oxygen demand (BOD) of clean water is typically:
A. Between 10 and 20 ppm
B. Less than 5 ppm
C. Greater than 500 ppm
D. Between 50 and 100 ppm
Answer: Option B
Solution (By JKSSB Mock Tests)
Biological Oxygen Demand (BOD) measures the amount of dissolved oxygen needed by aerobic organisms to break down organic material in water. Clean, unpolluted water has a low BOD, typically less than 5 ppm. Highly polluted water can have a BOD of 17 ppm or much higher, indicating severe organic contamination.
Explanation:
According to the reactivity series, zinc is more reactive than copper, so Zn displaces Cu: Zn + CuSO₄ → ZnSO₄ + Cu. Silver, gold, and mercury are less reactive than copper and cannot displace it. Reactivity: K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au. Displacement reactions are used in metallurgy and galvanic cells. The blue color of CuSO₄ fades as copper is deposited.
Explanation:
Elements in the same group (vertical column) have the same number of valence electrons, resulting in similar chemical properties. For example, all Group 1 elements have 1 valence electron (ns¹). The number of shells increases down the group. Atomic mass varies. This similarity was the basis for Mendeleev's grouping. Modern table uses atomic number for ordering.
Explanation:
Flooded rice paddies promote anaerobic decomposition of organic matter, producing methane (CH₄). It is a potent greenhouse gas. Livestock also emit methane.
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