The biological oxygen demand (BOD) of clean water is typically: MCQ with Answer and Explanation

Explanation:
NPK stands for Nitrogen (N), Phosphorus (P), and Potassium (K), the three primary macronutrients essential for plant growth. N promotes leaf growth, P aids root and flower development, K supports overall health. Fertilizer labels like 10-26-26 indicate the percentages of N, P₂O₅, and K₂O respectively. Calcium, magnesium, and sulfur are secondary nutrients, while Fe, Mn, Zn etc. are micronutrients.

Explanation:
Amphoteric oxides react with both acids and bases to form salts and water. Aluminum oxide (Al₂O₃) and Zinc oxide (ZnO) are classic examples. Na₂O and MgO are basic oxides that react only with acids, while SO₂ is an acidic oxide that reacts only with bases.

Explanation:
Mass number (A) = number of protons (Z) + number of neutrons (N). Electrons have negligible mass and are not counted. It is the total number of nucleons. Example: ¹²C has 6 protons + 6 neutrons = 12. Isotopes have same Z but different A due to varying N. Atomic mass (weight) is the weighted average.