Answer: Calcium and magnesium.
- A Calcium and magnesium
- B Sodium and potassium
- C Iron and copper
- D Sodium chloride only
Correct answer: A. Calcium and magnesium
Explanation: Hard water contains dissolved Ca<sup>2+</sup> and Mg<sup>2+</sup> salts (bicarbonates, sulfates, chlorides) that prevent lathering with soap.
Each water molecule's O-H bonds (covalent, solid) can hydrogen-bond (dashed) to neighbouring molecules, forming an extended network responsible for water's unusually high boiling point and density anomaly.
Concept context
The simplest and most abundant element in the universe. Study its unique position in the periodic table, isotopes (protium, deuterium, tritium), properties of water and hydrogen peroxide, and industrial uses.