Hardness of water is mainly due to the presence of

1. Analyze the Question

The question asks for the chemical compounds responsible for the "hardness" of water. Water hardness refers to the presence of specific dissolved mineral salts, primarily those of calcium and magnesium, which prevent soap from lathering easily.

2. Explain the Concept

Water hardness is categorized into two types:

• Temporary Hardness: Caused by the presence of calcium hydrogentrioxocarbonate (IV) or magnesium hydrogentrioxocarbonate (IV). This can be removed by boiling.
• Permanent Hardness: Caused by the presence of dissolved calcium or magnesium salts, specifically calcium tetraoxosulphate (VI) (calcium sulphate) or calcium chloride. These salts do not precipitate out by boiling.

In chemistry, "trioxocarbonate (IV)" is the IUPAC name for carbonate ($CO_3^{2-}$), and "tetraoxosulphate (VI)" is the IUPAC name for sulphate ($SO_4^{2-}$).

3. Show the Solution

• Option A: Calcium hydroxide and magnesium hydroxide are generally insoluble in water and are not the primary causes of water hardness.
• Option B: Calcium trioxocarbonate (IV) ($CaCO_3$) and calcium tetraoxosulphate (VI) ($CaSO_4$) are classic examples of salts that contribute to the hardness of water. While $CaCO_3$ is often associated with temporary hardness (in the form of hydrogentrioxocarbonate), these salts are the standard culprits identified in chemistry curricula for water hardness.
• Option C: Sodium hydroxide is a strong base and does not cause water hardness.
• Option D: While calcium chloride causes hardness, sodium chloride (common salt) does not cause water hardness. Therefore, this option is incorrect.

Comparing the options, Option B contains the salts most accurately associated with the mineral content that makes water "hard."

4. Identify the Answer

The correct option is B.

5. Exam Tip

When you see "trioxocarbonate (IV)" or "tetraoxosulphate (VI)" in an exam, don't panic! Just remember:

• Trioxocarbonate (IV) = Carbonate ($CO_3$)
• Tetraoxosulphate (VI) = Sulphate ($SO_4$)
• Memory Trick: Hard water is "heavy" with minerals. Think of Calcium and Magnesium as the "Hard" guys. If you see them paired with Sulphate or Carbonate, that's your answer!