**Question:** Softening of water is recommended when hardness exceeds

A. 70 mg/L of calcium carbonate (CaCO3)
B. 100 mg/L of calcium carbonate (CaCO3)
C. 300 mg/L of calcium carbonate (CaCO3)
D. 500 mg/L of calcium carbonate (CaCO3)

**Core Concept:** Hardness of water refers to the concentration of calcium and magnesium ions, which are responsible for the water's ability to dissolve soap and leave a residue. Softening agents, like sodium bicarbonate or potassium carbonate, are used to reduce hardness by exchanging calcium and magnesium ions with sodium or potassium ions.

**Why the Correct Answer is Right:** The correct answer, **A. 70 mg/L of calcium carbonate (CaCO3)**, is based on the guideline provided by the World Health Organization (WHO) for safe drinking water quality. According to WHO, water is considered hard if it contains more than 70 mg/L of calcium carbonate (CaCO3). Softening the water above this threshold is necessary to prevent soap scum and other issues associated with hard water.

**Why Each Wrong Option is Incorrect:**

**B. 100 mg/L of calcium carbonate (CaCO3)** is incorrect because it is higher than the recommended limit from the WHO guideline, making the water hard and requiring softening treatment.

**C. 300 mg/L of calcium carbonate (CaCO3)** is wrong as it is more than double the recommended limit set by WHO. Softening water at this level is unnecessary and could lead to improper elimination of essential minerals from the water.

**D. 500 mg/L of calcium carbonate (CaCO3)** is incorrect as it exceeds the maximum limit recommended by the WHO for safe drinking water quality. Softening water at this level is not necessary and may lead to the removal of essential minerals from the water.

**Core Concept:** Softening water is essential in regions with high hardness levels, as hard water can cause soap scum, skin irritation, and other health-related issues.

**Why Each Wrong Option is Incorrect:**

Option **B, C, and D** exceed the recommended limit of calcium carbonate (CaCO3) set by the World Health Organization (WHO) for safe drinking water quality. Softening water at these levels is unnecessary and can lead to the removal of essential minerals from the water, which is detrimental to human health.

**Core Concept:** The use of softening agents, such as sodium bicarbonate or potassium carbonate, is the primary method for reducing the hardness of water.

**Why the Correct Answer is Right:** Calcium carbonate (CaCO3) is the primary constituent causing hardness in water. By reducing the calcium carbonate level, softening agents can prevent the negative impacts of hard water on human health and daily activities like bathing, laundry, and dishwashing.

**Why Each Wrong Option is Inc