The degree of hardness of water is usually expressed in terms of

Options

A. g/L of `CaCO_3` and `MgCO_3` present
B. ppm by weight of `MgSO_4`
C. ppm of `CaCO_3` actually present in water
D. ppm by weight of `CaCO_3` irrespective of whether it is actually present

Answer

To determine how the degree of hardness of water is usually expressed, let's break down the concepts step by step:

### Step 1: Understanding Water Hardness
Water hardness is primarily caused by the presence of dissolved minerals, mainly calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These minerals can come from various sources, including the dissolution of limestone and other geological formations.

**Hint:** Water hardness is related to the concentration of certain ions in the water.

### Step 2: Units of Measurement
The degree of hardness of water can be expressed in several ways, including:
- Grains per gallon (gpg)
- Parts per million (ppm)
- Milligrams per liter (mg/L)

**Hint:** Different units can be used to express the same concept, but we need to focus on the most common one.

### Step 3: Commonly Used Reference
The most common reference for measuring water hardness is the concentration of calcium carbonate (CaCO₃) equivalent. This is because CaCO₃ is a standard reference compound for hardness.

**Hint:** Think about which compound is typically used as a standard for measuring hardness.

### Step 4: Specific Options Given
From the options provided:
- GL of CaCO₃ and MgCO₃ present
- ppm by weight of MgSO₄
- ppm of CaCO₃ actually present in water
- ppm by weight of CaCO₃ irrespective of whether it is actually present

The most appropriate choice is the last one, which states "ppm by weight of CaCO₃ irrespective of whether it is actually present."

**Hint:** Look for the option that mentions CaCO₃ and does not limit itself to what is actually present in the water.

### Conclusion
Therefore, the degree of hardness of water is usually expressed in terms of **ppm by weight of CaCO₃ irrespective of whether it is actually present or not**.

**Final Answer:** Option D: ppm by weight of CaCO₃ irrespective of whether it is actually present.