Degree of hardness is expressed by

Options

A. the number of parts by weight of `CaCo_(3)` present per one million parts by weight of water
B. grams of `CaCO_(3)` present in 1000 parts of water
C. grams of `MgCl_(2)` in 1000 parts of water
D. grams of `MgCO_(2)` in 1000 parts of water

Explanation

To determine how the degree of hardness in water is expressed, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Water Hardness**:
- Water hardness is primarily caused by the presence of dissolved minerals, mainly calcium (Ca²⁺) and magnesium (Mg²⁺) ions.

2. **Measurement Units**:
- Hardness is typically measured in terms of the concentration of these minerals in water. The most common way to express this concentration is in parts per million (ppm).

3. **Exploring the Options**:
- The options given are:
1. Number of parts by weight of calcium carbonate present per 1 million parts by weight of water.
2. Grams of calcium carbonate present in 1000 parts of water.
3. Grams of MgCl₂ in 1000 parts of water.
4. Grams of MgCO₃ in 1000 parts of water.

4. **Analyzing the Options**:
- **Option 1**: This option expresses hardness in terms of ppm, which is suitable for measuring low concentrations of substances in water.
- **Option 2, 3, and 4**: These options express hardness in grams per 1000 parts of water, which is not as effective for expressing very low concentrations.

5. **Conclusion**:
- The most appropriate way to express the degree of hardness in water is through the first option: "the number of parts by weight of calcium carbonate present per 1 million parts by weight of water." This is a standard method for measuring water hardness.

### Final Answer:
The degree of hardness is expressed by the number of parts by weight of calcium carbonate present per 1 million parts by weight of water.

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