Water hardness is typically measured in milligrams per liter ($\text{mg/L}$) and categorized based on the concentration of calcium ($\text{Ca}^{2+}$) and magnesium ($\text{Mg}^{2+}$) ions, often expressed as an equivalent concentration of calcium carbonate ($\text{CaCO}_3$). Understanding these classifications is key to identifying hard water.

Water Hardness Levels by Concentration

Different organizations may use slightly varying ranges, but a common classification scheme is as follows:

• Soft Water: Less than 60 mg/L as $\text{CaCO}_3$.
• Moderately Hard Water: 60 mg/L to 120 mg/L as $\text{CaCO}_3$.
• Hard Water: 120 mg/L to 180 mg/L as $\text{CaCO}_3$.
• Very Hard Water: Greater than 180 mg/L as $\text{CaCO}_3$.

Identifying Hard Water Range

The question asks for the hardness level characteristic of hard water. Based on the standard classifications:

• Option 1: 150-180 mg/L falls within the defined range for Hard Water (120-180 mg/L) and borders on Very Hard Water.
• Option 2: 60-150 mg/L includes Moderately Hard Water and extends into the Hard Water range.
• Option 3: < 60 mg/L represents Soft Water.
• Option 4: > 180 mg/L represents Very Hard Water.

The range 150-180 mg/L specifically represents the upper end of the hard water classification, aligning with common definitions.

Conclusion on Hard Water Measurement

Therefore, a hard water typically exhibits a hardness within the range of 150-180 mg/L as $\text{CaCO}_3$, according to the provided options and standard classifications.