**Core Concept**
Hardness of water is a measure of its content of calcium and magnesium ions, which contribute to its ability to precipitate soap and other detergents. Hard water is often associated with an increased risk of cardiovascular disease, due to the role of calcium in vascular calcification. The hardness of water is typically classified into three categories: soft, moderately hard, and hard.

**Why the Correct Answer is Right**
The hardness of water is primarily due to the presence of calcium and magnesium ions, which are introduced into the water through the dissolution of limestone and other minerals. These ions can also be introduced through the use of water softeners, which replace calcium and magnesium ions with sodium ions. The degree of water hardness is typically measured in terms of the concentration of calcium carbonate (CaCO3) equivalent in the water. Water hardness can have significant effects on the skin, hair, and plumbing systems, as well as on the taste and odor of the water.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is a true characteristic of hard water.
**Option B:** This statement is also true, as calcium and magnesium ions are the primary contributors to water hardness.
**Option C:** This statement is incorrect because water hardness is not directly related to the pH of the water.

**Clinical Pearl / High-Yield Fact**
A common way to remember the classification of water hardness is to use the mnemonic "S-M-H" for soft, moderately hard, and hard, respectively.

**Correct Answer:** D