Hardness of water refers to the presence of dissolved minerals, primarily calcium and magnesium ions, in the water. These minerals are responsible for the formation of scale deposits and can interfere with the effectiveness of soaps and detergents. The hardness of water is primarily due to dissolved substances, specifically calcium and magnesium ions.

Dissolved Substances as the Main Cause of Water Hardness:

1. Calcium and Magnesium Ions:
- Calcium and magnesium ions are the main contributors to water hardness.
- These ions are naturally present in the earth's crust and can dissolve in water sources such as rivers, lakes, and groundwater.
- When water passes through rocks and soil containing calcium and magnesium minerals, it picks up these ions and becomes hard.

2. Formation of Scale Deposits:
- When hard water is heated or evaporates, the dissolved calcium and magnesium ions can combine with carbonates and bicarbonates in the water to form insoluble compounds, mainly calcium carbonate (CaCO3) and magnesium carbonate (MgCO3).
- These compounds precipitate out of the water and form scale deposits on the surfaces of pipes, appliances, and fixtures.
- Scale deposits can reduce water flow, decrease the efficiency of water heaters, and clog plumbing systems.

3. Interference with Soap and Detergents:
- Hard water can interfere with the effectiveness of soaps and detergents.
- The calcium and magnesium ions in hard water react with the soap molecules, forming insoluble compounds known as soap scum.
- Soap scum reduces the lathering ability of soaps and detergents, making it harder to achieve a desired level of cleanliness.

4. Types of Water Hardness:
- Temporary Hardness: Temporary hardness is caused by the presence of bicarbonate ions (HCO3-) in water. It can be removed by boiling the water, which converts the bicarbonate ions into insoluble carbonates that can be precipitated.
- Permanent Hardness: Permanent hardness is caused by the presence of sulfate and chloride ions of calcium and magnesium. It cannot be removed by boiling and requires other methods such as ion exchange or water softening techniques.

In conclusion, the hardness of water is primarily due to the presence of dissolved substances, particularly calcium and magnesium ions. These minerals can form scale deposits and interfere with the effectiveness of soaps and detergents.