Hard water is water that contains high levels of dissolved salts, particularly calcium and magnesium ions. These ions can react with soap and cause a buildup of insoluble scum, leading to reduced effectiveness of cleaning agents.

Magnesium and Calcium
The correct answer is option B - magnesium and calcium. These are the two most common ions that contribute to water hardness. When water passes through rocks and soil, it can dissolve minerals such as calcium and magnesium, which then enter the water supply.

Effects of hard water
Hard water can have several negative effects, including:

1. Reduced effectiveness of soap and cleaning agents.
2. Increased buildup of scale in pipes and appliances, which can reduce their efficiency and lifespan.
3. Staining of clothing, dishes, and other surfaces due to the minerals in the water.
4. Reduced lathering and rinsing ability of shampoos and soaps, leading to skin and hair irritation.

Conclusion
In conclusion, water hardness is determined by the levels of dissolved calcium and magnesium ions. These ions can cause several negative effects and must be treated to avoid damage to appliances and other surfaces.

Hard water is water that has a high concentration of dissolved minerals, especially calcium and magnesium. These minerals can cause problems in plumbing systems and appliances, as well as interfere with soap and detergent performance.

Magnesium and Calcium ions make the water hard.

Explanation:

1. What is Hard Water?
Hard water is water that has a high concentration of dissolved minerals, especially calcium and magnesium. The hardness of water is usually measured in terms of the concentration of calcium carbonate (CaCO3) in the water.

2. Why is Hard Water a Problem?
Hard water can cause several problems, including:
- Scale buildup in plumbing systems, appliances, and fixtures
- Reduced efficiency and lifespan of water heaters, dishwashers, and other appliances
- Reduced effectiveness of soaps and detergents
- Stains on clothes, dishes, and surfaces

3. Which Ions Make Water Hard?
The most common ions that cause water hardness are:
- Calcium ions (Ca2+)
- Magnesium ions (Mg2+)

4. Why Do Calcium and Magnesium Ions Make Water Hard?
Calcium and magnesium ions are divalent cations, which means that they have a positive charge of +2. This makes them highly reactive with other ions in the water, including carbonate (CO32-) and bicarbonate (HCO3-) ions. When calcium and magnesium ions react with these ions, they form insoluble compounds that precipitate out of the water and form scale.

5. Which Group of Ions Makes Water Hard?
Out of the given options, the group of ions that make water hard is magnesium and calcium. Therefore, option B is the correct answer.

In conclusion, hard water is water that has a high concentration of dissolved minerals, especially calcium and magnesium. These minerals can cause several problems, including scale buildup, reduced efficiency of appliances, and reduced effectiveness of soaps and detergents. The group of ions that make water hard is magnesium and calcium.