The correct answer is option 1 - Calcium bicarbonate.

Hard water refers to water that contains high levels of dissolved minerals, particularly calcium and magnesium. These minerals can cause various problems such as scaling in pipes and appliances, reduced soap lather, and residue on fixtures and surfaces.

Calcium bicarbonate is one of the salts responsible for water hardness. When water containing calcium bicarbonate is heated or left standing, it undergoes a chemical reaction where the bicarbonate ions decompose into carbonate ions and release carbon dioxide. The carbonate ions then react with calcium ions, forming insoluble calcium carbonate, which is a primary component of scale buildup.

Option 2 - Calcium carbonate is a related compound but is not typically responsible for water hardness. It is a common mineral found in nature and can contribute to scale buildup, but it is not as soluble as calcium bicarbonate and does not cause as much hardness in water.

Option 3 - Sodium sulfate is not a salt that causes water hardness. It is a neutral compound that does not contain calcium or magnesium ions.

Option 4 - Potassium chloride is also not responsible for water hardness. It is commonly used as a substitute for sodium chloride in water softeners, which are devices designed to remove hardness-causing minerals from