Question

Water softening by Clarke's process uses (a) potash alum (b) calcium bicarbonate (c) calcium hydroxide (d) sodium bicarbonate

Short answer

(c) calcium hydroxide

Step-by-step solution

Understand the Function of Water Softening

Water softening is the process of removing calcium (Ca²⁺) and magnesium (Mg²⁺) ions that cause water hardness. This is typically done to make water suitable for various household and industrial applications.

Clarke's Process for Water Softening

Clarke's process is a chemical technique used to remove hardness from water. It specifically targets the temporary hardness, which is primarily caused by calcium bicarbonate (Ca(HCO₃)₂) in the water.

Identify the Chemical Used in Clarke's Process

Clarke's process involves adding a substance that reacts with calcium ions to form an insoluble compound, which can then be removed from the water. The substance used in this process is calcium hydroxide (Ca(OH)₂), also known as lime.

Select the Correct Option

Based on the understanding of Clarke's process, the correct answer for the chemical used in this process is calcium hydroxide. This helps in precipitating out calcium carbonate from calcium bicarbonate, thus softening the water.

Key concepts

Clarke's Process

Clarke's process is a traditional and effective method of water softening, which targets the removal of temporary hardness from water. Temporary hardness is primarily due to the presence of calcium bicarbonate, which dissolves in water. Consequently, Clarke's process specifically seeks to convert these dissolved compounds into a form that can be easily removed. The essence of this method lies in its ability to chemically alter the water composition, ensuring the water is more suitable for everyday use.

• Reduces scales and deposits in pipes and appliances
• Makes water ideal for cleaning and bathing
• Improves the efficiency of soaps and detergents

Through the addition of lime, also known as calcium hydroxide, the intricate chemical reactions take place, enabling the precipitation of insoluble compounds. The process not only softens the water but also makes it safer and more efficient for domestic and industrial applications.

Calcium Hydroxide

Calcium hydroxide, chemically denoted as Ca(OH)₂, is often referred to as lime or slaked lime. It plays a crucial role in Clarke's process by facilitating the removal of calcium ions that are responsible for water hardness. When added to the hard water, calcium hydroxide reacts with the calcium bicarbonate in the water. This reaction produces calcium carbonate, a solid that does not dissolve easily in water.

• Reacts with calcium bicarbonate to form insoluble calcium carbonate
• Assists in precipitating out components causing hardness
• Non-toxic and widely available

The formed calcium carbonate is heavy and settles at the bottom of the container. This substance can then be filtered or removed, effectively softening the water and eliminating temporary hardness.

Temporary Hardness

Temporary hardness in water is caused by dissolved calcium and magnesium bicarbonates. These compounds can precipitate and form scales under conditions of high temperature or pH, affecting water heaters and boilers. Unlike permanent hardness, temporary hardness can be addressed by boiling the water or more efficiently through chemical treatments such as Clarke's process.

• Caused by dissolved calcium and magnesium bicarbonates
• Can lead to scaling in hot water systems
• Removable by boiling or chemical precipitation

Through techniques like Clarke's process, the bicarbonates are converted into insoluble carbonates, which can be easily separated from the water. Understanding and addressing temporary hardness is crucial for maintaining the longevity and efficiency of household and industrial water systems.