Question

Water softening by Clarke's process uses: (a) Potash alum (b) Calcium bicarbonate (c) Calcium hydroxide (d) Sodium bicarbonate

Short answer

Calcium hydroxide (c) is used in Clarke's process for water softening.

Step-by-step solution

Understanding the Question

The question asks which compound is used in Clarke's process for softening water. Clarke's process is a method used to remove hardness in water caused by calcium and magnesium ions.

Analyzing the Options

Let's evaluate each option: - (a) Potash alum is used as a coagulating agent, not for softening water. - (b) Calcium bicarbonate is actually a cause of temporary hardness in water, not a solution for it. - (c) Calcium hydroxide, commonly known as slaked lime, can be used for reducing water hardness. - (d) Sodium bicarbonate is mainly used in baking and is not typically used for water softening.

Selecting the Correct Answer

Based on the analysis, calcium hydroxide (option c) is the correct choice. Clarke's process involves adding calcium hydroxide to water to precipitate the calcium bicarbonate as calcium carbonate, thereby softening the water.

Key concepts

Water Softening

Water softening is an essential process for making hard water suitable for use in households and industries. Hard water contains dissolved minerals, predominantly calcium and magnesium, which can cause a variety of problems like scaling in pipes, reduced lathering of soaps, and harmful effects on appliances and clothing.

By removing these minerals, the water softening process ensures that the water becomes "soft" and more usable. Soft water helps enhance the life span and efficiency of appliances and makes daily chores like cleaning easier due to improved detergent performance.

• The softening of water is significantly important in preventing scale accumulation and extending the life of plumbing systems.
• Various methods are employed for softening water including the use of ion-exchange resins, precipitation, and lime softening, also known as chemical softening.

Clarke's process is one such method, focusing specifically on the chemical softening of water using lime.

Calcium Hydroxide

Calcium hydroxide, also known as slaked lime, plays a crucial role in Clarke's process for water softening. It is a white powdery substance obtained by mixing calcium oxide (lime) with water. The chemical formula for calcium hydroxide is \( Ca(OH)_2 \).

When added to hard water, calcium hydroxide reacts with the bicarbonate hardness, precipitating it as insoluble calcium carbonate, which can be filtered and removed. This reaction is instrumental in reducing water hardness, especially with temporary hardness caused by bicarbonates.

• One of the benefits of using calcium hydroxide is its effectiveness in removing temporary hardness without significantly affecting other water properties.
• The addition of calcium hydroxide increases the pH initially, but it settles once the precipitation process is complete.

Through this chemical interaction, the water becomes softer, thereby reducing scale formation in pipes and enhancing the usability of water.

Temporary Hardness

Temporary hardness in water is mainly due to the presence of dissolved bicarbonate minerals, specifically calcium bicarbonate \( Ca(HCO_3)_2 \) and magnesium bicarbonate. It is called "temporary" because it can be easily removed by processes such as boiling or through chemical reactions.

In the context of Clarke's process, temporary hardness is primarily tackled by causing bicarbonates to react with calcium hydroxide, leading to the formation of calcium carbonate \( CaCO_3 \), a substance insoluble in water. This process helps in eliminating the bicarbonates from the water, thereby reducing its hardness.

• The removal of temporary hardness is critical for water systems, as it prevents the formation of scale and improves the lifespan of household appliances.
• Boiling is an alternative method to remove temporary hardness, as it causes bicarbonates to break down into carbon dioxide and carbonate, which precipitates out.

While boiling can be effective for small quantities of water, for larger scale applications, chemical methods like Clarke's process are more efficient and reliable.