Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The correct option is (c) calcium hydroxide.

Step 1: Understand Clarke's Process

Clarke's process is a method for softening hard water. The main idea is to remove calcium ions, which are responsible for water hardness, by converting them into an insoluble form.

Step 2: Analyze the Given Options

We have four options: (a) potash alum (b) calcium bicarbonate (c) calcium hydroxide (d) sodium bicarbonate. We know that Clarke's process requires a chemical that reacts with the calcium ions to form an insoluble compound.

Step 3: Identify the Correct Chemical

Among the options, calcium hydroxide reacts with dissolved calcium bicarbonate in water to form calcium carbonate, which is insoluble and precipitates out, thereby softening the water.

Step 4: Conclude with the Correct Option

Based on the analysis, the correct chemical used in Clarke's process for water softening is calcium hydroxide, which is option (c).

Key Concepts

Clarke's Process

Clarke's process is a straightforward way to soften water, particularly hard water. It targets calcium ions, which are a main cause of water hardness. The essence of the process is to transform these calcium ions into a form that won't dissolve in water. The process generally involves adding a chemical that facilitates this transformation, leading to the removal of the calcium ions from the solution. This method is helpful for both households and industries where soft water is necessary for appliances and processes.

• The primary goal is to make water useful and less harsh for cleaning and industrial purposes.
• Reducing the calcium content extends the life of water equipment.
• Softened water also improves the effectiveness of soap and detergents.

Calcium Hydroxide

Calcium hydroxide, often known as slaked lime, is the star of Clarke's process. It plays a crucial role in the chemical reaction that helps soften hard water. When added to water, calcium hydroxide reacts with calcium bicarbonate already in the water. This reaction produces calcium carbonate, which is not soluble in water. This means it cannot dissolve and will instead form a solid precipitate.

• Calcium hydroxide has the chemical formula Ca(OH)8.
• It is a white powder that readily mixes with water.
• The formation of calcium carbonate is key to removing excess calcium from water.

This reaction is the core mechanism in Clarke's process, leading to the precipitation of calcium carbonate and the resulting softening of the water.

Hard Water

Hard water is water that contains a high concentration of minerals, primarily calcium and magnesium ions. These minerals get dissolved into the water as it travels through rocks and soil, usually originating from limestone, chalk, or gypsum. Hard water is commonly the cause of scale build-up in pipes and makes cleaning chores more demanding as it decreases the effectiveness of soaps and detergents.

• The presence of calcium carbonate or magnesium carbonate leads to water hardness.
• Limescale deposits on appliances reduce efficiency and lifespan.
• Hard water is measured in grains per gallon or milligrams per liter.

Water softening aims to counter these effects, making water more compatible with various uses by reducing its mineral content.

Insoluble Compounds

An insoluble compound is a substance that does not dissolve in a solvent—in this context, water. When calcium hydroxide reacts with calcium bicarbonate in hard water, it forms an insoluble compound known as calcium carbonate. The excess calcium orthogonal to the water now becomes a solid, removing its part as a solute. This is crucial in the water-softening process.

• Calcium carbonate precipitates out of the water as a solid.
• This reduces the concentration of calcium ions, effectively softening the water.
• Insoluble compounds generally settle out of the water, making separation easy.

The shift from a dissolved state to an insoluble state is critical, as it allows for more efficient water treatment and management.