Question

Softening of hard water is done using sodium aluminium silicate (Zeolite). This causes: (a) Adsorption of \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) ions of hard water replacing \(\mathrm{Al}^{3+}\) ions (b) Adsorption of \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) ions of hard water replacing \(\mathrm{Na}^{+}\) ions (c) Both of these (d) None of these

Short answer

(b) Adsorption of \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) ions replacing \(\mathrm{Na}^{+}\) ions.

Step-by-step solution

Understanding the Problem

We need to determine how sodium aluminium silicate (Zeolite) softens hard water. Hard water typically contains high concentrations of calcium (\(\mathrm{Ca}^{2+}\)) and magnesium (\(\mathrm{Mg}^{2+}\)) ions, which are responsible for hardness.

Role of Zeolite in Water Softening

Zeolite is a mineral that contains sodium ions (\(\mathrm{Na}^{+}\)). Zeolites soften hard water by exchanging their \(\mathrm{Na}^{+}\) ions for the \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) ions in the water.

Exchanging Ions

During the process, \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) ions in the water are adsorbed and held by the zeolite structure, replacing the \(\mathrm{Na}^{+}\) ions. This ion exchange is essential for softening the water.

Analyzing Options

Option (a) suggests the replacement of \(\mathrm{Al}^{3+}\) ions which isn't involved in the process; thus, it's incorrect. Option (b) correctly describes the replacement of \(\mathrm{Na}^{+}\) ions by \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) ions. Option (b) is the correct outcome.

Key concepts

Ion Exchange

Ion exchange is a fundamental concept in chemistry used to remove ions from a solution and replace them with others of a similar charge. In the context of water softening, zeolite plays a crucial role through this mechanism. Zeolites contain sodium ions (\(\mathrm{Na}^{+}\)) loosely held within their structure. When hard water passes through a bed of zeolite, the sodium ions are released, and calcium (\(\mathrm{Ca}^{2+}\)) and magnesium (\(\mathrm{Mg}^{2+}\)) ions, which cause water hardness, adhere to the zeolite, effectively displacing the sodium ions.The process:

• Hard water passes over the zeolite.
• Zeolite releases \(\mathrm{Na}^{+}\) ions into the water.
• \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) ions are absorbed by zeolite.

This exchange significantly reduces water hardness, making ion exchange a highly effective and preferred method for water softening.

Hard Water

Hard water is water that contains high levels of dissolved minerals, particularly calcium (\(\mathrm{Ca}^{2+}\)) and magnesium (\(\mathrm{Mg}^{2+}\)) ions. These minerals are typically picked up by water as it moves through soil and rock, emerging as groundwater enriched with these elements. Hard water can cause several problems:

• It tends to form scale in pipes and appliances, reducing their lifespan and efficiency.
• It interferes with soap and detergent action, making it difficult to form lather.
• It may leave spots on dishes and fixtures.

To mitigate these issues, hard water often requires treatment such as water softening.

Sodium Aluminium Silicate

Sodium aluminium silicate, commonly known as zeolite, is a type of mineral abundant in nature or synthetically manufactured. Its structure consists of a framework of silicon, aluminum, and oxygen, housing sodium ions (\(\mathrm{Na}^{+}\)). The key characteristics of zeolite in water softening are:

• It functions as a natural cation exchanger, replacing calcium and magnesium ions with sodium ions.
• It is highly porous, which increases its surface area for efficient ion exchange.
• After some time, zeolite becomes saturated. It requires regeneration with a concentrated salt solution to restore its ion exchange capacity.

Zeolite's ability to be regenerated makes it an economical and sustainable option for water treatment applications.

Calcium and Magnesium Ions

Calcium (\(\mathrm{Ca}^{2+}\)) and magnesium (\(\mathrm{Mg}^{2+}\)) ions are the primary contributors to water hardness. Originating mainly from limestone and other mineral deposits, these ions dissolve into the water as it flows through underground formations. Once dissolved, these ions can:

• React with soap to form a curdy precipitate that prevents effective cleaning; this is often seen as soap scum.
• Precipitate out as scale inside pipelines and appliances, hindering water flow and heat exchange.

Water softening processes aim to reduce the concentration of these ions through methods such as ion exchange, which swaps them for less disruptive sodium ions, hence making water much more manageable for consumer and industrial use.