Question

By which of the following process permanent hardness of water can be removed? (a) washing soda (b) soda lime (c) sodium chloride (d) sodium bicarbonate

Short answer

Permanent hardness is removed by washing soda (option a).

Step-by-step solution

Identify the Problem

We need to find out which substance can remove permanent hardness from water. Permanent hardness is primarily caused by the presence of calcium and magnesium sulfates, which do not precipitate out as easily as carbonates.

Understand Permanent Hardness

Permanent hardness of water cannot be removed by simply boiling the water, as it involves sulfates and chlorides of calcium and magnesium which remain dissolved even after boiling.

Evaluate Options

Out of the given options, (a) washing soda (sodium carbonate) can precipitate calcium and magnesium ions as carbonates, thus softening the water by removing the sulfates and chlorides responsible for hardness.

Select the Correct Option

Given the functions of the substances listed, washing soda is the correct choice as it reacts with calcium and magnesium sulfates to form insoluble carbonates, effectively removing permanent hardness.

Key concepts

Washing Soda

Washing soda, chemically known as sodium carbonate, is a key player in the battle against permanent water hardness. This compound effectively removes the hardness by converting calcium and magnesium ions into insoluble carbonates. Permanent hardness is often due to the presence of calcium and magnesium sulfates which do not precipitate out through simple boiling. Instead, they require a chemical reaction to be removed.

• Washing soda reacts with the magnesium and calcium ions present in the water.
• This reaction results in the formation of insoluble calcium carbonate and magnesium carbonate.

These insoluble carbonates can then be filtered out, leaving behind softer water. This process is effective because the new compounds (calcium and magnesium carbonates) do not dissolve in water, unlike their sulfate counterparts. Washing soda doesn't just make water more pleasant by making it softer. It also makes soaps and detergents more effective by reducing the number of ions that would otherwise interact negatively with cleaning agents.
Without washing soda, the process of eliminating these troublesome ions would be much more complex or costly.

Calcium Sulfate

Calcium sulfate is one of the key culprits when it comes to causing permanent hardness in water. It is a naturally occurring mineral, often found dissolved in hard water. This compound is the reason why merely heating or boiling water does not remove hardness. Instead, a chemical reaction is needed to solve this problem. Unlike calcium carbonate, calcium sulfate's solubility does not significantly decrease with temperature increase, hence boiling has no effect.

• Calcium sulfate remains dissolved even when the water is boiled, highlighting why boiling is ineffective for permanent hardness.
• To remove it, washing soda can be introduced to the system.

When washing soda is added, a chemical reaction transforms the soluble calcium sulfate into insoluble calcium carbonate. This precipitate can then settle out of the water or be filtered, addressing the hardness issue efficiently. Understanding the role of calcium sulfate helps in understanding why certain water softening methods work and why others don't.

Magnesium Sulfate

Magnesium sulfate is another common cause of permanent hardness in water. Like calcium sulfate, magnesium sulfate remains dissolved in water even after boiling, and thus requires a more proactive approach for removal. This compound contributes significantly to the persistent hardness that affects household water systems. The introduction of washing soda offers a solution. Through a chemical reaction, washing soda turns soluble magnesium sulfate into insoluble magnesium carbonate.
This transformation is crucial for removing the permanent hardness of water.

• Magnesium sulfate does not precipitate out by normal methods like heating; it needs a chemical aid.
• Washing soda ensures that magnesium ions form a solid with carbonate ions, removing the ions from the water as solid particles.

This interaction between magnesium sulfate and washing soda is an essential part of water treatment, which is why washing soda is a favored household remedy for hard water. By converting the problematic compounds to a form that can be physically removed, washing soda simplifies the task of water purification immensely.