Problem 1 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The one that is NOT suitable for the removal of permanent hardness of water is : [Main Sep. 05, 2020 (II)] (a) Clark's method (b) Ion-exchange method (c) Calgon's method (d) Treatment with sodium carbonate

Step-by-Step Solution

Answer

Clark's method is not suitable for removing permanent hardness.

Step 1: Understanding Permanent Hardness

Permanent hardness of water is primarily due to the presence of soluble chlorides and sulfates of calcium and magnesium. It cannot be removed by simple boiling and requires chemical treatment.

Step 2: Clark's Method

Clark's method involves adding lime (calcium hydroxide) to precipitate calcium and magnesium ions. However, this method is typically used for removing temporary hardness, not permanent hardness.

Step 3: Ion-exchange Method

The ion-exchange method uses resin materials to exchange calcium and magnesium ions with sodium or potassium ions, effectively removing both temporary and permanent hardness.

Step 4: Calgon's Method

Calgon's method uses sodium hexametaphosphate to form complexes with calcium and magnesium ions. This method is specifically designed to remove permanent hardness by keeping these ions in solution but ineffective in adding free salts.

Step 5: Treatment with Sodium Carbonate

Treatment with sodium carbonate (washing soda) involves precipitating out calcium and magnesium ions as insoluble carbonates. This is effective for removing both temporary and permanent hardness.

Step 6: Determine the Unsuitable Method

We conclude that Clark's method (option a) is not suitable for removing permanent hardness, as it primarily targets temporary hardness through lime addition.

Key Concepts

Permanent Hardness

Water hardness is a measure of the concentration of calcium and magnesium salts in water. Permanent hardness refers to the portion of total hardness that remains even after boiling. This type of hardness is mainly caused by the presence of

• calcium sulfate (CaSO₄)
• magnesium sulfate (MgSO₄)
• calcium chloride (CaCl₂)
• magnesium chloride (MgCl₂)

These compounds remain dissolved in water at boiling temperatures and do not precipitate out as solid salts. As a result, they require chemical treatment to remove. Boiling is not effective here, because the ions responsible for permanent hardness are unaffected by heat. To tackle permanent hardness, we rely on chemical methods that either exchange or neutralize these ions by converting them into insoluble forms.

Clark's Method

Clark's method is a technique that employs lime, or calcium hydroxide, to treat water hardness. Although effective for temporary hardness, it proves inadequate for permanent hardness. Here's how the method works:

• The addition of lime reacts with bicarbonate ions to form carbonate ions.
• The carbonate ions then precipitate calcium and magnesium as insoluble carbonates.

This method is successful for addressing temporary hardness because bicarbonates are largely responsible. However, in the case of permanent hardness, where calcium and magnesium are bound with chlorides and sulfates, lime has no effective precipitating action.

Ion-exchange Method

The ion-exchange method is an advanced and widely used technique for removing both temporary and permanent hardness. It involves using a resin material that can exchange ions. Here's how it operates:

• The resin contains sodium or potassium ions.
• As hard water passes through, calcium and magnesium ions bind to the resin, replacing them with sodium or potassium ions.
• This exchange process effectively eliminates the hardness-causing ions from the water.

The resin is eventually regenerated using a strong sodium chloride solution to replace the exchanged sodium ions. This method is popular for its efficiency and ability to handle all kinds of hardness without needing frequent replacement.

Calgon's Method

Calgon's method employs sodium hexametaphosphate, commonly known as Calgon, to tackle water hardness. This method works via a sequestration process:

• Sodium hexametaphosphate forms complex molecules with calcium and magnesium ions.
• These complexes keep the ions in solution, preventing them from solidifying as compounds.

This approach is specifically tailored for permanent hardness, where traditional precipitation techniques may not suffice. By sequestering the ions, it ensures that calcium and magnesium do not interfere with water's functionality in processes like washing or heating.

Sodium Carbonate Treatment

The sodium carbonate treatment method utilizes the reaction of washing soda, or sodium carbonate, to soften water. Here's the mechanism:

• When added to water, sodium carbonate reacts with calcium and magnesium ions.
• This reaction forms insoluble carbonates, such as calcium carbonate and magnesium carbonate, which precipitate out.

This straightforward and effective method tackles both temporary and permanent hardness. It's particularly advantageous for large-scale operations because it directly reduces hardness ions while leaving no residual effects, thereby enhancing water's usability for domestic and industrial applications.