Problem 87 - A Complete Resource Book in Chemistry for JEE Main

Question

Polyphosphates are used as water softening agents because they (a) form soluble complexes with cationic species (b) precipitate cationic species (c) precipitate anionic species (d) form soluble complexes with anionic species

Step-by-Step Solution

Answer

(a) form soluble complexes with cationic species.

Step 1: Understanding the Role of Polyphosphates

Polyphosphates work by interacting with ions in water to prevent them from causing hardness. They need to interact specifically with ions that cause hardness, which are typically cations such as calcium (Ca²⁺) and magnesium (Mg²⁺).

Step 2: Recognizing Cations and Anions

In water chemistry, cations are positively charged ions, while anions are negatively charged ions. Water hardness is mainly caused by cations like calcium and magnesium.

Step 3: Analyzing Possible Interactions

Option (a) states that polyphosphates form soluble complexes with cationic species, which would prevent cations from precipitating as scale. Option (b) suggests precipitation, but this is more likely for anions. Options (c) and (d) suggest interactions with anions, which are not the cause of water hardness.

Step 4: Choosing the Correct Answer

Given that polyphosphates are used to soften water by preventing the precipitation of cations as scale, forming soluble complexes with cationic species aligns with this functionality. Thus, the best choice is option (a).

Key Concepts

Polyphosphates

Polyphosphates play a crucial role in water softening systems. These chemical compounds are commonly used because they can interact with ions in water. Specifically, they interact with cationic species to prevent them from causing water hardness.
To understand their function, it's important to know that polyphosphates can form soluble complexes with metal ions. In the context of water softening, they bind with cations like calcium (Ca²⁺) and magnesium (Mg²⁺), which are responsible for water hardness.

• These complexes are water-soluble, which means they don't precipitate out of the water.
• This allows the ions to remain dissolved, preventing the formation of scale.

Polyphosphates are especially favored in applications where maintaining the dissolved state of ions is preferable over removing them altogether, ensuring that pipes and appliances remain free from crusty deposits that can impair functionality.

Cationic Species

In water chemistry, the terms cation and anion are used to describe the type of charge an ion carries. Cationic species are ions with a positive charge.
These include important contributors to water hardness like calcium (Ca²⁺) and magnesium (Mg²⁺). They are often found in natural water sources and are brought into homes through tap water.

• Cations are attracted to negative charges, which makes them susceptible to forming scales when they react with other ions.
• This is why understanding the role of cationic species is essential for water treatment processes.

When hard water is heated, or when it evaporates, these cations can combine with other substances to form solid deposits, or scale, on surfaces such as pipes, boilers, and water heaters.
Water softening solutions aim to prevent these positive ions from combining with other ions, thus avoiding the formation of scale.

Water Hardness

Water hardness is a common term used to describe the presence of certain ions in water, mainly calcium (Ca²⁺) and magnesium (Mg²⁺).
These ions enter water sources as rainwater percolates through soil and rock, dissolving minerals and carrying them into the water supply.

• Hard water can cause several issues; it reduces the effectiveness of soaps and detergents, leading to residue on clothes and dishes.
• It can also cause scaling, which clogs pipes and reduces the efficiency of appliances like water heaters.

While hard water is not harmful to health, it is a nuisance when it comes to household uses and industrial processes.
Addressing water hardness involves methods such as chemical treatments to prevent scale formation or the application of water softening agents, like polyphosphates, to keep these ions dissolved.