Animated Solution for Chemistry - s and p-Block Elements: Calgon is used for water treatment. Which of the following statement is not true about Calgon?

Water softening is a classic topic in chemistry, and one of the most fascinating chemicals used for this purpose is Calgon. The name "Calgon" is actually a commercial portmanteau derived from the phrase "Calcium gone," which perfectly describes its primary function!

Chemically, Calgon is known as sodium hexametaphosphate. Its formula is often written as $(NaP{O}_3{)}_6$ or, more structurally accurately, as $N{a}_2[N{a}_4(P{O}_3{)}_6]$. It consists of a beautiful, large ring structure made of alternating phosphorus and oxygen atoms. But what makes this molecule so special, and how does it interact with the elements of our planet?

Let's evaluate the first claim: does Calgon contain the second most abundant element in the Earth's crust? To answer this, we need to recall our basic geochemistry. The Earth's crust is predominantly made of silicate minerals. By weight, the most abundant elements are:

1. Oxygen (O) at approximately $46.6\%$ 2. Silicon (Si) at approximately $27.7\%$ 3. Aluminum (Al) at approximately $8.1\%$

So, the second most abundant element is Silicon. Now, let's look back at the formula for Calgon: $(NaP{O}_3{)}_6$. It contains Sodium (Na), Phosphorus (P), and Oxygen (O). There is absolutely no Silicon in Calgon! Therefore, the statement that Calgon contains the second most abundant element in the Earth's crust is blatantly false.

What about the other properties of Calgon? Is it a polymer? Yes! Calgon is an inorganic polymer. The $(P{O}_3{)}_6^{6-}$ anion is a hexameric ring, meaning it is made of repeating phosphate units linked together. Because of its ionic nature, it is highly soluble in water.

Historically, this compound was discovered by the Scottish chemist Thomas Graham in 1833. He prepared it by heating sodium dihydrogen phosphate, which is why sodium hexametaphosphate is still commonly referred to as Graham's salt. Thus, the statements claiming it is a water-soluble polymer and known as Graham's salt are perfectly true.

No Precipitation!

Finally, we arrive at the most chemically elegant part of Calgon: how it actually softens water. Hard water is a nuisance because it contains dissolved $C{a}^{2+}$ and $M{g}^{2+}$ ions, which react with soap to form a sticky scum.

Many water softeners, like washing soda ($N{a}_{2}C{O}_3$), work by reacting with these ions to form an insoluble solid precipitate (like $CaC{O}_3$), which settles out of the water. However, Calgon works differently. It uses a process called sequestration.

When Calgon is added to hard water, its large polyphosphate ring wraps around the $C{a}^{2+}$ and $M{g}^{2+}$ ions, trapping them inside a complex anion:

Notice the $(aq)$ state! The newly formed calcium complex is highly soluble in water. The calcium ions are effectively "hidden" or sequestered from the soap, but they remain dissolved in the solution. No solid precipitate is formed. Therefore, the statement that Calgon does not remove $C{a}^{2+}$ ions by precipitation is absolutely true.

The question asks us to identify the statement that is not true. Since we have established that Calgon does not contain Silicon (the second most abundant element in the Earth's crust), option (a) is the incorrect statement, making it our correct answer. Always read the question carefully to ensure you are hunting for the false statement rather than the true one!