Question

(a) Which of the following ionic species could be. responsible for hardness in a water supply: \(\mathrm{Ca}^{2+}, \mathrm{K}^{+}\), \(\mathrm{Mg}^{2+}, \mathrm{Fe}^{2+}, \mathrm{Na}^{+} ?\) (b) What properties of an ion determine whether it will contribute to water hardness?

Short answer

(a) The ions responsible for water hardness among the given species are \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\). (b) The two main properties of an ion that determine whether it will contribute to water hardness are: 1. Valency: Divalent cations like \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) tend to cause water hardness due to their stronger bonds with anions present in water (such as bicarbonates, carbonates, and sulfates), leading to the formation of insoluble precipitates. 2. Solubility: Ions that form insoluble compounds with anions found in water, like calcium and magnesium ions forming precipitates with soap, bicarbonates, carbonates, and sulfates, are more prone to causing water hardness and scale formation.

Step-by-step solution

(a) Identifying ions responsible for water hardness

Water hardness is primarily caused by the presence of divalent metal cations, particularly calcium ions \(\mathrm{Ca}^{2+}\) and magnesium ions \(\mathrm{Mg}^{2+}\). These ions form insoluble precipitates with soap, which create the hardness and also cause the formation of scales in pipes and appliances. Potassium \(\mathrm{K}^{+}\), sodium \(\mathrm{Na}^{+}\), and iron \(\mathrm{Fe}^{2+}\) do not significantly affect the hardness of water. So, the ions responsible for water hardness among the given species are \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\).

(b) Properties of an ion that determine its contribution to water hardness

There are two main properties of an ion that determine whether it will contribute to water hardness: 1. Valency: Divalent cations like \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) are more likely to cause water hardness as they form stronger bonds with the anions present in water (such as bicarbonates, carbonates, and sulfates), leading to the formation of insoluble precipitates. 2. Solubility: Ions that form insoluble compounds with anions found in water are more prone to causing water hardness. Calcium and magnesium ions form insoluble precipitates with soap, bicarbonates, carbonates, and sulfates, leading to water hardness and scale formation. In conclusion, the ions responsible for water hardness are mainly divalent cations like \(\mathrm{Ca}^{2+}\) and \(\mathrm{Mg}^{2+}\) due to their valency and tendency to form insoluble precipitates with anions present in water.

Key concepts

Divalent Cations

To understand water hardness, it's crucial to know about divalent cations. These are ions with a charge of +2. Common examples include calcium (\( Ca^{2+}\)) and magnesium (\( Mg^{2+}\)), which are particularly important in the context of water hardness.

Divalent cations have two positive charges, allowing them to readily bond with negatively charged ions (anions). This bonding capability means they can form various compounds in water, contributing significantly to hardness.

• Attraction to anions: Due to their double charge, divalent cations have a strong attraction to anions, forming salts and other compounds.
• Contribution to scale: When these ions combine with certain anions, they form precipitates that can deposit as scale in pipes and appliances.

Understanding divalent cations is key when studying how certain ions make water hard.

Insoluble Precipitates

In the water hardness context, insoluble precipitates play a major role. When divalent cations like calcium (\( Ca^{2+}\)) and magnesium (\( Mg^{2+}\)) mix with anions, they form compounds that do not dissolve well in water. Instead, these compounds settle out as solid particles known as precipitates.

Why is this important?

• Soap's interaction: Insoluble precipitates are why soap doesn't lather well in hard water. The ions react with soap to form a curd-like substance.
• Scale formation: The solid form of these compounds can lead to scale buildup in pipes and appliances, hindering efficiency and leading to maintenance issues.

The formation of these precipitates is a key characteristic that distinguishes hard water.

Valency and Solubility

Valency, or the number of charges on an ion, directly affects how ions interact in water. Divalent cations, like \( Ca^{2+}\) and \( Mg^{2+}\), have a valency of two, making them effective at forming stable, often insoluble compounds. This combination of valency and solubility is crucial for understanding water hardness.

Here's why:

• **Higher valency, stronger bonds**: The two positive charges allow for the formation of tightly bonded compounds with available anions in water.
• **Solubility dynamics**: Some of these compounds are less soluble, creating the insoluble precipitates that lead to hardness.

Thus, both valency and solubility determine how ions contribute to water hardness.

Calcium Ions and Magnesium Ions

In the realm of water hardness, calcium (\( Ca^{2+}\)) and magnesium (\( Mg^{2+}\)) ions are infamous as principal culprits. They are the main divalent cations causing hardness.

Here's more about their role:

• **Hardness contributors**: These ions react with anions like bicarbonates and sulfates to form hard-to-dissolve compounds.
• **Widespread presence**: Calcium and magnesium are common in natural waters, coming from rock and soil dissolution.
• **Impact on daily life**: They are the reasons your soap won’t lather and your kettle forms scales over time.

By understanding these ions, you grasp why hard water is a common issue in many regions.