Problem 1 - Chemistry in 30 days

Question

Temporary hardness of water is due to the presence of (a) \(\mathrm{Mg}\left(\mathrm{HCO}_{3}\right)_{2}\) and \(\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}\) (b) \(\mathrm{MgCl}_{2}\) and \(\mathrm{MgSO}_{4}\) (c) \(\mathrm{CaCl}_{2}\) and \(\mathrm{CaSO}_{4}\) (d) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) and \(\mathrm{MgCl}_{2}\)

Step-by-Step Solution

Answer

Temporary hardness is caused by (a) \( \mathrm{Mg}\left(\mathrm{HCO}_{3}\right)_{2} \) and \( \mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2} \).

Step 1: Understanding Temporary Hardness

Temporary hardness is a type of water hardness caused by the presence of dissolved bicarbonate minerals, specifically calcium bicarbonate \( \mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2} \) and magnesium bicarbonate \( \mathrm{Mg}\left(\mathrm{HCO}_{3}\right)_{2} \). Temporary hardness is called so because it can be removed by boiling the water, which causes the bicarbonates to decompose into carbonate, water, and carbon dioxide.

Step 2: Identifying Chemical Compounds

From the given options, we need to identify which compounds are responsible for temporary hardness. Option (a) lists \( \mathrm{Mg}\left(\mathrm{HCO}_{3}\right)_{2} \) and \( \mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2} \), which are precisely the bicarbonate compounds causing temporary hardness.

Step 3: Analyzing Each Option

We analyze each option: - Option (a) contains bicarbonate salts, which directly relate to temporary hardness. - Option (b), (c), and (d) list chloride, sulfate, and nitrate salts, which are associated with permanent hardness, as they are not removed by boiling water.

Step 4: Conclusion and Selection

The compounds that cause temporary hardness are \( \mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2} \) and \( \mathrm{Mg}\left(\mathrm{HCO}_{3}\right)_{2} \), which are found in option (a).

Key Concepts

Bicarbonates

Bicarbonates are a type of chemical compound that play an important role in water chemistry. Specifically, they are salts containing the bicarbonate ion, \( ext{HCO}_3^-\). These compounds typically form from the combination of a metallic element like calcium or magnesium with the bicarbonate ion. For example, calcium bicarbonate \( ext{Ca(HCO}_3 ext{)}_2 \) and magnesium bicarbonate \( ext{Mg(HCO}_3 ext{)}_2 \) are common in natural water sources.
The presence of bicarbonates in water contributes to temporary water hardness. This type of hardness is temporary because it can be removed by heating the water. When heated, bicarbonates decompose to form carbonates, water, and carbon dioxide. This is a useful characteristic, as boiling can be an easy method to soften water at home.
In summary, bicarbonates like calcium and magnesium bicarbonates are responsible for temporary hardness, which can be resolved simply by boiling the water.

Water Hardness

Water hardness refers to the concentration of dissolved minerals in water, which affects its quality and usability. There are mainly two types of hardness: temporary and permanent. Temporary hardness is primarily caused by bicarbonates, such as calcium bicarbonate \( ext{Ca(HCO}_3 ext{)}_2 \) and magnesium bicarbonate \( ext{Mg(HCO}_3 ext{)}_2 \).
Permanent hardness, on the other hand, is caused by dissolved sulfates, chlorides, and nitrates like calcium sulfate or magnesium chloride. This type of hardness cannot be resolved simply by boiling the water. It often requires more involved processes like ion exchange or chemical treatment.
Water hardness affects various applications from household chores, like laundry and cleaning, to industrial processes. Hard water can leave mineral deposits on fixtures and appliances, which can decrease their efficiency and lifespan.

Chemical Compounds

Chemical compounds are substances consisting of two or more different elements bonded together. In the context of water hardness, we often discuss compounds like bicarbonates and chlorides.
Bicarbonates, as mentioned before, are responsible for temporary hardness. Chemical compounds such as calcium bicarbonate \( ext{Ca(HCO}_3 ext{)}_2 \) break down when heated to remove hardness.
In contrast, chlorides are associated with permanent hardness. Salts like magnesium chloride \( ext{MgCl}_2 \) do not decompose upon heating, and they require more sophisticated treatment methods to be removed from water. Understanding these differences in chemical behavior is crucial for addressing water hardness issues effectively.

Decomposition by Boiling

Decomposition by boiling is a physical process used to tackle temporary hardness in water. When water that contains bicarbonate compounds is boiled, a chemical reaction occurs. These bicarbonates break down into more simple compounds: carbonates, water, and carbon dioxide. The carbonates formed are usually insoluble in water, resulting in their precipitation as solid particles.
For instance, the decomposition of calcium bicarbonate \( ext{Ca(HCO}_3 ext{)}_2 \) by boiling results in calcium carbonate \( ext{CaCO}_3 \), water, and carbon dioxide gas. This reaction is summarized as:
\[ ext{Ca(HCO}_3 ext{)}_2 ext{ }(aq) \rightarrow ext{CaCO}_3 ext{ }(s) + ext{H}_2 ext{O} ext{ }(l) + ext{CO}_2 ext{ }(g)\]
This is why temporary hardness is called 'temporary'—because boiling is an effective way to remove it. Once the water cools, the precipitated carbonates settle at the bottom, leaving softer water above that is free from the temporary hardness.