Problem 31 - A Complete Resource Book in Chemistry for JEE Main

Question

A variety of water which contains soluble salts of \(\mathrm{Ca}\) and \(\mathrm{Mg}\) is known as (a) soft water (b) heavy water (c) conductivity water (d) hard water

Step-by-Step Solution

Answer

The correct answer is (d) hard water.

Step 1: Identify the Option

This exercise asks us to identify a term used for water that contains soluble salts of calcium (Ca) and magnesium (Mg).

Step 2: Understand the Terms

Soft water typically refers to water with low concentrations of calcium and magnesium ions. Heavy water is water where the hydrogen atoms are replaced by deuterium. Conductivity water refers to water used in experiments for measuring electrical conductivity.

Step 3: Define Hard Water

Hard water is the type of water that contains significant quantities of dissolved minerals, namely calcium and magnesium ions, which is exactly what the exercise describes.

Step 4: Select the Correct Option

Since hard water fits the description given in the exercise, the correct answer is option (d) hard water.

Key Concepts

Calcium Ions

Calcium ions are an essential component of many water sources. They are simply calcium atoms that have lost two electrons, giving them a positive charge, denoted as \( \text{Ca}^{2+} \). These ions contribute significantly to water hardness.
Calcium ions come from natural sources like the dissolution of limestone or gypsum into the water supply. When hard water enters your home, these calcium ions can lead to the formation of scale in pipes and appliances.
This scaling problem often arises in kettles, boilers, and even on shower heads, often referred to as limescale. It is a mineral build-up caused when hard water evaporates and leaves behind the calcium.

• Causes scale build-up
• Contributes to water hardness
• Originates from natural rocks like limestone

Knowing about calcium ions helps us understand why water treatment methods, such as ion exchange or water softeners, often target these in efforts to "soften" hard water.

Magnesium Ions

Similar to calcium ions, magnesium ions contribute to the hardness of water by being present in the water as positively charged ions, \( \text{Mg}^{2+} \). They too come from natural soil and rock formations that dissolve into the water.
Magnesium ions have some unique effects. For one, they can lead to the creation of insoluble compounds when they react with soap, which is why soaps do not lather well in hard water conditions.
Besides affecting soap's effectiveness, a presence of excessive magnesium ions may give water a bitter taste. In some instances, when magnesium salts like magnesium sulfate can accumulate, they might also have a laxative effect on some people.

• Cause poor soap lather
• Contribute to water hardness
• Can cause bitter taste in water

Treating water to reduce magnesium ions often improves both the taste and functionality of the water for household use.

Water Hardness

Water hardness is a term that refers to the concentration of calcium and magnesium ions in the water.
Hard water does not cause health problems directly, but it can affect household cleaning and appliance efficiency. When these ions are present in high concentrations, they interact with soaps and detergents, which can lead to soap scum and less effective cleaning.
Further, water hardness is often measured in grains per gallon (gpg) or milligrams per liter (mg/L), both representing the concentration of calcium carbonate. Anything above 7 grains per gallon is usually considered hard water.

• Measures calcium and magnesium content
• Affects cleaning effectiveness
• Measured in grains per gallon or mg/L

By understanding water hardness, one can address and correct issues caused by it, such as using water softeners or alternative cleaning products designed to work better with hard water.