Problem 27 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following hydroxides is insoluble in water? (a) \(\mathrm{Ca}(\mathrm{OH})_{2}\) (b) \(\mathrm{Ba}(\mathrm{OH})_{2}\) (c) \(\mathrm{Mg}(\mathrm{OH})_{2}\) (d) \(\mathrm{Sr}(\mathrm{OH})_{2}\)

Step-by-Step Solution

Answer

Mg(OH)₂ is insoluble in water.

Step 1: Understanding Solubility

First, we need to recall the solubility rules for hydroxides in water. Generally, hydroxides of alkali metals are soluble, while alkaline earth metal hydroxides become less soluble as you move up the group. Calcium, strontium, and barium hydroxides have higher solubility compared to magnesium hydroxide.

Step 2: Evaluating Magnesium Hydroxide

Magnesium hydroxide, Mg(OH)₂, is known to be sparingly soluble in water. This means it does not dissolve well, forming a suspension when mixed with water. It is commonly known as milk of magnesia.

Step 3: Evaluating Other Hydroxides

Looking at the solubility of the other hydroxides: - Calcium hydroxide, Ca(OH)₂, is moderately soluble. - Barium hydroxide, Ba(OH)₂, and Strontium hydroxide, Sr(OH)₂, are more soluble in water compared to magnesium hydroxide. This confirms that magnesium hydroxide is the least soluble among them.

Step 4: Conclusion

From the steps above, we can conclude that among the given options, magnesium hydroxide, Mg(OH)₂, is the hydroxide that is insoluble in water, compared to the others that have some degree of solubility.

Key Concepts

Solubility Rules

Understanding solubility rules is crucial in determining which compounds will dissolve in water and to what extent. These rules act like guidelines to predict solubility based on the type of ions involved. For hydroxides, the rules are quite specific:

• Hydroxides of alkali metals (like sodium and potassium) are generally soluble in water. They dissolve easily and completely.
• Hydroxides of alkaline earth metals, which include the second column of the periodic table, become increasingly insoluble as you move up the group. This means that the further up you go in this group, the less likely the hydroxide will dissolve in water.

Recognizing these patterns helps you quickly identify the solubility characteristics of different hydroxides without needing to test them all. This is why we can conclude that magnesium hydroxide is less soluble than calcium, strontium, and barium hydroxides.

Magnesium Hydroxide

Magnesium hydroxide, represented by the chemical formula \(\mathrm{Mg(OH)}_{2}\), is a fascinating compound with unique solubility properties. It is known as a sparingly soluble substance, meaning it does not dissolve well in water. When \(\mathrm{Mg(OH)}_{2}\) is mixed with water, it forms a white suspension, often referred to as "milk of magnesia".

• This suspension is used in medicine to treat digestive issues like heartburn and indigestion due to its ability to neutralize stomach acid.
• The low solubility is due to the strong interaction between magnesium ions \((\mathrm{Mg}^{2+})\) and hydroxide ions \((\mathrm{OH}^{-})\), preventing them from dissolving readily in water.

While sparingly soluble, magnesium hydroxide still has important applications in both medical and industrial fields, showcasing the relevance of understanding its solubility.

Alkaline Earth Metals

Alkaline earth metals belong to Group 2 of the periodic table and include elements like beryllium, magnesium, calcium, strontium, barium, and radium. They are known for their distinct properties and trends, especially in relation to solubility:

• Unlike alkali metals in Group 1, alkaline earth metals form oxides and hydroxides that are generally less soluble in water.
• As you move down the group from beryllium to radium, the metals become more reactive, but their hydroxides' solubility increases. However, magnesium stands out with its hydroxide being notably less soluble.

These metals are used in various industries, from medical applications to construction materials, largely due to their reactivity and the properties of their compounds. By understanding where they sit on the periodic table, we can predict their behavior in chemical reactions, including their solubility patterns.