Question

A metal \(\mathrm{M}\) readily forms water soluble sulphate \(\mathrm{MSO}_{4}\), water insoluble hydroxide \(\mathrm{M}(\mathrm{OH})_{2}\) and oxide MO which becomes inert on heating. The hydroxide is soluble in \(\mathrm{NaOH}\). The \(\mathrm{M}\) is: (a) \(\mathrm{Mg}\) (b) \(\mathrm{Sr}\) (c) \(\mathrm{Be}\) (d) \(\mathrm{Ca}\)

Short answer

The metal \( \mathrm{M} \) is (c) \( \mathrm{Be} \).

Step-by-step solution

Analyze the Solubility of Sulphate

The metal \( \mathrm{M} \) forms a water-soluble sulphate \( \mathrm{MSO}_{4} \). Among the given options, beryllium (\( \mathrm{Be} \)) sulphate is highly soluble in water, making option (c) \( \mathrm{Be} \) a possibility. However, strontium (\( \mathrm{Sr} \)), magnesium (\( \mathrm{Mg} \)), and calcium (\( \mathrm{Ca} \)) also have soluble sulphates.

Consider the Hydroxide Insolubility

The hydroxide \( \mathrm{M(OH)}_{2} \) is water-insoluble. Magnesium hydroxide \( \mathrm{Mg(OH)}_{2} \) is less soluble in water compared to beryllium, strontium, and calcium hydroxides, ruling out options \( \mathrm{Sr} \) and \( \mathrm{Ca} \), which have hydroxides relatively more soluble. Hence, we consider \( \mathrm{Be} \).

Check Inert Behavior of Oxide

The oxide \( \mathrm{MO} \) becomes inert on heating. Beryllium oxide \( \mathrm{BeO} \) is inert at high temperatures, forming a stable substance that does not easily react further. This behavior rules out \( \mathrm{MgO} \), which is not completely inert when heated.

Hydroxide Solubility in NaOH

The hydroxide \( \mathrm{M(OH)}_{2} \) is soluble in \( \mathrm{NaOH} \), forming a complex ion. Beryllium hydroxide dissolves in sodium hydroxide to form beryllate ions. This is a unique property to beryllium, confirming it as the right choice.

Key concepts

Water Soluble Sulphates

Sulphates are compounds containing the sulphate ion, \( \mathrm{SO}_{4}^{2-} \), paired with a metal. Many metal sulphates are soluble in water, making them important in various chemical processes.
When identifying a metal based on its sulphate's solubility, it's critical to know which metals form water-soluble sulphates.
Some commonly soluble metal sulphates include those of magnesium, calcium, and beryllium.

• Magnesium sulphate ( \( \mathrm{MgSO}_{4} \)) is well-known for its solubility, often used in medical and industrial applications.
• Beryllium sulphate ( \( \mathrm{BeSO}_{4} \)) is also highly soluble, an exception in the general trend of beryllium compounds.

The differing solubility of sulphates can help determine the identity of the metal involved when presented with similar options.

Hydroxide Solubility

The solubility of a metal hydroxide in water can provide critical clues when trying to identify unknown metals. Hydroxides consist of the hydroxyl ion \( \mathrm{OH}^{-} \) bound to a metal.
In the case of this exercise, the metal hydroxide formed is insoluble in water but can dissolve in \( \mathrm{NaOH} \).

• Magnesium hydroxide \( \mathrm{Mg(OH)}_{2} \) is sparingly soluble in water, often forming a heterogeneous mixture in aqueous solutions.
• Beryllium hydroxide \( \mathrm{Be(OH)}_{2} \) behaves differently. It dissolves in \( \mathrm{NaOH} \) to form complex ions called beryllates, while being largely insoluble in plain water.

This unique solubility trait of beryllium hydroxide is crucial in identifying beryllium as the metal in question when faced with similar options.

Inert Oxides

Some metal oxides display a unique behavior of becoming inert upon heating, meaning they don't easily react further. When a metal oxide is heated and remains non-reactive, it indicates that the oxide forms a stable compound.
Beryllium oxide \( \mathrm{BeO} \) is particularly noted for being chemically inert at high temperatures, unlike many other oxides.

• When heated, \( \mathrm{BeO} \) remains stable and won't readily decompose or react with other substances.

This inert behavior is useful for various high-temperature applications and is a key identifier for beryllium in chemical problem-solving contexts.

Beryllium Chemistry

Beryllium, represented by the symbol \( \mathrm{Be} \), shows distinct chemical behavior compared to other elements in the alkaline earth metal group. Its chemistry is characterized by the relatively high solubility of its sulphate and the unique properties of its hydroxide and oxide.
Beryllium hydroxide and oxide both play vital roles in distinguishing beryllium from other metals:

• Beryllium hydroxide \( \mathrm{Be(OH)}_{2} \) is notable because it forms beryllate ions when reacted with \( \mathrm{NaOH} \), displaying solubility that is atypical of other group 2 metal hydroxides.
• Beryllium oxide \( \mathrm{BeO} \) serves as a robust, inert material when subjected to high heat, staying stable and unreactive.

The distinct nature of beryllium compounds is essential in applications where chemical stability or specific solubility traits are needed, and they offer important clues when determining unknown metals in chemical exercises.