How does the solubility trend of Group 2 hydroxides compare to that of Group 2 sulfates?

They are opposite. Hydroxide solubility increases as you go down the group (from Mg to Ba), whereas sulfate solubility decreases as you go down the group.

Why does the pH of saturated Group 2 hydroxide solutions increase down the group?

As solubility increases down the group, more $OH^-$ ions are released into the solution. A higher concentration of hydroxide ions results in a more alkaline solution and a higher pH value.

What is the specific purpose of adding $HCl$ before $BaCl_2$ when testing for sulfate ions?

The acid reacts with and removes any carbonate ions ($CO_3^{2-}$) present in the sample. This prevents the formation of Barium Carbonate, which is also a white precipitate and would cause a false positive result.

Why is Barium sulfate safe to use as a 'Barium meal' despite Barium ions being toxic?

Barium sulfate is extremely insoluble in water and stomach acid. Because it does not dissolve, the toxic $Ba^{2+}$ ions are not absorbed into the blood and simply pass through the digestive system safely.

What observation is made when $BaCl_2$ is added to a solution containing $SO_4^{2-}$ ions?

A thick white precipitate of Barium sulfate ($BaSO_4$) forms immediately. This solid is the result of the low solubility of sulfates at the bottom of Group 2.

Which Group 2 hydroxide is commonly used in agriculture to treat acidic soil, and why?

Calcium hydroxide ($Ca(OH)_2$) is used because it is a base that can neutralize acid. It is preferred because it is more soluble than magnesium hydroxide but still safe to handle.

What happens to the reaction rate when Barium metal is placed in sulfuric acid compared to hydrochloric acid?

The reaction with sulfuric acid will quickly slow down or stop because an insoluble layer of $BaSO_4$ forms on the metal surface. In $HCl$, the resulting $BaCl_2$ is soluble, so the reaction continues vigorously.

Define 'sparingly soluble' in the context of Magnesium hydroxide.

It means that only a very small amount of the solid dissolves in water to form a solution. Most of the compound remains as a solid precipitate, resulting in a weakly alkaline solution.

What error occurs if a student uses sulfuric acid to acidify a sample before testing for sulfate ions?

The student would be introducing sulfate ions from the acid itself. This would cause a white precipitate to form with the Barium reagent regardless of whether the original sample contained sulfates, making the test useless.

Predict the solubility of Radium sulfate ($RaSO_4$) relative to Barium sulfate.

Radium is below Barium in Group 2, and sulfate solubility decreases down the group. Therefore, Radium sulfate would be even less soluble than Barium sulfate.

Library Podcasts

Courses

Referral & Rewards

Solubility of Group 2 Compounds: Hydroxides & Sulfates

Summary

The solubility of Group 2 compounds follows distinct, opposing trends down the group. While the solubility of hydroxides increases, leading to more alkaline solutions, the solubility of sulfates decreases, which is a property exploited in analytical chemistry for ion identification.

1. Solubility Trends of Group 2 Hydroxides

Diagram showing the opposing solubility trends of Group 2 hydroxides and sulfates down the group.

2. Solubility Trends of Group 2 Sulfates

Trend Direction: In contrast to hydroxides, the solubility of Group 2 sulfates decreases as you move down the group. Magnesium sulfate ( $MgSO_4$ ) is highly soluble, while Barium sulfate ( $BaSO_4$ ) is virtually insoluble.
Precipitation Observations: When a solution containing sulfate ions is mixed with a solution of a Group 2 metal lower in the group (like Barium), a dense white precipitate forms immediately. This lack of solubility is a defining characteristic of the heavier Group 2 sulfates.
General Formula: These compounds follow the general formula $XSO_4$ , where $X$ represents the Group 2 metal. The decrease in solubility is primarily due to the changes in lattice enthalpy and hydration enthalpy as the ionic radius of the metal cation increases.

3. Analytical Testing for Sulfate Ions

4. Medical and Industrial Applications

5. Key Distinctions for Problem Solving

6. Exam Strategy & Common Pitfalls