Question

Describe a chemical test you could use to determine whether a white solid is \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) or \(\mathrm{Na}_{2} \mathrm{S}_{2} \mathrm{O}_{3} .\) Explain the basis of this test using a chemical equation or equations.

Short answer

A chemical test distinguishing between Sodium Sulphate and Sodium Thiosulphate could involve adding dilute Hydrochloric acid. Sodium Thiosulphate will react to form a yellow precipitate of Sulphur, whereas Sodium Sulphate will not.

Step-by-step solution

Identify Suitable Tests

The initial task is to identify a test that would distinguish between sodium sulphate \(\mathrm{Na}_{2} \mathrm{SO}_{4}\) and Sodium thiosulphate \(\mathrm{Na}_{2} \mathrm{S}_{2} \mathrm{O}_{3}\). One suitable test would be the addition of dilute Hydrochloric acid (HCl) to each compound. Sodium sulphate would have no visible reaction, while sodium thiosulphate would produce a yellow precipitate of Sulphur.

Write Down the Chemical Reactions

After identifying the tests, the chemical reactions should be written for Sodium thiosulphate and Hydrochloric acid: \(\mathrm{Na}_{2} \mathrm{S}_{2} \mathrm{O}_{3} + 2\mathrm{HCl} \rightarrow 2\mathrm{NaCl} + \mathrm{S} + \mathrm{SO}_{2} + \mathrm{H}_{2}\mathrm{O}\). Here, Sulphur (\(\mathrm{S}\)) is the yellow precipitate observed.

Explain the basis of the Test

The basis of this test is that Sodium thiosulphate reacts with dilute Hydrochloric acid to form a yellow precipitate of Sulphur. Sodium sulphate does not produce this reaction, so there will be no observed change. Hence, after performing this test, observation of a yellow precipitate signifies Sodium thiosulphate; the absence of a yellow precipitate indicates Sodium Sulphate.

Key concepts

Sodium Sulphate

Sodium sulphate, \( \mathrm{Na}_{2} \mathrm{SO}_{4} \), is a white, crystalline solid that is often used in the production of detergents and in paper manufacturing. Being soluble in water, it easily dissociates into sodium ions \( (\mathrm{Na}^+) \) and sulphate ions \( (\mathrm{SO}_4^{2-}) \).
This compound is stable and does not react with dilute acids under normal conditions, making it a good candidate to differentiate between sodium sulphate and sodium thiosulphate when performing chemical tests.

• Does not produce visible reaction with dilute HCl.
• Used as a drying agent in laboratories.
• Non-reactive, making it easy to identify by exclusion.

Sodium Thiosulphate

Sodium thiosulphate, \( \mathrm{Na}_{2} \mathrm{S}_{2} \mathrm{O}_{3} \), is another white, crystalline solid frequently employed in photographic development and as a dechlorinator in pools.
When treated with dilute hydrochloric acid (HCl), it reacts to form a series of products.
In this reaction, the thiosulphate ion \( (\mathrm{S}_{2}\mathrm{O}_{3}^{2-}) \) is decomposed. This decomposition results in the formation of sulphur, which appears as a yellow precipitate.

• Commonly used to neutralize bleach in environments.
• Reacts visibly with dilute HCl, distinguishable in tests.
• Important in the iodine clock reaction, a classical chemical demonstration.

Hydrochloric Acid Reaction

Hydrochloric acid (HCl) is a strong, clear acid widely used in laboratories for its simplicity and reactivity.
When it comes to distinguishing between sodium sulphate and sodium thiosulphate, its role is crucial.
HCl reacts with sodium thiosulphate in the following chemical equation:
\[ \mathrm{Na}_{2} \mathrm{S}_{2} \mathrm{O}_{3} + 2\mathrm{HCl} \rightarrow 2\mathrm{NaCl} + \mathrm{S} + \mathrm{SO}_{2} + \mathrm{H}_{2}\mathrm{O} \]
This reaction leads to the formation of sulfur dioxide (\( \mathrm{SO}_{2} \)) gas, water, and a visibly yellow sulfur precipitate. This distinct reaction does not occur with sodium sulphate, making the test rapid and effective.

• Produces no visible change with sodium sulphate.
• Causes yellow sulphur formation with thiosulphate.
• The production of sulphur dioxide can be detected by its sharp smell.

Sulphur Precipitate

The formation of a sulphur precipitate is a key indicator in chemical testing.
This yellow solid is a result of a reaction involving sodium thiosulphate and hydrochloric acid \((\mathrm{HCl})\).
When sodium thiosulphate is exposed to HCl, elemental sulphur \((\mathrm{S})\) is produced, forming a visible yellow precipitate in the solution.
This specific reaction is both qualitative and visually discernible, which aids in identifying sodium thiosulphate among similar white solids.

• Yellow appearance helps in visual identification.
• Indicative of sodium thiosulphate in the presence of dilute HCl.
• Sulphur precipitates are insoluble, making them easy to observe.