Question

Excess of dilute sodium hydroxide solution is gradually added with shaking to an aqueous solution of zinc sulphate or aluminium sulphatc. What would you obscrve? (1) A permanent white precipitate is formed immediately. (2) A white precipitate appears which dissolves to give a colourless solution. (3) No change in the beginning but later a white precipitate is formed on standing. (4) No change takes place and the solution remains clear.

Short answer

Option (2) is correct: A white precipitate appears which dissolves to give a colourless solution.

Step-by-step solution

- Identify the Reactants

Determine the substances involved in the reaction. In this case, the reactants are dilute sodium hydroxide (NaOH) and zinc sulphate (ZnSO₄) or aluminium sulphate (Al₂(SO₄)₃).

- Determine Potential Reactions

When sodium hydroxide is added to these sulphates, it can form hydroxides. The possible reactions are:For zinc sulphate: ZnSO₄ + 2NaOH → Zn(OH)₂ + Na₂SO₄For aluminium sulphate: Al₂(SO₄)₃ + 6NaOH → 2Al(OH)₃ + 3Na₂SO₄

- Analyze Solubility and Behavior

Both Zn(OH)₂ and Al(OH)₃ initially form white precipitates. Zn(OH)₂ is moderately soluble in excess NaOH forming a colorless, clear solution: Zn(OH)₂ + 2NaOH → Na₂[Zn(OH)₄]Al(OH)₃ dissolves in excess NaOH to form a colorless, clear solution: Al(OH)₃ + NaOH → Na{Al(OH)₄]

- Predict the Observations

Since both Zn(OH)₂ and Al(OH)₃ precipitates dissolve in excess NaOH, a white precipitate appears initially, which dissolves into a colorless solution upon adding excess NaOH.

- Select the Correct Option

Based on the analysis, the observed phenomenon matches option (2): 'A white precipitate appears which dissolves to give a colourless solution.'

Key concepts

formation of precipitates

When sodium hydroxide (NaOH) is added to an aqueous solution of zinc sulphate (ZnSO₄) or aluminium sulphate (Al₂(SO₄)₃), a series of reactions occur. In chemistry, a precipitate refers to an insoluble solid that emerges from a liquid solution.

Initially, the hydroxide ions (OH⁻) from the sodium hydroxide react with the metal ions (Zn²⁺ or Al³⁺). This reaction produces insoluble hydroxides, Zn(OH)₂ and Al(OH)₃. These hydroxides cannot dissolve in water and, therefore, form a white precipitate.

In practical terms, this means if you observe the solution, you will see a cloudy mixture or sediment due to the formation of these solid hydroxides. This is the visual cue for precipitation.

solubility of hydroxides

The solubility of a substance indicates how well it can dissolve in a particular solvent, such as water. In the case of our hydroxides—zinc hydroxide (Zn(OH)₂) and aluminium hydroxide (Al(OH)₃)—this property is crucial in understanding the resulting solutions added to the sodium hydroxide (NaOH).

When the sodium hydroxide is added in excess, these metal hydroxides interact further. For zinc hydroxide, the reaction can be:
Zn(OH)₂ + 2NaOH → Na₂[Zn(OH)₄]

For aluminium hydroxide, it can be:
Al(OH)₃ + NaOH → Na[Al(OH)₄]

Both reactions lead to the formation of complex anions that are soluble in water. The initial white precipitate dissolves, producing a clear and colorless solution. This increased solubility upon adding excess NaOH is a key observation in these chemical reactions.

chemical reactions

Chemical reactions involve the transformation of reactants into products through the breaking and forming of chemical bonds. In our case, the reactants are sodium hydroxide (NaOH) and either zinc sulphate (ZnSO₄) or aluminium sulphate (Al₂(SO₄)₃).

These reactions follow these steps:

• Sodium hydroxide provides hydroxide ions (OH⁻).
• These ions react with metal ions (Zn²⁺ or Al³⁺) to form insoluble hydroxides (Zn(OH)₂ or Al(OH)₃).
• Initially, a white precipitate forms as these hydroxides separate out from the solution.
• Upon adding excess sodium hydroxide, these hydroxides dissolve forming soluble complex ions.

The chemical equations representing these reactions are:

• For zinc: ZnSO₄ + 2NaOH → Zn(OH)₂ + Na₂SO₄ followed by Zn(OH)₂ + 2NaOH → Na₂[Zn(OH)₄]
• For aluminium: Al₂(SO₄)₃ + 6NaOH → 2Al(OH)₃ + 3Na₂SO₄ followed by Al(OH)₃ + NaOH → Na[Al(OH)₄]

Understanding these chemical reactions helps predict the sequence of visual changes in the mixtures and the reasoning behind the solution becoming clear again after initially becoming cloudy.