Distilled water is produced by boiling water and collecting the condensation. This process removes most impurities, but it may not remove all ions. It contains very few dissolved substances, but some ionic impurities can still be present.

Deionised water has had all the ions removed. This is achieved by passing water through an ion exchange resin, which exchanges the ions in the water with hydrogen and hydroxide ions. Deionised water is therefore essentially pure H₂O.

Deionised water is sometimes used in chemistry experiments when extremely high purity is required. This is because distilled water may still contain trace amounts of ions that could interfere with certain reactions or analyses. For example:

• Electrochemistry: In electrochemical experiments, the presence of even small amounts of ions can affect the voltage and current readings.
• Certain Analytical Techniques: Some analytical techniques, such as ion chromatography, require deionised water to avoid interference from other ions.
• Reactions Sensitive to Ions: Reactions that are highly sensitive to the presence of specific ions (e.g., precipitation reactions) may require deionised water to ensure accurate results.

While distilled water is often sufficient for many practical chemistry experiments, deionised water provides a higher level of purity and is essential for more demanding applications.

Chemical Equation:

CuSO₄(s) + xH₂O(l) → CuSO₄·xH₂O(aq)

Explanation:

The colour change from white to pale blue is evidence of the formation of hydrated copper(II) sulfate because water molecules (H₂O) are being incorporated into the crystal lattice of the copper(II) sulfate. The 'x' in CuSO₄·xH₂O represents a variable number of water molecules that can be associated with each copper(II) sulfate ion. When water molecules are added, they coordinate with the copper(II) ion, forming a hydrated crystal structure. This hydrated form, CuSO₄·xH₂O, has a different electronic structure and therefore absorbs light differently, resulting in the characteristic pale blue colour. The colour change is a direct consequence of the formation of this hydrated compound.

Apparatus:

• Anhydrous copper(II) sulfate (CuSO₄)
• Beaker (e.g., 50 cm³)
• Dropper

Procedure:

1. Place a small amount of anhydrous copper(II) sulfate (approximately 1-2 cm³) into a clean, dry beaker.
2. Carefully add a few drops of distilled water to the copper(II) sulfate in the beaker.
3. Observe the copper(II) sulfate solution for any colour change.

Expected Observations:

The anhydrous copper(II) sulfate will turn from a white colour to a pale blue colour. This indicates the presence of water, as the water reacts with the copper(II) sulfate to form hydrated copper(II) sulfate (CuSO₄·xH₂O), which is blue.