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Testing for Water with Anhydrous Copper II Sulfate

Updated July 2026

The presence of water can be chemically identified using anhydrous copper(II) sulfate. This qualitative test relies on a distinctive colour change from white to blue as the salt becomes hydrated. Mastering this test is essential for the ESAT Chemistry section, particularly for distinguishing between chemical presence and physical purity.

Core concept

The test for the presence of water involves the reaction of water with white anhydrous copper(II) sulfate (CuSO4CuSO_{4}) to form blue hydrated copper(II) sulfate (CuSO4⋅5H2OCuSO_{4} \cdot 5H_{2}O).

The Chemical Identification of Water

In laboratory chemistry, it is often necessary to confirm the presence of water in a sample. While physical observations like boiling point can indicate identity, chemical tests provide a definitive result for the presence of H2OH_{2}O molecules. The standard chemical test for the presence of water utilizes the properties of copper(II) sulfate.

The Reagent: Anhydrous Copper(II) Sulfate

The term anhydrous means without water. Anhydrous copper(II) sulfate, with the chemical formula CuSO4CuSO_{4}, is a dry, white powder. In this state, the substance lacks the water of crystallisation that usually gives copper salts their characteristic colour. Because it is highly hygroscopic, it readily reacts with any water it comes into contact with.

Procedure and Observations

To conduct the test, a student adds a few drops of the liquid in question to a sample of white anhydrous copper(II) sulfate powder.

  1. If water is present in the liquid, the powder will change colour from white to blue.
  2. This colour change is due to the formation of hydrated copper(II) sulfate.
  3. If the liquid does not contain water, the powder will remain white.

The Chemical Reaction

The reaction involves water molecules becoming integrated into the crystalline lattice of the copper(II) sulfate. This process is represented by the following chemical equation:

CuSO4(s)+5H2O(l)→CuSO4⋅5H2O(s)CuSO_{4} (s) + 5H_{2}O (l) \rightarrow CuSO_{4} \cdot 5H_{2}O (s)

The product, CuSO4⋅5H2OCuSO_{4} \cdot 5H_{2}O, is known as hydrated copper(II) sulfate. This reaction is exothermic, which means heat is released as the water molecules bond with the salt, often causing a noticeable temperature rise in the test tube.

Reversibility of the Test

The hydration of copper(II) sulfate is a reversible process. If the blue hydrated crystals are placed in a crucible and heated strongly, the water of crystallisation is evaporated and released as water vapour. As the water is lost, the blue crystals turn back into the white anhydrous powder:

CuSO4⋅5H2O(s)⇌CuSO4(s)+5H2O(g)CuSO_{4} \cdot 5H_{2}O (s) \rightleftharpoons CuSO_{4} (s) + 5H_{2}O (g)

Presence versus Purity

A critical distinction in the ESAT syllabus is the difference between testing for the presence of water and testing for the purity of water.

  • Presence: The anhydrous copper(II) sulfate test only tells you if water is there. It will give a positive blue result even if the water is impure, such as a mixture of water and ethanol.
  • Purity: To determine if water is pure, you must measure its physical constants. Pure water has a sharp boiling point of 100∘C100^{\circ}C and a melting point of 0∘C0^{\circ}C at standard pressure. Impurities will raise the boiling point and lower the melting point.

Key takeaways

  • Anhydrous copper(II) sulfate is a white powder with the formula CuSO4CuSO_{4}.
  • A positive test for water is a colour change from white to blue.
  • The blue product is hydrated copper(II) sulfate, CuSO4⋅5H2OCuSO_{4} \cdot 5H_{2}O.
  • This chemical test confirms the presence of water but does not prove its purity.
Tips

In the ESAT, if you are asked how to prove a sample of water is pure, do not suggest using anhydrous copper(II) sulfate. Instead, state that you would measure its boiling point to see if it is exactly 100∘C100^{\circ}C.

Cautions

Be careful not to confuse the reagent for water with the reagent for other gases. Copper(II) sulfate is specifically for water, while substances like limewater are for carbon dioxide.

Insight

The formation of hydrated copper(II) sulfate is a hydration reaction. The 'dot' in the formula CuSO4⋅5H2OCuSO_{4} \cdot 5H_{2}O signifies that the water molecules are part of the crystal structure but are not covalently bonded to the sulfate group itself.

Frequently asked questions

What does the term anhydrous mean in this context?

Anhydrous means the substance contains no water. In the case of CuSO4CuSO_{4}, it refers to the white powder that has had all water of crystallisation removed.

Can this test be used to measure the amount of water present?

No, this is a qualitative test used to recognize the presence of water. It is not used for quantitative analysis of the exact mass of water.

Why does the copper(II) sulfate turn blue?

The colour change occurs because water molecules bond with the copper(II) ions in the crystal lattice, forming a hydrated complex that absorbs different wavelengths of light, appearing blue.

Will anhydrous copper(II) sulfate turn blue if added to a mixture of water and alcohol?

Yes, it will turn blue because water is present in the mixture. This highlights why the test is for presence, not purity.

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